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Behavioral responses by overwintering raptors to human approach

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Behavioral responses by overwintering raptors to human approach
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Norman, Celia Danielle
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ix, 27 leaves : illustrations ; 29 cm

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Birds of prey -- Behavior ( lcsh )
Birds of prey -- Effect of habitat modification on ( lcsh )
Birds of prey -- Behavior ( fast )
Birds of prey -- Effect of habitat modification on ( fast )
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bibliography ( marcgt )
theses ( marcgt )
non-fiction ( marcgt )

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Includes bibliographical references (leaves 25-27).
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Submitted in partial fulfillment of the requirements for the degree, Master of Arts, Biology.
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by Celia Danielle Norman.

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University of Florida
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Full Text
BEHAVIORAL RESPONSES BY OVERWINTERING RAPTORS
TO HUMAN APPROACH
by
Celia Danielle Norman
B.S., California Polytechnic State University, 1990
A thesis submitted to the
University of Colorado at Denver
in partial fulfillment
of the requirements for the degree of
Master of Arts
Biology
1996


This thesis for the Master of Arts
degree by
Celia Danielle Norman
has been approved
Date
Mil?


Norman, Celia Danielle (M.A., Biology)
Behavioral Responses by Overwintering Raptors to
Human Approach
Thesis directed by Charles R. Preston
ABSTRACT
Encroachment of human populations into natural areas increases confrontations
between humans and wild species. To mitigate negative impacts of human
intrusion into natural areas, it is useful to know how species adapt to human
activity.
To determine if buteonine hawks occurring in urban areas respond differently
to human intrusion than those in rural areas, I measured the flush distance of 84
individuals of three species wintering in eastern Colorado between 15 December
1994 and 26 February 1995.
Virtually all (97%) Rough-legged Hawks observed occurred in rural sites,
whereas observations of Ferruginous Hawks and Red-tailed Hawks were
distributed more evenly between rural (47 % Ferruginous Hawks, 41 % Red-tailed
Hawks) and urban sites. The results of a Kruskal-Wallis test performed on data
pooled from urban and rural sites showed a significant difference in flush response
among species (X2= 16.243; 2 df; P=0.0003). Scheffes test indicated that
in


Rough-legged Hawks flushed at a greater mean distance from human approach
than did the other two species. There was no difference in mean flush distance
between Ferruginous Hawks and Red-tailed Hawks. Both Ferruginous Hawks and
Red-tailed Hawks flushed at greater mean distances in rural than in urban sites, but
the difference was not statistically significant (Z= 1.189; P=0.2344). Thus, there
is no indication that familiarity with human activity increases the tolerance of these
species to human approach.
IV


CONTENTS
Figures...........................................................vi
Tables............................................................vii
Acknowledgements.............................................. viii
Dedication....................................................... ix
Chapters
1. Introduction.....................................................1
2. Study Design
Study Area...................................................... 5
Methods..........................................................6
Data Analysis................................................... 9
3. Results.........................................................12
4. Discussion......................................................20
Practical Applications..........................................23
Literature Cited.......................................................25
v


FIGURES
Figures
2.1. Representation of the state of Colorado. Observation points of
overwintering raptors at both urban and rural sites, 1994-1995........7
2.2. Representation of the state of Colorado. Observation points of
overwintering raptors at urban sites, 1994-1995.......................8
3.1 Number of overwintering raptors observed at urban and rural sites
in eastern Colorado, 1994-1995........................................14
3.2 Mean flush distances of overwintering raptors observed at all sites
in eastern Colorado, 1994-1995........................................15
vi


TABLES
Tables
2.1. Variables measured for possible influence on flush distance
of overwintering raptors in eastern Colorado, 1994-1995. ,..........11
3.1. Mean flush distances of overwintering raptors observed at U=urban;
R=rural; A=all sites in eastern Colorado,
1994-1995...........................................................16
3.2. Results of Wilcoxon-two-sample test for the flush distance of
overwintering F=Ferruginous Hawks; R=Red-tailed Hawks; P=pooled
sample of both species observed in eastern Colorado,
1994-1995......................................................... 17
3.3. Results of Wilcoxon-two-sample test for the flush distance of
overwintering Rough-legged Hawks observed in eastern Colorado,
1994-1995......................................................... 18
3.4. Results of Kruskal-Wallis test for flush distance of overwintering hawks
observed in eastern Colorado, 1994-1995.............................19
Vll


ACKNOWLEDGEMENTS
I am grateful for the guidance and encouragement of my committee members;
Alan Brockway for his enthusiasm and insight into the world of biology, Cheri
Jones for her moral support and knowledge of grassland mammals, and especially,
Charles Preston for his ecological sense, firm, yet gentle prodding throughout this
project, and his belief in my abilities. I am also grateful for advice from Frank
Hein, Ron Beane, and Kevin Felberg. Equipment was provided by the Denver
Museum of Natural History and financial awards from the University of Colorado
at Denver helped buffer the cost of this project.
I wish to acknowledge all the people of the eastern plains who helped me in
innumerable ways throughout my journeys and especially Glenn Shirley for
repairing my car on an early Sunday morning.
I would like to thank members of my family for their support. Mom, thank
you for sitting for many hours in my car with me watching hawk behavior. Dad,
thank you for your wisdom and encouragement. Esther, Hannah, Shirley and
Larry, thank you for your company on preliminary trips to study hawks and
general assistance. Thanks to the rest of my family for moral support. Finally, to
my husband Doug, thank you for being my pillar of strength and supporting me
always.
vm


DEDICATION
This work is dedicated to my nephew Sky, who helped
me explore the wonders of nature and quietly
succumbed to Her majesty.
tx


CHAPTER 1
INTRODUCTION
Due to the rapid encroachment of human populations into natural areas, wild
species are forced to encounter a wide variety of disturbances ranging from daily
vehicular traffic to recreational activities and urbanization. Pomerantz et al.
(1988) discussed possible impacts of such activities on wildlife, including: 1)
influencing the individuals energy budget; 2) decreasing productivity with
subsequent impacts on the entire population; 3) changing community composition
due to variable responses of its component species; 4) changing behavior
(Pomerantz, Decker, Goff, and Purdy 1988). Many species are simply forced out
of their natural ranges while others are able to cope with the co-occurrence of man
through immediate behavioral changes, behavioral changes provoked by additive
disturbances (Fraser, Frenzel, and Mathisen 1985), or habituation (Lee 1981,
Andersen and Rongstad 1989). Some species have even benefited by human-
caused disturbances. Limited construction and agriculture may expose previously
closed areas, thus creating a more suitable habitat for certain prey species while
allowing aerial hunters easy access to them (Littlefield, Thompson, and Johnstone
1992). Conversely, these opened areas may eliminate other prey species and
subsequently cause the exodus of raptors which are heavily dependent on them
(Schmutz 1989). Several diurnal raptors in eastern Colorado exist in close
1


proximity to human disturbance. These include Red-tailed Hawk (Buteo
jamaicensis), Bald Eagle (Haliaeetus leucocephalus), Ferruginous Hawk (Buteo
regalis), Rough-legged Hawk (Buteo lagopus), American Kestrel (Falco
sparverius), Prairie Falcon (Falco mexicanus), and Swainsons Hawk {Buteo
swainsoni). Of these species, the Red-tailed Hawk, Ferruginous Hawk, and
Rough-legged Hawk co-occur during the winter season from approximately
November through February (Preston and Beane 1996) These three species may
exhibit a variety of behavioral responses to human activity at their overwintering
sites.
In one study, raptors increased the size of their activity area, increased the
number of extra home range movements, and shifted the centers of their home
range away from nearby disturbances (Andersen, Rongstad, and Mytton 1990).
Red-Tailed Hawks have been shown to avoid areas completely when occupied by
humans (Andersen, Rongstad, and Mytton 1982). By showing increased timid
responses (Knight and Knight 1984), and greater flush distances when presented
with human stimuli (Holmes, Knight, Stegall, and Craig 1993) or ground level
disturbance (Knight and Knight 1984), certain behaviors may be altered
indefinitely (Stone, Snell, and Snell 1994). Flush distances appear to be related to
the age of the raptor, with older birds flushing earlier than younger, perhaps less
experienced birds (Stalmaster and Newman 1978). These studies indicate that as
the bird becomes accustomed to human presence, it learns of the dangers that
2


humans represent.
In a study of radio-tagged raptors, Andersen et al. (1990) found that Red-
tailed Hawks and Bald Eagles did not completely move out of their home ranges
when military training maneuvers commenced and one of two Ferruginous Hawks
and a Swainsons Hawk left the disturbed area completely, but returned the
following season once the disturbance had ceased (Andersen et al. 1990). This
study indicates that certain types of disturbances may only be a nuisance to the
individual bird and may not affect its behavior indefinitely. In addition,
habituation, the decreased responsiveness to repeated stimuli (McFarland 1985),
may occur in some species existing in frequently disturbed areas (Lee 1981,
Andersen and Rongstad 1989).
Although humans generally present problems for wild species, human-caused
disturbance has increased the habitat for certain types of prey species. Rough-
legged Hawks prefer previously grazed or mowed lands more than expected,
probably due to the increased number of rodent species associated with this type of
habitat (Littlefield et al. 1992). Ferruginous Hawk densities were shown to
increase where cultivation of an area was <30% (Schmutz 1989). Certain man-
made structures such as telephone poles and fenceposts are used by raptors as
perches for hunting (Schnell 1968, Fischer, Ellis, and Meese 1984), perhaps
increasing their hunting ranges beyond what was available to them naturally.
Northern Goshawks (Accipiter gentilis) and Coopers Hawks (Accipiter cooperii)
3


have shown substantial tolerance for disturbance near their nesting sites, although
there appears to be inter- and intrasexual variation in their wariness
responses (Lee 1981).
Wariness of a bird to disturbance can be quantified by measuring the
behavioral responses of raptors via flush responses, i.e. whether or not a raptor
flees from disturbances and at what distance (Holmes et al. 1993, Stone et al.
1994). Although the flush response may be modified through habituation to
disturbance or through individual selection for a specific habitat, the flush response
is an important measure of the effects of disturbances on raptor behavior. The
primary purpose of this study was to determine if there is a difference in wariness
of overwintering raptors to approach by humans in heavily populated urban areas
versus less populated rural areas. I tested the null hypothesis that there is no
difference in the flush behavior of raptors between these areas.
4


CHAPTER 2
STUDY DESIGN
Study Area
The study was conducted on the eastern plains of Colorado and along the
eastern side of the Rocky Mountains, encompassing an area of approximately
145,039 km2 (Kingery 1988) and ranging in altitude from approximately 1,020 m
along the eastern border to 1,525 m adjacent to the Front Range (Chronic and
Chronic 1972). Shortgrass prairie dominates the plains and is characterized by
perennial native grasses such as blue grama (Bouteloua gracilis) and buffalo grass
(Buchloe dactyloides) (Brown 1989).
Each point of observation within the study area was characterized as prairie
dog towns, crops, or grasslands. Prairie dog towns were recorded as areas with
the presence of active black-tailed prairie dog (Cynomys ludovicianus) burrows as
indicated by tracks in the snow leading to and from burrow entrances, fresh feces,
or visual observations. Crops were indicated by rows of cultivated
vegetation such as alfalfa and winter wheat. Grasslands include native shortgrass
prairies and pastures, as indicated by fenced off areas or the presence of grazing
mammals and fields or weedy growth consisting of species such as crested
wheatgrass (Agropyron desertum), thistle (Cirsium vulgare), and little bluestem
5


(Andropogon scoparius) (Dennis and Antonio 1980, Kingery 1988), all apparently
ungrazed or uncultivated.
Urban areas were characterized by frequent pedestrian traffic (1 pass min'1),
high levels of vehicle traffic (10-35 passes min'1), man-made structures within 250
m of the observed hawk, and other disturbances such as dogs barking or bicyclists
in the immediate area. Rural areas were defined as areas with low levels of
pedestrian traffic (<_1 passe min'1) or other disturbances and low to medium levels
of vehicle passes (0-5 passes min1).
Methods
I collected data between 15 December 1994 and 26 February 1995. All
observations were recorded between 09:00 h and 16:00 h. Observations in
different counties of the rural plains minimized the risk of repeated encounters
with the same individuals (Figure 2.1). It is uncertain that this risk was
minimized in urban areas due to the close proximity of data points and difficulties
in following the raptors once they had been flushed (Figure 2.2). At rural sites,
random routes were driven and raptors were spotted within 500 m of each side of
the road. Due to winter defoliation of trees and the open landscape of the
grasslands, raptors were relatively easy to find. I measured flush distances alone
in order to avoid observer bias (Wiens 1989). In all cases, I wore the same color
of clothing (a dark green jacket and blue jeans) to avoid possible difference in
hawk behavior
6


Figure 2.1. Representation of the state of Colorado. Observation points of
Overwintering raptors at urban and rural sites, 1994-1995.
108 107 106 105 104 103n
T-Re d-tailed Hawk
L-Rough-legged Hawk F-Ferruginous Hawk


00
Figure 2.2. Representation of the state of Colorado. Observation points of
overwintering raptors at urban sites, 1994-1995.
106
105
104
41
40
39
T- Re d-tailed
L-Rough-legged
F-Ferruginous


due to different visual stimulus (Gutzwiller and Marcum 1993).
Each hawk was approached at a standardized rate of 1.2 m sec'1. Once the
hawk was flushed, I observed it until it perched again or flew out of sight. The
initial perch angle was taken with an altimeter and distance to the base of the
perch was measured using a rangefinder which was calibrated using a set distance
of 100 m before each field trip. Based on these measurements, calculations were
performed to determine the flush distance to the raptor. The following parameters
were measured for each of 84 hawks: temperature using a standard liquid
thermometer to the nearest 1 degree-C; wind velocity using a Davis wind meter to
the nearest 0.5 m Sec"1; and visual observations of cloud cover and snow cover as
>50% or <.50%. Time of day (morning or afternoon), presence or absence of
shadows or black-tailed prairie dogs, and habitat type (prairie dog town, crops, or
grasslands) and age (immature, adult, or unknown) were also recorded.
Data Analysis
All species data were pooled and a nonparametric analysis of variance
(Kruskal-Wallis) (Zar 1974) was performed to determine if there was a significant
difference in mean flush distance among species. If there was a significant
difference, Scheffes test (Zar 1974) was then used to determine which species
differed in mean flush distance. Ferruginous Hawk and Red-tailed Hawk data
were pooled and Wilcoxon-two sample test (Zar 1974) was performed to determine
9


if there was a significant difference in the mean flush distance of these raptors at
urban and rural sites. Other potentially confounding variables, i.e. time of day,
temperature, wind velocity, cloud and snow cover, and presence or absence of
shadow and prairie dogs were screened using a Wilcoxon-two-sample test
(Kruskal-Wallis for habitat and age) in order to determine possible influences on
flush distances (Table 2.1). Data were then separated by species and Wilcoxon-
two-sample test was performed to determine if mean flush distances differed
between urban and rural sites for Ferruginous Hawks and Red-tailed Hawks and to
determine if other possibly confounding variables influenced the flush distances of
individual species (Table 1). The data were analyzed using SAS statistical
software (Procedures NPAR1WAY: Wilcoxon test and GLM) (Zar 1974, SAS
Institute, Inc. 1988).
10


Table 2.1. Variables measured for possible influence on flush distance of
overwintering raptors in eastern Colorado, 1994-1995.
Variable Description
Age I=Immature A=Adult U=Unknown
Time of Day M=Moming (Before 12:00 Noon)
A=Afternoon (After 12:00 Noon)
Temperature H=High (> 7 Degrees Celsius)
L=Low (<_ 7 Degrees Celsius)
Wind Velocity H=High (> 2 Meters per Second)
L=Low (<. 2 Meters per Second)
Cloud Cover H=High (>50% Sky Coverage)
L=Low (<_ 50% Sky Coverage)
Snow Cover H=High ( > 50% Ground Coverage)
L=Low (_<_ 50% Ground Coverage)
Shadow P=Presence A=Absence
Disturbance Type U=Urban R=Rural
Habitat Type C=Crops G=Grasslands P=Prairie Dog Towns
Prairie Dogs P=Presence A=Absence
11


CHAPTER 3
RESULTS
Data were analyzed for 84 hawks: 35 Ferruginous Hawks, 30 Rough-legged
Hawks, and 19 Red-tailed Hawks. Ferruginous Hawks and Red-tailed Hawks
were frequently encountered at urban sites, whereas Rough-legged Hawks were
observed almost exclusively at rural sites (Figure 3.1). Rural sites were
distributed across the eastern plains (Figure 2.1), whereas most urban sites were
clustered primarily around the Greater-Denver Metro area (Figure 2.2).
Mean flush distances were significantly different among species (2^= 16.23; 2
df; P=0.0003) (Figure 3.2 and Table 3.1). Scheffes test indicated that the mean
flush distance for Rough-legged Hawks was significantly greater than the mean
flush distance for both Ferruginous Hawks and Red-tailed Hawks. The mean flush
distances of Ferruginous Hawks and Red-tailed Hawks did not differ significantly
from one another. Rough-legged Hawk data were not included in pooled analysis
of flush distances between urban and rural locations due to the lack of observations
at urban sites. Though both Ferruginous Hawks and Red-tailed Hawks might have
appeared to have flushed at greater mean distances at rural than urban sites (Table
3.1), the difference was not statistically significant (Ferruginous Hawks:
Z=1.1954, P=0.2319; Red-tailed Hawks: Z=0.4961, P=0.6198). The pooled
12


observations for the two species were analyzed and again, no significant difference
was detected between the mean flush distance at urban and rural sites (Z= 1.1890,
P=0.2344) (Table 3.2). Flush distances were independent of other potentially
confounding variables for all species tests at both urban and rural sites (P > 0.05
for all tests) (Tables 3.2, 3.3, and 3.4).
13


Figure 3.1. Number of overwintering raptors observed at urban
and rural sites in eastern Colorado, 1994-1995.
30
25
20
t-<
X>
E 15
s
Z
10
i '
Urban
Rural
Ferruginous & Red-tailed Rough-legged
14


Meters
Figure 3.2. Mean flush distances of overwintering raptors
observed at all sites in eastern Colorado, 1994-1995
Ferruginous Red-tailed Rough-legged
15


Table 3.1. Mean flush distances of overwintering raptors observed at
U=urban; R=rural; and A=all sites in eastern Colorado,
1994-1995.
Species N Mean Flush Distance (m) +/- Standard Deviation Range
U R A U R A U R A
Ferrrug inous Hawk 22 13 35 54.18 +/- 48.81 67.08 +/- 38.27 58.97 +/- 45.04 1- 171 11- 160 1- 171
Rough-legged Hawk 1 29 30 NA 124.4 4 +/- 65.55 120.7 0 +/~ 67.60 NA 48- 350 12- 350
Red-tailed Hawk 11 8 19 70.73 +/- 40.12 76.75 +/- 36.31 73.26 +/ 37.68 16- 151 46- 160 16- 160


Table 3.2. Results of Wilcoxon-two-saxnple test (SAS Institute Inc.,
1988) for flush distance of overwintering F=Ferruginous Hawks,
T=Red-tailed Hawks, and P=pooled sample of both species observed
in eastern Colorado, 1994-1995.
N Z P
Variance F T P F T P F T P
Time:A 17 12 29 0.693 -0.677 -0.998 0.488 0.498 0.318
Time:M 18 7 25
Temp:L 13 2 15 -1.315 -0.665 -1.594 0.189 0.947 0.111
Temp:H 22 17 39
Cloud:L 24 10 34 -0.426 -0.531 -0.591 0.670 0.595 0.551
Cloud:H 11 9 20
Snow:L 24 3 40 -.0978 -0.336 -1.036 0.328 0.737 0.300
Snow:H 11 16 14
P-dogs:P 19 5 19 X2=. 716 -1.297 -0.485 0.699 0.194 0.664
P-dogs:A 10 14 16
Shadow:P 25 12 37 -0.420 -0.804 -0.671 0.674 0.421 0.502
Shadow:A 10 7 17
Disturb: U 22 11 33 1.195 0.496 1.189 0.232 0.620 0.234
Disturb: R 13 8 21


Table 3.3. Results of Wilcoxon-two-sample test (SAS Institute Inc.,
1988) for flush distance of overwintering Rough-legged Hawks
observed in eastern Colorado, 1994-1995.
Variance Description N Z P
Time A 17 -1.6118 0.1070
M 13
Temperature L 2 1.5390 0.1239 .
H 28
Cloud cover L 16 -1.0600 0.2890
H 14
Snow cover L 22 0.0704 0.9439
H 8
Prairiedogs P 0 NA NA
A . 30
Shadow P 20 -0.2420 0.8087
A 10
Disturbance U 1 NA NA
R 29


Table 3.4. Results of Kruskal-Wallis test (SAS Institute Inc., 1988)
for flush distance of overwintering hawks observed in eastern
Colorado, 1994-1995.
Species Variance Description N Y2 A df=2 P
Ferruginous Hawk Habitat C, G, P 6,10,19, respectively 0.7160 0.6991
Age I,A,U, 6,17,12 respectively 4.5583 0.1024
Rough-legged Hawk Habitat C,G 11,19 respectively 0.0568 0.4512
Age I,A,U 6,11,13 respect ively 2.4054 0.3004
Red-tailed Hawk Habitat C,G,P 4,9,6 respectively 0.5207 0.7708
Age I, A,U 5,5,9 respectively 0.0088 0.9956


CHAPTER 4
DISCUSSION
This study showed that Red-Tailed Hawks and Ferruginous Hawks did not
show a significant difference from one another in mean flush distances. However,
both Red-tailed Hawks and Ferruginous Hawks demonstrated a shorter mean flush
distance than Rough-legged Hawks. This supports work by Holmes et al. (1993)
which showed that Rough-legged Hawks flush at significantly greater distances
than Ferruginous Hawks (Holmes et al. 1993). Because the entire population of
overwintering Rough-legged Hawks migrates from the relatively less inhabited
region of upper Canada, this species may be shier than Ferruginous Hawks or
Red-tailed Hawks.
In addition, Schnell (1968) showed that Rough-legged Hawks are more active
than Red-tailed Hawks, which could explain why Rough-legged Hawks are more
readily flushed than either Red-tailed Hawks or Ferruginous Hawks in this study.
Though Ferruginous Hawks and Red-tailed Hawks tended to flush more
readily at rural sites, neither species in this study showed significantly different
mean flush distances between urban and rural sites. A larger sample size would
be necessary to determine if greater flush distances at rural sites are indeed
20


significant. Greater flush distances indicate that habituation to human stimulus as
predicted in studies of accipiters (Lee 1981) may not occur in these species, as
would be indicated by a decreased mean flush distances in heavily human
populated areas.
Raptors that apparently do not tolerate human activity, such as Rough-legged
Hawks, will simply not occupy areas with substantial human populations; 29 out
of 30 (97%) Rough-legged Hawks observed in this study were at rural sites.
Sixty-seven out of 79 (85% ) Rough-legged Hawks observed in a study of habitat
selection in wintering raptors were more than 0.8 km from a human-caused
disturbance (Fischer et al. 1984). Although Ferruginous Hawks, Rough-legged
Hawks, and Red-tailed Hawks are similar, they may respond differendy to similar
disturbances or habitat alterations (Lee 1981, Schmutz 1989). With more
infringement of human populations in natural areas, over time, less tolerant birds
may become less abundant (Andersen et al. 1990).
Studies have indicated that Rough-legged Hawks have benefited from human-
caused disturbance by increasing accessibility to certain prey through reduced
ground cover (Littlefield et al. 1992) or by increasing perch sites for hunting such
as lining roads with telephone poles (Schnell 1968). So while human activity in
the immediate area might not be acceptable to these hawks, past disturbance may
not pose substantial problems. In addition, most Rough-legged Hawks in this
study were presented with frequent vehicle disturbance; they were observed on
power lines paralleling two-lane roadways and appeared to have habituated to
21


traffic in rural areas, because most did not flush when vehicles passed. Such
habituation to common stimuli supports previous studies which indicate that raptors
are more likely to flush by a human on foot than by vehicle traffic (Holmes et al.
1993).
The type of disturbance presented may affect the flush response and flush
distance of raptors. In previous studies, disturbances have included
vehicles (Holmes et al. 1993), skiers or snowmobilers (Lee 1981), boaters
(Stalmaster and Newman 1978), one and multiple pedestrians (Holmes et al.
1993), and military activities (Andersen et al. 1982, Andersen et al. 1990).
Unusual disturbance may cause an exaggerated reaction (McFarland 1985),
whereas normal daily stimuli may not cause any quantitative reaction. For
example, a single observer on foot may cause a different behavioral reaction than a
military helicopter. In addition, tolerance to human activity may be related to a
specific location of disturbance (Stalmaster and Newman 1978).
The variable flush behavior of raptors may be due to other factors such as time
of year, geographic location, the amount of accessible prey in the vicinity, and the
foraging style of the particular species. Wintering populations of raptors have
decreased in density in response to prey declines, whereas breeding birds either
exhibit no such response or show lag times after the decline (Keith, Todd, Brand,
Adamcik, and Rusch 1977, Phelan and Robertson 1978). Generally, raptors in
higher latitudes exhibit stronger numerical responses to prey declines than lower-
latitude birds (Luttich, Keith, and Stefenson 1971). Red-tailed Hawks however,
22


showed no change in density when an epizootic plague eliminated over 90% of the
Gunnisons prairie dogs (C. gunnisoni) in a study region. Red-tailed Hawks
simply switched to an alternate prey species, but Ferruginous Hawks in the same
area declined during the plague due to their heavy reliance on prairie dogs (Cully
1991). Therefore, wintering birds or those at higher latitudes may flush sooner
than breeding or low-latitude birds. Similarly, generalist raptor species may flush
sooner than specialists simply because they would be able to find another suitable
prey item in an adjacent area. Future studies may focus on how the local prey
base influences flush behavior.
Other environmental factors and possible yearly variation in behavioral
responses which were not addressed in this study may have influenced results.
Practical Applications
Recent wildlife management practices include establishing buffer zones in
publicly-owned natural areas (Knight and Skagen 1988). These zones are areas
where people are not allowed to trespass, theoretically, giving wildlife a "buffer"
between their habitat and human disturbance. Sizes of buffer zones have not
generally been based on quantitative assessments of disturbance distances, and
variations in flush distances of wintering raptors pose a problem for the managerial
practice of establishing a universally-sized buffer zone (Fraser et al. 1985).
Resource managers may be able to use behavior studies such as this one to
23


establish buffer zones based on 1) the behavior of specific species in an area, and
2) the common disturbance types that wildlife may encounter in that area.
24


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Andersen, D.E. and O.J. Rongstad. 1989. Responses of nesting red-tailed hawks
to helicopter overflights. The Condor. 91:296-299.
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raptors exposed to increased human activity levels in southeastern Colorado.
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Holmes, T.L., R.L. Knight, L. Stegall, and G.R. Craig. 1993. Responses of
wintering grassland raptors to human disturbance. Wildlife Society Bulletin.
21:461-468.
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Keith, L.B., A.W. Todd, CJ. Brand, R.S. Adamcik, and D.H. Rusch. 1977. An
analysis of predation during a cyclic fluctuation of snowshoe hares. 13th
International Congress for Game Biology.
Kingery, H.E. ed. 1988. Colorado bird distribution latilong study. Colorado
Division of Wildlife, Denver.
Knight, R.L., and S.K. Knight. 1984. Responses of wintering bald eagles to
boating activity. Journal of Wildlife Management. 48:999-1004.
Knight, R.L., and S.K. Skagen. 1988. Effects of recreational disturbance
on birds of prey: a review. Pages 355-359 in R.L. Glinski, B.G. Pendleton,
M.B. Moss, M.N. LeFranc, Jr., B.A. Millsap,and S.W. Hoffman, eds.
Proceedings of the southwest raptor management symposium and workshop.
National Wildlife Federation, Washington D.C.
Lee, J.A. 1981. Habituation to human disturbance in nesting accipiters. Raptor
Research. 15:48-52.
Littlefield, C.D., S.P. Thompson, and R.S. Johnstone. 1992. Rough-legged hawk
habitat selection in relation to livestock grazing on Malheur National Wildlife
Refuge. Northwestern Naturalist.73:80-84.
Luttich, S.N., L.B. Keith, and J.D. Stefenson. 1971. Population dynamics of the
red-tailed hawk at Rochester Alberta. Auk 88:75-87.
McFarland, D. 1985. Animal behavior. The Benjamin/Cummings Publishing
Company, Inc., Menlo Park.
Phelan, F.J.S., and R.J. Robertson. 1978. Predatory responses of a raptor guild to
changes in prey density. Canadian Journal of Zoology. 56:2565-2572.
Pomerantz, G.A., D.J. Decker, G.R. Goff, and K.G. Purdy. 1988. Assessing
impact of recreation on wildlife: a classification scheme. Wildlife Society
Bulletin. 16:58-62.
Preston, C.R., and R.D. Beane. 1996. Occurrence and distribution of diurnal
raptors in relation to human activity and other factors at Rocky Mountain
Arsenal, Colorado. Pages 365-374 in D. Bird and J. Negro eds. Raptors in
human landscapes. Academic Press, London.
SAS Institute, Inc. 1988. SAS/STAT users guide. 6.03 ed. SAS Institute, Inc.
Carey, North Carolina.
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Schmutz, J.K. 1989. Hawk occupancy of disturbed grasslands in relation to
models of habitat selection. The Condor. 91:362-371.
Schnell, G.D. 1968. Differential habitat utilization by wintering rough-legged and
red-tailed hawks. The Condor. 70: 373-377.
Stalmaster, M.V., and J.R. Newman. 1978. Behavioral responses of wintering
bald eagles to human activity. Journal of Wildlife Management. 42:506-513.
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diversity: introduced cats and lava lizard wariness. Conservation Biology.
8(2):569-573.
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Full Text

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BEHAVIORAL RESPONSES BY OVERWINTERING RAPTORS TO HUMAN APPROACH by Celia Danielle Norman B.S., California Polytechnic State University, 1990 A thesis submitted to the University of Colorado at Denver in partial fulflliment of the requirements for the degree of Master of Arts Biology 1996 gg

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This thesis for the Master of Arts degree by Celia Danielle Norman has been approved c21 Hritz Date

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Norman, Celia Danielle (M.A., Biology) Behavioral Responses by Overwintering Raptors to Human Approach Thesis directed by Charles R Preston ABSTRACT Encroachment of human populations into natural areas increases confrontations between humans and wild species. To mitigate negative impacts of human intrusion into natural areas, it is useful to know how species adapt to human activity To determine if buteonine hawks occurring in urban areas respond differently to human intrusion than those in rural areas, I measured the flush distance of 84 individuals of three species wintering in eastern Colorado between 15 December 1994 and 26 February 1995. Virtually all (97 %) Rough legged Hawks observed occurred in rural sites, whereas observations of Ferruginous Hawks and Red-tailed Hawks were distributed more evenly between rural (47 % Ferruginous Hawks, 41 % Red-tailed Hawks) and urban sites. The results of a Kruskal-Wallis test performed on data pooled from urban and rural sites showed a significant difference in flush response among species (X2= 16.243; 2 df; P=O.OO03). Scheffe's test indicated that iii

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Rough-legged Hawks flushed at a greater mean distance from human approach than did the other two species. There was no difference in mean flush distance between Ferruginous Hawks and Red-tailed Hawks. Both Ferruginous Hawks and Red-tailed Hawks flushed at greater mean distances in rural than in urban sites, but the difference was not statistically significant (Z= 1.189; P=O.2344). Thus, there is no indication that familiarity with human activity increases the tolerance of these species to human approach. This abstract accurately represents the content of the Call1Ol
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CONTENTS FigUres ............................................................................. vi Tables .............. : ............................ .................................. vii Acknowledgements ....................................... ....................... viii Dedication ............. : .................. .......................................... ix Chapters 1. Introduction ..................................................................... 1 2. Study Design Study Area ....................................................................... 5 Methods .......................................................................... 6 Data Analysis .................................................................... 9 3. Results .......................................................................... 12 4. Discussion ...................................................................... 20 Practical Applications ......................................................... 23 Literature Cited ........................................................................ 25 v

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FIGURES Figures 2.1. Representation of the state of Colorado. Observation points of overwintering raptors at both urban and rural sites, 1994-1995 ............... 7 2.2 Representation of the state of Colorado. Observation points of overwintering raptors at urban sites, 1994-1995 ................................. 8 3.1 Number of overwintering raptors observed at urban and rural sites in eastern Colorado, 1994-1995 ................................................... 14 3.2 Mean flush distances of overwintering raptors observed at all sites in eastern Colorado, 1994-1995 ................................................... 15 vi

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TABLES Tables 2.1. Variables measured for possible influence on flush distance of overwintering raptors in eastern Colorado, 1994-1995 ..................... 11 3.1. Mean flush distances of overwintering raptors observed at U = urban; R=rural; A=allsites in eastern Colorado, 1994-1995 ............................................................................ 16 3.2. Results of Wilcoxon-two-sample test for the flush distance of overwintering F = Ferruginous Hawks; R = Red-tailed Hawks; P = pooled sample of both species observed in eastern Colorado, 1994-1995 ..... ...................................................................... 17 3.3. Results of test for the flush distance of overwintering Rough-legged Hawks observed in eastern Colorado, 1994-1995 .. .......................................................................... 18 3.4. Results of Kruskal-Wallis test for flush distance of overwintering hawks observed in eastern Colorado, 1994-1995 .................................. ; 19 vii

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ACKNOWLEDGEMENTS I am grateful for the guidance and encouragement of my committee members; Alan Brockway for his enthusiasm and insight into the world of biology, Cheri Jones for her moral support and knowledge of grassland mammals, and especially, Charles Preston for his ecological sense, firm, yet gentle prodding throughout this project, and his belief in my abilities. I am also grateful for advice from Frank Hein, Ron Beane, and Kevin Felberg. Equipment was provided by the Denver Museum of Natural History and financial awards from the University of Colorado at Denver helped buffer the cost of this project. I wish to acknowledge all the people of the eastern plains who helped me in innumerable ways throughout my journeys and especially Glenn Shirley for repairing my car on an early Sunday morning. I would like to thank members of my family for their support. Mom, thank you for sitting for many hours in my car with me watching hawk behavior. Dad, thank you for your wisdom and encouragement. Esther, Hannah, Shirley and Larry, thank you for your company on preliminary trips to study hawks and general assistance. Thanks to the rest of my family for moral support. Finally, to my husband Doug, thank you for being my pillar of strength and supporting me always. viii

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DEDICATION This work is dedicated to my nephew Sky, who helped me explore the wonders of nature and quietly succumbed to Her majesty. ix

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CHAPTER 1 INTRODUCTION Due to the rapid encroachment of human populations into natural areas, wild species are forced to encounter a wide variety of disturbances ranging from daily vehicular traffic to recreational activities and urbanization. Pomerantz et al. (1988) discussed possible impacts of such activities on wildlife, including: 1) influencing the individual's energy budget; 2) decreasing productivity with subsequent impacts on the entire population; 3) changing community composition due to variable responses of its component species; 4) changing behavior (Pomerantz, Decker, Goff, and Purdy 1988). Many species are simply forced out of their natural ranges while others are able to cope with the co-occurrence of man through immediate behavioral changes, behavioral changes provoked by additive disturbances (Fraser, Frenzel, and Mathisen 1985), or habituation (Lee 1981, Andersen and Rongstad 1989). Soine species have even benefited by human caused disturbances. Limited construction and agriculture may expose previously closed areas, thus creating a more suitable habitat for certain prey species while allowing aerial hunters easy access to them (Littlefield, Thompson, and Johnstone 1992). Conversely, these opened areas may eliminate other prey species and subsequently cause the exodus of raptors which are heavily dependent on them (Schmutz 1989). Several diurnal raptors in eastern Colorado exist in close 1

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proximity to human disturbance. These include Red-tailed Hawk (Buteo jamaicensis), Bald Eagle (Haliaeetus leucocephalus), Ferruginous Hawk (Buteo regalis), Hawk (Buteo lagopus), American Kestrel (Falco sparverius), Prairie Falcon (Falco mexicanus), and Swainson's Hawk (Buteo swainsoni). Of these species, the Red-tailed Hawk, Ferruginous Hawk, and Rough-legged Hawk co-occur during the winter season from approximately November through February (Preston and Beane 1996). These three species may exhibit a variety of behavioral responses to human activity at their overwintering sites. In one study, raptors increased the size of their activity area, increased the number of extra home range movements, and shifted the centers of their home range away from nearby disturbances (Andersen, Rongstad, and Mytton 1990). Red-Tailed Hawks have been shown to avoid areas completely when occupied by humans (Andersen, Rongstad, and Mytton 1982). By showing increased timid responses (Knight aild Knight 1984), and. greater flush distances when presented with human stimuli (Holmes, Knight, Stegall, and Craig 1993) or ground level disturbance (Knight and Knight 1984), certain behaviors may be altered indefmitely (Stone, Snell, and Snell 1994) Flush distances appear to be related to the age of the raptor, with older birds flushing earlier than younger, perhaps less experienced birds (Stalmaster and Newman 1978). These studies indicate that as the bird becomes accustomed to human presence, it learns of the dangers that 2

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humans represent. In a study of radio-tagged raptors, Andersen et al. (1990) found that Red tailed Hawks and Bald Eagles did not completely. move out of their home ranges when military training maneuvers commenced and one of two Ferruginous Hawks and a Swainson's Hawk left the disturbed area completely, but returned the following season once the disturbance had ceased (Andersen et al. 1990). This study indicates that certain types of disturbances may only be a nuisance to the individual bird and may not affect its behavior indefinitely. In addition, habituation, the decreased responsiveness to repeated stimuli (McFarland 1985), may occur in some species existing in frequently disturbed areas (Lee 1981, Andersen and Rongstad 1989). Although humans generally present problems for wild species, human-caused disturbance has increased the habitat for certain types of prey species. Rough legged Hawks prefer previously grazed or mowed lands more than expected, probably due to the increased number of rodent species associated with this type of habitat (Littlefield et al. 1992). Ferruginous Hawk densities were shown to increase where cultivation of an area was .$.30% (Schmutz 1989). Certain man made structures such as telephone poles and fenceposts are used by raptors as perches for hunting (Schnell 1968, Fischer, Ellis, and Meese 1984), perhaps increasing their hunting ranges beyond what was available to them naturally. Northern Goshawks (Accipiter gentilis) and Cooper's Hawks (Accipiter cooperii) 3

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have shown substantial tolerance for disturbance near their nesting sites, although there appears to be inter-:-and intrasexual variation in their wariness responses (Lee 1981). Wariness of a bird to disturbance can be quantified by measuring the behavioral responses of raptors via flush responses, i.e. whether or not a raptor flees from disturbances and at what distance (Holmeset al. 1993, Stone et al. 1994). Although the flush response may be modified through habituation to disturbance or through indiyidual selection for a specific habitat,the flush response is an important measure of the effects of disturbances on raptor behavior. The primary purpose of this study was to determine if there is a difference in wariness of overwintering raptors to approach by humans in heavily populated urban areas versus less populated rural areas. I tested the null hypothesis that there is no difference ill the flush behavior of raptors between these areas. 4

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CHAPTER 2 STUDY DESIGN Study Area The study was conducted on the eastern plains of Colorado and along the eastern side of the Rocky Mountains, encompassing an area of approximately 145,039 km2 (Kingery 1988) and ranging in altitude from approximately 1,020 m along the eastern border to 1,525 m adjacent to the Front Range (Chronic and Chronic 1972). Shortgrass prairie dominates the plains and is characterized by perennial native grasses such as blue grama (Bouteloua gracilis) and buffalo grass (Buchloe dactyloides) (Brown 1989). Each point of observation within the study area was characterized as prairie dog towns, crops, or grasslands. Prairie dog towns were recorded as areas with the presence of active black-tailed prairie dog (Cynomys ludovicianus) burrows as indicated by tracks in the snow leading to and from burrow entrances, fresh feces, or visual observations. Crops were indicated by rows of cultivated vegetation such as alfalfa and winter wheat. Grasslands include native shortgrass prairies and pastures, as indicated by fenced off areas or the presence of grazing mammals and fields or weedy growth consisting of species such as crested wheatgrass (Agropyron desertum), thistle (Cirsium vulgare), and little bluestem 5

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(Andropogon scoparilis) (Dennis and Antonio 1980, Kingery 1988), all apparently ungrazed or uncultivated. Urban areas were characterized by frequent pedestrian traffic (1 pass min-I), high levels of vehicle traffic (10-35 passes min-I), man-made structures within 250 m of the observed hawk, and other disturbances such as dogs barking or bicyclists in the immediate area. Rural areas were defined as areas with low levels of pedestrian traffic passe min -I) or other disturbances and low to medium levels of vehicle passes (0-5 passes min -I). Methods I collected data between 15 December 1994 and 26 February 1995 All observations were recorded between 09:00 hand 16:00 h. Observations in different counties of the rural plains minimized the risk of repeated encounters with the same individuals (Figure 2.1). It is uncertain that this risk was minimized in urban areas due to the close proximity of data points and difficulties in fOllowmg the raptors once they had been flushed (Figure 2.2). At rural sites, random routes were driven and raptors were spotted within 500 m of each side of the road. Due to winter defoliation of trees and the open landscape of the grasslands, raptors were relatively easy to fmd. I measured flush distances alone in order to avoid observer bias (Wiens 1989). In all cases, I wore the same color of clothing (a dark green jacket and blue jeans) to avoid possible difference in hawk behavior 6

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Figure 2.1. Representation of the state of Colorado. Observation points of Overwintering rap tors at urb an and rural site s, 1994-1995. L L T T-Red-tailed Hawk L-Rough-legged Hawk F LL-4 F L TL L L L L LF F L L L L F T F-2 T T T TL T /L-2 LF L L F-Ferruginous Hawk

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00 Figure 2.2. Representation of the state of Colorado. Observation points of overwintering raptors at urb an site s, 199.4-1995. T-Re d-taile d T T L F T F-2 F-J-2 F-14 T-8 Denver-Metro Are a L -Rough-Iegge d F L 1040 L F L L F-Ferruginous

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due to different visual stimulus (Gutzwiller and Marcum 1993). Each hawk was approached at a standardized rate of 1.2 m sec!. Once the hawk was flushed, I observed it until it perched again or flew out of sight. The initial perch angle was taken with an altimeter and distance to the base of the perch was measured using a rangefinder which was calibrated using a set distance of 100 m before each field trip. Based on these measurements, calculations were performed to determine the flush distance to the raptor. The following parameters were measured for each of 84 hawks: temperature using a standard liquid thermometer to the nearest 1 degree-C; wind velocity using a Davis wind meter to the nearest 0.5 m sec!; and visual observations of cloud cover and snow cover as > 50% or Time of day (morning or afternoon), presence or absence of shadows or black-tailed prairie dogs, and habitat type (prairie dog town, crops, or grasslands) and age (immature, adult, or unknown) were also recorded. Data Analysis All species data were pooled and a nonparametric analysis of variance (Kruskal-Wallis) (Zar 1974) was performed to determine if there was a significant difference in mean flush distance among species. If there was a significant difference, Scheffe's test (Zar 1974) was then used to determine which species differed in mean flush distance. Ferruginous Hawk and Red-tailed Hawk data were pooled and Wilcoxon-two sample test (Zar 1974) was performed to determine 9

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if there was a significant difference in the mean flush distance of these raptors at urban and rural sites. Other potentially confounding variables, i.e. time of day, temperature, wind velocity, cloud and snow cover, and presence or absence of shadow and prairie dogs were screened using a Wilcoxon-two-sample test (Kruskal-Wallis for habitat and age) in order to determine possible influences on flush distances (Table 2.1). Data were then separated by species and Wilcoxon two-sample test was performed to determine if mean flush distances differed between urban and rural sites for Ferruginous Hawks and Red-tailed Hawks and to determine if other possibly confounding variables influenced the flush distances of individual species (Table 1). The data were analyzed using SAS statistical software (Procedures NPARIWA Y : Wilcoxon test and GLM) (Zar 1974, SAS Institute, Inc. 1988). 10

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Table 2.1. Variables measured for possible influence on flush distance of overwintering raptors in eastern Colorado, 1994-1995. Variable Description Age I = Immature A=Adult U=Unknown Time of Day M=Morning (Before 12:00 Noon) A=Afternoon (After 12:00 Noon) Temperature H = High (> 7 Degrees Celsius) L= Low 7 Degrees Celsius) Wind Velocity H = High (> 2 Meters per Second) L = Low 2 Meters per Second) Cloud Cover H=High (> 50% Sky Coverage) L=Low Coverage) Snow Cover H = High (> 50% Ground Coverage) L=Low Ground Coverage) Shadow P=Presence A = Absence Disturbance U=Urban R=Rural Type Habitat Type C=Crops G = Grasslands P=Prairie Dog Towns Prairie Dogs P=Presence A=Absence 11

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CHAPTER 3 RESULTS Data were analyzed for 84 hawks: 35 Ferruginous Hawks, 30 Rough-legged Hawks, and 19 Red-tailed Hawks. Ferruginous Hawks and Red-tailed Hawks were frequently encountered at urban sites, whereas Rough-legged Hawks were observed almost exclusively at rural sites (Figure 3.1). Rural sites were distributed across the eastern plains (Figure 2.1), whereas most urban sites were clustered primarily around the Greater-Denver Metro area (Figure 2.2). Mean flush distances were significantly different among species ()f= 16.23; 2 df; P=0.OOO3) (Figure 3.2 and Table 3.1). Scheffe's test indicated that the mean flush distance for Rough-legged Hawks was significantly greater than the mean flush distance for both Ferruginous Hawks and Red-tailed Hawks. The mean flush distances of Ferruginous Hawks and Red-tailed Hawks did not differ significantly from one another. Rough-legged Hawk data were not included in pooled analysis of flush distances between urban and rural locations due to the lack of observations at urban sites. Though both Ferruginous Hawks and Red-tailed Hawks might have appeared to have flushed at greater mean distances at rura. than urban sites (Table 3.1), the difference was not statistically significant (Ferruginous Hawks: Z=1.1954, P=0.2319; Red-tailed Hawks: Z=0.4961, P=0.6198). The pooled 12

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observations for the two species were analyzed and again, no significant difference was detected between the mean flush distance at urban and rural sites (Z= 1.1890, P=0.2344) (Table 3.2). Flush distances were independent of other potentially confounding variables for all species tests at both urban and rural sites (P > 0.05 for all tests) (Tables 3.2, 3.3, and 3.4). 13

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Figure 3.1. Number of overwintering raptors observed at urban and rural sites in eastern Colorado, 1994-1995. 20 1 0 -1------1. : 5 Urban Rural 10 Ferruginous iii Red-tailed [] Rough-legged I 14

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Figure 3.2. Mean flush distances of overwintering raptors observed at all sites in eastern Colorado, 1994-1995 60 1----;::;:::==::::;----20 O....L...--.......... Ferruginous Red-tailed Rough-legged 15

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Table 3.1. Mean flush distances of overwintering raptors observed at U=urbani R=rurali and A=all sites in eastern Colorado, 1994-1995. species N Mean Flush Distance Rangeem) +/-Standard Deviation U R A U R A U R A Ferrruginous 22 13 35 54.18 67.08 58.97 1-11-1-Hawk +/+/+/-171 160 171 48.81 38.27 45.04 Rough-legged 1 29 30 NA 124.4 120. 7 NA 48-12-Hawk 4 0 350 350 +/+/I-' 65.55 67.60 0\ Red-tailed 11 8 19 70.73 76.75 73.26 16-46-16-Hawk. +/+/-+/-151 160 160 40.12 36.31 37.68

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Table 3.2. Results of Wilcoxon-two-sample test (SAS Institute Inc. 1988) for flush distance of overwintering F=Ferruginous.Hawks, T=Red-tailed Hawks, and P=pooled sample of both species observed in eastern Colorado, 1994-1995. N Z E Variance F T P F T p F T P Time:A 17 12 29 0.693 -0.677 -0.998 0.488 0.498 0.318 Time:M 18 7 25 Temp:L 13 2 15 -1. 315 -0.665 -1.594 0.189 0.947 0.111 Temp:H 22 17 39 Cloud:L 24 10 34 -0.426 -0.531 -0.591 0.670 0.595 0.551 ..... ...., Cloud:H 11 9 20 Snow:L 24 3 40 -.0978 -0.336 -1.036 0.328 0.737 0.300 Snow:H 11 16 14 P-dogs:P 19 5 19 X2= -1.297 -0.485 0.699 0.194 0.664 P-dogs:A 10 14 16 Shadow:P .25 12 37 -0.420 -0.804 -0.671 0.674 0.421 00502 Shadow: A 10 7 17 Disturb: 22 11 33 1.195 0.496 1.189 0.232 0.620 0.234 U Disturb: 13 8 21 R

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..... 00 Table 3.3. Results of Wilcoxon-two-sample test (SAS Institute Inc., 198'8) for flush distance of overwintering Rough-legged Hawks observed in eastern Colorado, 1994-1995. variance Description, N Z E Time A 17 ':"'1.6118 0.1070 M 13 Temperature L 2 1. 5390 0.1239 H 28 Cloud cover L 16 -1.0600 0.2890 H 14 Snow cover L 22 0.0704 0.9439 H 8 Prairiedogs P 0 NA NA A 30 Shadow P 20 -0.2420 0.8087 A 10 Disturbance U 1 NA NA R 29

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Table 3.4. Results of Kruskal-Wallis test (SAS Institute Inc., 1988) for flush distance of overwintering hawks observed in eastern Colorado, 1994-1995. Species Variance Description N E Ferruginous Hawk Habitat C, G, P 6,10,19, 0.7160 0.6991 respectively Age Rough-legged Habitat Hawk Age Red-tailed Habitat Hawk Age I,A,U, 6,17,12 4.5583 0.1024 respectively C,G 11,19 0.0568 0.4512 respectively I,A,U 6,11,13 2.4054 0.3004 respectively C,G,P 4,9,6 -0.5207 0.7708 respectively I,A,U 5,5,9 0.0088 0.9956 respectively

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CHAPTER 4 DISCUSSION This study showed that Red-Tailed Hawks and Ferruginous Hawks did not show a significant difference from one another in mean flush distances. However both Red-tailed Hawks and Ferruginous Hawks demonstrated a shorter mean flush distance than Rough-legged Hawks This supports work by Holmes et al. (1993) which showed that Rough-legged Hawks flush at significantly greater distances than Ferruginous Hawks (Holmes et al. 1993). Because the entire population of overwintering Rough-legged Hawks mIgrates from the relatively less inhabited region of upper Canada, this species may be shier than Ferruginous Hawks or Red-tailed Hawks. In addition, Schnell (1968) showed that Rough-legged Hawks are more active thari Red-tailed Hawks, which could explain why Rough-legged Hawks are more readily flushed than either Red-tailed Hawks or Ferruginous Hawks in this study Though Ferruginous Hawks Red':'tailed Hawks to flush more readily at rural sites, neither species in this study showed significantly different mean flush distances between urban and rural sites A larger sample size would be necessary to determine if greater flush distances at rural sites are indeed 20

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significant. Greater flush distances indicate that habituation to human stimulus as predicted in studies of accipiters (Lee 1981) may not occur in these species, as would be indicated by a decreased mean flush distances in heavily human populated areas. Raptors that apparently do not tolerate human activity, such as Rough-legged Hawks, will simply not occupy areas with substantial human populations; 29 out of 30 (97%) Rough-legged Hawks observed in this study were at rural sites. Sixty-seven out of 79 (85%) Rough-legged Hawks observed in a study of habitat selection in wintering raptors were more than 0.8 km from a human-caused disturbance (Fischer et al. 1984). Although Ferruginous Hawks, Rough-legged Hawks, and Red-tailed Hawks are similar, they may respond differently to similar disturbances or habitat alterations (Lee 1981, Schmutz 1989). With more infringement of human populations in natural areas, over time, less tolerant birds may become less abundant (Andersen et al. 1990). Studies have indicated that Rough-legged Hawks have benefited from human caused disturbance by increasing accessibility to certain prey through reduced ground cover (Littlefield et al. 1992) or by increasing perch sites for hunting such as lining roads with telephone poles (Schnell 1968). So while human activity in the immediate area might not be acceptable to these hawks, past disturbance may not pose substantial problems. In addition, most Rough-legged Hawks in this study were presented with frequent vehicle disturbance; they were observed on power lines paralleling two-lane roadways and appeared to have habituated to 21

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traffic in rural areas, because most did not flush when vehicles passed. Such habituation to common stimuli supports previous studies which indicate that raptors are more likely to flush by a human on foot than by vehicle traffic (Holmes et aI. 1993). The type of disturbance presented may affect the flush response and flush distance of niptors. In previous studies, disturbances have included vehicles (Holmes et al. 1993), skiers or snowmobilers (Lee 1981), boaters (Stalmaster and Newman 1978), one and multiple pedestrians (Holmes et al. 1993), and military activities (Andersen et al. 1982, Andersen et al. 1990). Unusual disturbance may cause an exaggerated reaction (McFarland 1985), whereas normal daily stimuli may not cause any quantitative reaction. For example, a single observer on foot may cause a different behavioral reaction than a military helicopter. In addition, tolerance to human activity may be related to a specific location of disturbance (Stalmaster and Newman 1978). The variable flush behavior .ofraptors may be due to other factors such as time of year, geographic location,the amount of accessible prey in the vicinity, and the foraging style of the particular species. Wintering populations of raptors have decreased in density in response to prey declines, whereas breeding birds either exhibit no such response or show lag times after the decline (Keith, Todd, Brand, Adamcik, and Rusch 1977, Phelan and Robertson 1978). Generally, raptors in higher latitudes exhibit stronger numerical responses to prey declines than lower latitude birds (Luttich, Keith, and Stefenson 1971). Red-tailed Hawks however, 22

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showed no change in density when an epizootic plague eliminated over 90% of the Gunnison's prairie dogs (c. gunnisoni) in a study region. Red-tailed Hawks simply switched to an alternate prey species, but Ferruginous Hawks in the same area declined during the plague due to their heavy reliance on prairie dogs (Cully 1991). Therefore, wintering birds or those at higher latitudes may flush sooner than breeding or low-latitude birds. Similarly, generalist raptor species may flush sooner than specialists simply because they would be able to find another suitable prey item in an adjacent area. Future studies may focus on how the local prey base influences flush behavior. Other environmental factors and possible yearly variation in behavioral responses which were not addressed in this study may have influenced results. Practical Applications Recent wildlife management practices include establishing buffer zones in publicly-owned natural areas (Knight and Skagen 1988). These zones are areas where people are not allowed to trespass, theoretically, giving wildlife a "buffer" between their habitat and human disturbance. Sizes of buffer zones have not generally been based on quantitative assessments of disturbance distances, and variations in flush distances of wintering raptors pose a problem for the managerial practice of establishing a universally-sized buffer zone (Fraser et al. 1985). Resource managers may be able to use behavior studies such as this one to 23

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establish buffer zones based on 1) the behavior of specific species in an area, and 2) the common disturbance types that wildlife may encounter in that area. 24 !i,,

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Literature Cited Andersen, D.E., 0.1. Rongstad, and W.R. Mytton. 1982. The behavioral response of a red-tailed hawk to military training activity. Raptor Research. 20:65-68. Andersen, D.E. and 0.1. Rongstad 1989. Responses of nesting red-tailed hawks to helicopter overflights. The Condor. 91:296-299. Andersen, D.E., O.J. Rongstad, andW.R. Mytton. 1990. Home range changes in raptors exposed to increased human activity levels in southeastern Colorado. Wildlife Society Bulletin. 18:134-142. Brown, L. 1989. Grasslands. Press, New York, New York Chronic, 1. and H. Chronic. 1972. prairie, peak and plateau: a guide to the geology of Colorado. Colorado Geological Survey Bulletin 32. Cully, J.F., If. 1991. Response of raptors to reduction of a Gunnison's prairie dog population by plague. American .Midland Naturalist. 125: 140-149. Dennis, E.C., and D.W. Antonio. 1980. Colorado range plants (paniallist). USDA. Soil Conservation Service. Denver, Colorado. Fischer,p.L., K.L. Ellis, and R.I. Meese. 1984. Winter habitat selection of diurnal raptors in central Utah. Raptor Research. 18:98-102. Fraser; J.D., L.D. Frenzel, and J.E. Mathisen. 1985. The impact of human activities on breeding bald eagles in north-central Minnesota. Journal of Wildlife Management. 49:98-lOi Gutzwiller, K.J., and H.A. Marcum. 1993. Avian responses to observer clothing color: caveats from winter point counts. Wilson Bulletin. 105:628-636. Holmes, T.L., R.L. Knight, L. Stegall, and G.R. Craig. 1993. Responses of wintering grassland raptors to human disturbance. Wildlife Society Bulletin. 21:461-468. 25

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