Citation
Seasonal foraging dynamics and the importance of pine seeds in the diet of Steller's jays (cyanocitta stelleri)

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Title:
Seasonal foraging dynamics and the importance of pine seeds in the diet of Steller's jays (cyanocitta stelleri)
Creator:
Breindel, Sara
Publication Date:
Language:
English
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vii, 39 leaves : ; 28 cm

Subjects

Subjects / Keywords:
Steller's jay -- Food ( lcsh )
Forage ( lcsh )
Pine -- Seeds ( lcsh )
Genre:
bibliography ( marcgt )
theses ( marcgt )
non-fiction ( marcgt )

Notes

Bibliography:
Includes bibliographical references (leaves 35-39).
General Note:
Publications does not include any punctuation.
General Note:
Department of Integrative Biology
Statement of Responsibility:
by Sara Breindel.

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Source Institution:
|University of Colorado Denver
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Auraria Library
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
47122327 ( OCLC )
ocm47122327
Classification:
LD1190.L45 2000m .B73 ( lcc )

Full Text
SEASONAL FORAGING DYNAMICS AND THE IMPORTANCE OF PINE
SEEDS IN THE DIET OF STELLERS JAYS (CYANOCITTA STELLERI)
by
Sara Breindel
B. A., University of Virginia, 1992
A thesis submitted to the
University of Colorado at Denver
in partial fulfillment
of the requirements for the degree of
Master of Arts
Biology
2000


This thesis for the Master of Arts
degree by
Sara M. Breindel
has been approved
by
Diana F. Tomback
Leo P. Bruederle
30 /?60
Date


Breindel Sara M (M A Biology)
Seasonal Foraging Dynamics And The Importance of Pine Seeds In The Diet Of
Steller s Jays (Cyanocitta stelleri)
Thesis directed by Professor Diana F Tomback
ABSTRACT
I examined the year round variation in foraging substrate preferences of
Steller s Jays (Cyanocitta stelleri) focusing on the importance of pine seeds in their
diet To determine foraging patterns jays were observed year round from July 1998
to December 1999 at two study sites along the Front Range of the Colorado Rocky
Mountains Pine seeds were not available at these sites Two other sites were added
where pine seeds were available one in fall 1999 with ponderosa pine (Pinus
ponderosa) seeds and one in fall 2000 with limber pine (P flexilis) seeds When pine
seeds were available seed foraging rates in seconds per seed were recorded as well
as pine cone ripening phenology and ground density of wind blown seeds Data were
analyzed for patterns in substrate preference and pine seed use During this study no
one foraging substrate was used to the exclusion of all others The jays showed
seasonal variation in substrate preference variation in substrate preference among
different habitats and a definite preference for ponderosa pine seeds when available
Trees were the predominant foraging substrate in winter and summer During spring
and falls when pine seeds were not available general ground foraging predominated
When ponderosa pine seeds were available seed foraging began in September and
increased until it predominated in early November Jays carried ponderosa seeds in
their throats and flew off presumably caching them Once seeds were depleted in
December the jays returned to tree foraging The amount of time foraging for
ponderosa seeds corresponded to changes in ground seeds density and to changes in
seeds foraging rates The jays initially made attempts to forage for limber seeds but
limber seeds and cones were removed by other seed predators before jays could
forage for them From this it appears the jays are efficient opportunists that
preferentially forage on and potentially disperse pine seeds when available
This abstract accurately represents the content
its publication
Signed
mend
in
Diana F Tomback


ACKNOWLEDGEMENT
My thanks first and foremost to my advisor Diana F Tomback for her interest in me
as a student and a scientist and her support and tutelage during my tenure as her
student I also wish to thank Leo P Bruederle and Cheri A Jones for their
willingness to be on my thesis committee and for their valued support and guidance


CONTENTS
Figures......................................................................vi
Tables.......................................................................vii
Chapter
1. Introduction..............................................................1
2. Methods...................................................................6
2.1 Study Sites...............................................................6
2.2 Data Collection..........................................................8
2.3 Data Analysis............................................................9
3. Results..................................................................11
3.1 Seasonal patterns without pine seeds....................................11
3.2 Foraging patterns during fall with pine seeds...........................13
3.3 Fall comparisons........................................................16
4. Discussion...............................................................26
References
35


FIGURES
Figure
3 1 Seasonal substrate preferences at Kriley Pond
3 2 Seasonal substrate preferences at Knott Creek
3 3 Monthly substrate preferences at Gordon Creek
3 4 Ponderosa seed foraging dynamics
22
23
24
25
vi


TABLES
Table
3 1 Steller s Jay substrate preferences between like seasons in 1998 and 1999
at Kriley Pond and Knott Creek 18
3 2 Comparison of substrate preference of Steller s Jays within a season at
Kriley Pond and Knott Creek 19
3 3 Overall significant differences among seasons in Steller s Jay substrate
preferences at each study site 20
3 4 Comparisons of Steller s Jay substrate preferences between paired seasons
at Kriley Pond and Knott Creek 21
3 5 Comparisons Steller s Jay substrate use between paired months at
Gordon Creek 21
vii


1 Introduction
Steller s Jays (Cyanocitta stelleri family Corvidae) are territorial
opportunistic foragers that inhabit western North American montane forests Their
diet has been examined in some habitats and the make up of their diet differs among
different habitat types (Greene et al 1998) In general little is known about how the
jays use their environment to forage As sedentary year round residents of a range of
forest types the resources available to jays may differ greatly among these different
habitats The jays consume pine seeds in some habitats (Vander Wall and Baida
1981 Greene et al 1998 references therein) but it is not known how important pine
seeds are in their diet Pine sees a food resource provided by many Colorado forests
vary in availability quality and abundance among different habitats seasons and
years Seasonal and yearly changes in pine seed availability and abundance may
cause change in the jays foraging behavior In this study I examined year round
variation in foraging substrate preferences of Steller s Jays at several sites in the
Colorado Front Range of the Rocky Mountains focusing on how these preferences
change when pine seeds are available
Previous studies have shown that birds alter foraging behavior with changes
in resources In ponderosa pine (Pinus ponderosa) forests in Colorado Mountain
Chickadees (Parus gambeli) and Pygmy Nuthatches (Sitta pygmaea) vary their use of
foraging substrates between and among seasons (Manolis 1977 Ewell and Cruz
1998) Nuthatches (Sittidae) and woodpeckers (Picidae) in this habitat change
foraging locations between reproductive and non reproductive periods (Stallcup
1968 ) Avian species in other habitats also change foraging behavior when seasonal
environmental changes alter food abundance and distribution (Jackson 1970 Conner
1


1981 Morrison et al 1985 Lundquist and Manuwal 1990 Ford et al 1990 Hejl and
Vemer 1990 Kelly and Wood 1996 Wanless et al 1998) Foraging behavior also can
be influenced by diurnal intraseasonal and intersexual behavioral differences as well
as by the requirements of territorial behavior and predation avoidance (Maccarone
and Montevecchi 1986 Hejl and Vemer 1990 Sakai and Noon 1990 Kelly and
Wood 1996 Morris and Thompson 1998 Pulido and Diaz 2000) Apparently
foraging resource choice for many species is a plastic behavior that varies among
years at one site and among different sites occupied by the same species (Szaro et al
1990) As permanent year round residents in a variable environment Steller s Jays
would be expected to alter their foraging behavior with seasonal environmental
variation
Steller s Jays inhabit coniferous forests from southern Alaska to Central
America and from California to Colorado at the widest part of their range in North
America (Greene et al 1998) The jays live at elevations between 1 000 and 3 500 m
and maintain territories as permanently mated pairs (Brown 1963 Greene et al
1998) They are short distance fliers (Vander Wall and Baida 1981) with short
broad wings most suitable for maneuvering beneath forest canopy (Gill 1995) They
do not migrate however first year jays disperse to suitable habitats (Brown 1963
Westcott 1969) The social system of the Steller s Jay is based on site related
dominance where dominance decreases with distance from the nest with territories
overlapping at the edges (Brown 1963) Steller s Jays forage mainly in pairs within
their territories but will forage in groups in overlapping areas of territories (Brown
1963 Vander Wall and Baida 1981) Steller s Jay is often sympatric with other
corvids such as Clark s Nutcrackers (Nucifraga columbiana) Pinyon Jays
('Gymnorhinus cyanocephalus) and Western Scrub Jays (Aphelocoma californica)
(Westcott 1969 Vander Wall & Baida 1981) In Colorado Steller s Jays breed in
ponderosa pine Douglas fir (Pseudotsuga menziesii) pinyon juniper (Pinus edulis -
2


Juniperus spp) lodgepole pine (Pinus contorta) and spruce fir forests (Picea spp -
Abies spp ) (Greene et al 1998)
Steller s Jays do not range far for food and use a wide variety of food sources
(Brown 1963 Vander Wall & Baida 1981 Greene et al 1998) Steller s Jays forage
on the ground and in bushes and trees sometimes carrying larger food items to higher
perches before consuming them (Greene et al 1998) Their diet includes insects
grains conifer seeds fruits acorns bird eggs and occasionally other animal matter
such as the remains of small reptiles and mammals (Bent 1964 Vander Wall and
Baida 1981 Tomback and Linhart 1990 Greene et al 1998) Steller s Jays have been
described as probable nest robbers eating eggs and nestlings (Bent 1964) and have
been observed pirating the caches of Gray Jays {Perisoreus canadensis) (Burnell and
Tomback 1985) Thus their flexible feeding habits seem to be tailored to making the
best of staying in one place
Pine seeds may be an important food source for Steller s Jays (Greene et al
1998) Steller s Jays in pinyon pine communities carry pinyon pine seeds from cones
and cache the seeds within their territories (Vander Wall and Baida 1981) usually
making one seed caches which they share with mate and young Along the Colorado
Front Range of the Rocky Mountains likely sources of pine seeds include limber (P
flexilis) ponderosa and pinyon pines Corvids that eat pine seeds like Clark s
Nutcracker may prefer large wingless seeds like limber pine as opposed to small
seeds like those of lodgepole (P contorta) and bristlecone (P aristata) pine
(Tomback 1978) Pine seeds are a high energy but also unreliable food resource
(Janzen 1971 Richardson 1998) During this study there were no successful cone
crops of limber pine or ponderosa pine in 1998 at potential study sites examined
Ponderosa pines produced seeds at one site in 1999 limber pines produced seeds at
one site in 2000 As pine seed cachers Steller s Jays may play a role in the dispersal
3


of pine seeds a key factor affecting the recruitment density and composition of pine
forests (Janzen 1971 Tomback and Linhart 1990)
In this study I first determined how Steller s Jays allocate time among
foraging substrates in the absence of pine seeds This information served as a
baseline for comparison to jay foraging behavior when pine seeds an unpredictable
resource became available I hypothesized that seasonal patterns would be found in
substrate preference and that pine seeds when available would comprise a
significant part of their fall diet altering previous patterns of fall substrate use I
expected that the jays would use pine seeds in a manner that optimized energetic
efficiency as previously shown with nutcrackers (Vander Wall 1988 Torick 1995
Tomback et al unpub obs ) These data were used to address the following questions
First how do the jays use their habitat to forage and does this use change seasonally?
Second do foraging patterns vary among different habitat types? Third when pine
seeds are available are they used by jays as an important food resource? Fourth do
patterns of pine seed foraging suggest optimization of energetic efficiency? Fifth
could the jays act as secondary seed dispersers?
Jays were observed year round from June 1998 to December 1999 at two
study sites in the Front Range of the Colorado Rocky Mountains These two study
sites differed in habitat type and experienced little pine seed production during this
period Observations were also made from September to December 1999 at a third
site which produced a good crop of ponderosa pine seeds and in August and
September of 2000 at a fourth site that produced limber pine seeds Data were
compared among seasons at each site to determine whether the jays showed seasonal
substrate preferences Data also were compared between the two sites without seeds
but with different habitat types and between the sites with and without seeds From
these comparisons it was found that Steller s Jays do have seasonal substrate
preferences and that foraging substrate preferences vary among different habitat
4


types Pine seeds were preferred in fall when available and appeared to be used in an
efficient manner suggesting that the jays may be making foraging choices at least
partially based on energy return
5


2 Methods
2 1 Study Sites
Steller s Jays were observed at four study sites in the Front Range of the
Colorado Rocky Mountains Two study sites Kriley Pond and Knott Creek were
located in Golden Gate Canyon State Park in Jefferson and Gilpin Counties These
two sites were studied from July 1998 to December 1999 The Kriley Pond site
(elevation 2 600 m about 39 50 N 105 25 W) covers about 1 3 km2 This site
consists of a long west facing ridge covered with dense coniferous forest with
relatively well developed understory The primarily Doulgas fir forest (Benedict
1991 Mutel and Emerick 1992) includes limber pine ponderosa pine lodgepole pine
and at its highest elevations Engelmann spruce (Picea engelmannii) The forest
opens in the middle into a meadow with aspen (Populus tremuloides) stands along a
small creek
Jays may occur at a breeding population density of around 19 pairs per 0 4
km2 (Brown 1964) Based on this the Kriley Pond site could have included up to 62
pairs of jays although this could not be investigated in my study since jays were not
2
banded This is probably an overestimate for my sites because the 19 pairs/0 4 km
estimate was made in an area with tame jays who fed at picnic tables This may also
be an overestimate because it is not likely that all parts of the study site are suitable
jay habitat At most six jays were seen at one time at any of my study sites and so
that is the highest number that can be confirmed
The Knott Creek site (elevation 2 400 m about 39 51 N 105 21 W) covers
about 3 8 km2 and consists of a complex topography of hills and valleys with open
grassland Open spaces are interspersed with park like ponderosa pine dominated
6


stands with sparse shrub understory (Benedict 1991 Mutel and Emerick 1992)
Taller shrubs include mountain mahogany (Cerocarpus montanus) and common
juniper (Juniperus communis) At the edge of the site is a small creek with fir and
aspen Lodgepole pine and Douglas fir forests occur on the north and west faces of
slopes around this site
The Gordon Creek site covers about 2 km2 and is located northeast of
Nederland in Arapaho National Forest in Boulder Co (elevation 2 500 m about 40
00 N 105 30 W) Jays were observed at this site only during the last four months
of 1999 This site is similar in habitat structure and vegetation to Knott Creek but is
surrounded on the north and south sides by dense lodgepole pine forests
The Tremont Mountain site (elevation 3 000 m about 39 52 N 105 26 W)
in the northern part of Golden Gate Canyon State Park in Gilpin Co served as a
study site for two weeks at the end of August through the first two weeks of
September 2000 This site covers about 2 km2 and includes a densely forested north
facing ridge The forest is dominated by limber pine and Douglas fir (Benedict 1991
Mutel and Emerick 1992) with stands of aspen along stream banks and washes
All sites were chosen because of their resident jays and stands of ponderosa or
limber pine trees that could potentially produce a fall cone crop The Kriley Pond site
has both ponderosa pine at lower elevations and limber pine at its upper elevations
neither of which produced cones during this study The Knott Creek site produced a
ponderosa pine cone crop in fall of 1998 which ultimately failed because of insect
infestation The cones appeared to be infested by either the beetle larvae of the genus
Conophthorus as described by Hedlin et al (1981) or by moth larvae of the genus
Dioryctria as described by Schoettle and Negron (2000) The cones were shriveled
dark reddish brown and smaller than normal cones The cones failed to open and
were filled with frass and a few empty white seed coats Gordon Creek ponderosa
pines produced cones which yielded seeds in the fall of 1999 and the site was thus
7


added to the study at that time The Tremont Mountain site was added in late
summer 2000 when it was discovered that limber cones were being produced
2 2 Data Collection
This study was conducted from July 1998 to December 1999 and for four
weeks in August and September 2000 Developing pine cones were monitored for
cone opening phenology and insect infestation (see earlier description of cone
damage) Potential fall cone crops were initially identified in this manner
Observations of jay foraging behavior were made year round at Kriley Pond and
Knott Creek from July 1998 to December 1999 from September through December
1999 at Gordon Creek and from mid August to mid September 2000 at Tremont
Mountain Kriley Pond and Knott Creek were visited every other week in winter
weather permitting Observations began by 8 00 a m and ended by 1 00 to 2 00 p m
Sites were visited more frequently when pine seeds were available with two to three
days of observation per week at Gordon Creek and Tremont Mountain Jays were
located by sight and by calls and observed as encountered both with and without
binoculars
Once a jay was sighted I started a stopwatch and recorded the duration of
each new activity in seconds until the jay moved out of sight Duration of foraging
behavior substrate used in foraging and any foods taken were recorded A jay was
considered to be foraging once it probed a substrate with its bill Substrates were
defined by the locations of observed foraging behavior which included all parts of
live trees dead snags fallen logs rock surfaces shrubs stumps and various ground
surfaces These data were used to determine what foraging substrates jays used and
how the jays allocated foraging time among substrates
Cone ripening phenology was monitored at Gordon Creek and Tremont
Mountain Ten cone producing trees were marked with surveyor s tape and cone
8


counts were taken weekly Counted cones were categorized as open partly open
closed or aborted following the procedure of earlier studies (Torick 1995 Forbes
Tomback and Breindel unpublished data 1996) Ground density of ponderosa pine
seeds was also sampled weekly at Gordon Creek I assessed density by randomly
selecting a spot and counting all the ponderosa pine seeds in a one meter square plot
sampling one plot a week
Seed foraging rate was also recorded when a jay harvested seeds In this
study the jays harvested ponderosa pine seeds only from the ground but extracted
limber seeds directly from cones Seed foraging rate was recorded in seconds per
seed starting when the search began until time of departure from the patch of ground
or group of cones The total number of seeds taken was recorded The rate was
calculated from the time the bird began searching a patch of ground and picking up
seeds until it began another activity divided by the number of seeds taken during that
time
2 3 Data Analysis
For each foraging observation a frequency was calculated for each substrate
as the number of seconds spent foraging on this substrate divided by total seconds
foraging during that observation These data were combined into seasonal samples
i e summer 1998 fall 1998 winter 1998 99 etc Data were grouped in this manner
for two reasons First the combination of observations into larger data sets was
necessary for statistical analysis Second it was necessary to single out the time
period over which pine seeds were available for comparison with periods over which
pine seeds were not available Fall is defined as September through November the
season when pine seeds may be available The other seasons followed in three month
periods e g winter covers December through February
9


After all data were gathered foraging substrates were combined into four
categories The first category tree foraging included foraging on and under tree bark
on branches and trunks as well as on and under needles or leaves combining all tree
species used The second category ground foraging included any foraging directly
on the surface of the ground The remaining substrates including shrubs fallen logs
and rock surfaces were categorized as other The fourth category ground seed
foraging consisted of foraging for pine seeds from the ground This fourth category
was separated from other ground foraging once seeds were available to determine the
effect of seed foraging on substrate preferences during the period without seeds
Substrate preference was first described by substrate most frequently used
during a season Data were then analyzed by means of nonparametric statistics
Comparisons were made with the Kruskal Wallis one way analysis of variance and
the Mann Whitney test (Siegel and Castellan 1988) Among seasons comparisons of
substrate use and seed foraging rate were made with Kruskal Wallis analysis The
Mann Whitney test was used for pairwise comparisons
10


3 Results
3 1 Seasonal patterns without pine seeds
From June 1998 to December 1999 there were 174 foraging observations
recorded at Kriley Pond ( Kriley ) and Knott Creek ( Knott ) with 83 at Kriley and
91 at Knott The majority of foraging involved either ground or tree substrates (Figs
3 1 and 3 2) ponderosa and lodgepole pine comprised the large majority of the trees
used for foraging (89%) at Kriley Knott and Gordon Creek ( Gordon ) The
remaining species included Douglas fir spruce and aspen Lodgepole pine and
Douglas fir comprised the large majority of trees used for foraging (87%) at Tremont
Mountain ( Tremont ) Jays also were observed foraging year round on other
substrates such as fallen logs the branches of shrubs and from rock surfaces
The jays generally gleaned items from rock surfaces and from the leaves and
branches of shrubs Jays sometimes gleaned food items from the ground but also
probed into the ground and took items from just below the surface or moved ground
litter to find items Jays foraged on trees by multiple methods as well Jays gleaned
items from the surface of bark on branches or the trunk at all heights as well as
probed into crevices in the bark and removed bark to expose insects under the
surface The frequency of use of other substrates besides trees and the ground
remained low (between 0 10 and 0 20) with no statistically significant changes in use
among seasons (Mann Whitney test p > 0 05)
Substrate preferences at both sites show similar seasonal patterns Seasons
can be characterized by the most frequently chosen foraging substrate with two
exceptions (Figs 3 1 3 2 ) At both Kriley and Knott trees were the predominant
substrate in winter and summer except during part of the winter at Knott Spring was
11


characterized by mixed use of ground and tree substrates Ground foraging
predominated at both sites during fall except in fall 1998 at Knott when trees were
the preferred substrate During this study no foraging substrate was used exclusively
during any season
Outside of these general seasonal patterns substrate use differed between the
sites The jays at Knott used ground and tree substrates more equally overall but
were more likely to forage on the ground with a frequency of 0 46 for tree use and
0 51 for ground use At Kriley trees were more strongly preferred with a frequency
of 0 56 versus ground at 0 32 Substrate use at the two sites also differed in that
seasonal patterns were more consistent from year to year at Kriley than at Knott
(Table 3 1 Figs 3 1 3 2) Summer and fall substrate preferences were the same
during both years at Kriley with no statistically significant differences between the
frequency of ground and tree foraging from summer to summer or fall to fall (Table
3 1) At Knott substrate preferences differed significantly between summers with
much more ground foraging during the second summer although trees were still the
preferred substrate Fall substrate preferences also differed significantly at Knott
between years Ground foraging at Knott during the second fall was significantly
higher and tree foraging was significantly lower than the other fall seasons at the two
sites From this it appears the jays at Knott Creek in fall 1999 had significantly
different substrate preferences than the previous falls at either site
Jays at the two sites also differed in time of year at which their substrate
preferences were strongest and in the amount of seasonal variation In general the
jays at Kriley showed strong substrate preference in summer and winter whereas jays
at Knott showed strong substrate preference mainly in fall (Table 3 2) At Kriley
trees were used significantly more than ground in summer and winter but there was
no difference in use of the two substrates in spring and fall In contrast at Knott
preferences were strongest in fall although the preferred substrate was not consistent
12


with trees used significantly more in fall 1998 and ground in fall 1999 Overall
variation in substrate preference occurred at both sites in ground foraging and at
Knott in tree foraging as well (Table 3 3) However changes in substrate preference
from one season to the next were greater at Kriley than at Knott Mann Whitney test
analysis of paired seasons at Kriley showed that ground substrate preferences
changed significantly between three of five paired seasons {p < 0 05) (Table 3 4) and
that tree substrate preference changed significantly from fall 1998 to winter
However at Knott there is only one significant seasonal change the increase in
ground foraging from summer to fall 1998 (Table 3 4)
3 2 Foraging patterns during fall with pine seeds
In fall of 1999 the Gordon Creek site was added because it produced a
ponderosa pine cone crop Data were organized as previously described except that
they were grouped by month from September through December 1999 instead of by
season since observations only covered four months The category of ground seed
foraging was added to the categories analyzed once seeds were available At this site
the jays also exhibited a new behavior As cones ripened jays perched at the end of
branches with ripening cones and probed the cones with their bills The jays did not
forage for seeds directly from the ponderosa cones and did not attempt to break into
closed cones They sometimes foraged for insects on the same branch or in the same
part of the tree before and after probing cones
The jays harvested ponderosa pine seeds only from the ground A jay
foraging for seeds would hop down from a tree perch to the ground and hop with bill
pointed down while turning its head from side to side The jay occasionally stuck its
bill into ground litter and extracted a seed and then sometimes moved the seed
around in its bill Using binoculars I could distinguish between seeds and other
items like insects Seeds were sometimes dropped and retrieved When a jay
13


manipulated a seed in its bill the seed wing if present fell off To crack the seed
the jay opened and closed its bill a few times Seeds were either cracked and eaten or
placed into the jay s expanded esophagus for transport When placing a seed in the
esophagus for transport the jay tipped its head back pointing its slightly open bill
upwards Jays consuming seeds did not tip their heads back The jays flew away
with throats bulging with seeds presumably to cache the seeds The jays carrying
seeds flew north of the study site onto private land where I was unable to follow
them This occurred at the end of most of the ten seed foraging bouts observed
There were 76 foraging observations recorded at Gordon Creek The jays
substrate preferences during the fall changed with the presence of pine seeds on the
ground Tree foraging frequency remained relatively steady from September through
November sharply increasing in December (Fig 3 3) Ground foraging frequency
(not including seed harvesting) was relatively equal to tree foraging in September and
then steadily decreased through December to less than 0 01 as the ground harvest of
seeds increased from September to its peak in November Ground seed foraging was
concentrated during the early part of November when it reached its highest
frequency From the 2nd through the 14th of November 1999 ground seed foraging
was the predominant foraging behavior at a frequency of 0 38 (ground = 0 36 tree =
0 26) with 84% of all ground seed foraging observations were made during this
period Analysis of within season patterns at this site showed that substrate
preference changed significantly between November and December that is between
the fall and winter seasons There was an overall statistically significant difference
among months in use of ground substrate (Kruskal Wallis p = 0 038) (Table 3 3)
The only significant changes in paired comparisons between months were the
increase in tree foraging and decrease in ground foraging between November and
December (Table 3 5) This was at a time when seed foraging was declining and the
ground was covered with snow
14


There was also an overall statistically significant change in ground seed
foraging rate during the fall (Kruskal Wallis p 0 001) and a significant change
between November and December (Mann Whitney p = 0 0004) with foraging rate
becoming slower Ground seed foraging began at the end of October and increased to
its peak at the beginning of November (Fig 3 3) Seed foraging rate ground seed
density and the frequency of ground seed foraging all reached a turning point
between November 2nd and November 14th (Fig 3 4) Seed foraging rate became
faster from the end of October until the middle of November as ground seed density
increased with the fastest average foraging rate coinciding with the highest ground
seed density The frequency of seed foraging as a substrate preference also reached
its peak during the beginning of November increasing from zero in September and
dropping off in December The average foraging rate during this period was
significantly faster than the average foraging rate during the rest of the fall (Mann
Whitney p = 0 008) There was only one observation of seed foraging in October
and six in December with almost all seed foraging occurring only at the peak of
ground seed density in November (31 observations) Jays chose seed foraging most
frequently and were most efficient in foraging for seeds during the peak of seed
density (Fig 3 4) In December there were a few observations of jays taking seeds
Then the jays foraged for seeds from tall piles of needle litter which just broke the
surface of the snow around the base of ponderosa pine trees Upon inspection of
these piles I found them to include numerous loose pine seeds and cones that still
held some seeds These piles were likely cone middens made by red squirrels
(Tamasciurus hudsonicus) (Finley 1969)
In summer of 2000 limber pine trees at the Tremont Mountain site produced
cones Data were organized as previously described with the same substrate
categories There were 28 foraging observations over four weeks from August 26 to
September 16 The jays probed the ripening limber cones during the first week of
15


observations During the first week jays also foraged on the branches and trunks of
limber pine trees using other tree species after that time The jays at this site foraged
mostly on the ground and in trees other than limber pine (ground frequency = 0 49
tree frequency = 0 37 other substrates frequency = 0 12) and substrate use did not
differ significantly during the four weeks of observations (Kruskal Wallis p > 0 05)
(Table 3 3) Pairwise comparisons showed a significant change in preferred foraging
substrate only between the third and fourth weeks
Cone ripening phenology was monitored at Tremont and all cones were open
by the third week (September 8th) However the total number of cones on tagged
trees dramatically decreased after the first week from 316 to 153 cones and cones
continued to disappear during the following weeks There were three observations of
jays harvesting seeds directly from cones during the first two days of observations
Clark s Nutcrackers and red squirrels actively harvested seeds and whole cones
throughout the first two weeks During the third week seed harvest by nutcrackers
slowed until no nutcrackers were observed during the last week Red squirrels were
observed taking cones throughout the observations period The jays did not take
seeds nor probe cones after the first week
3 3 Fall comparisons
Fall data were compared among all study sites except Tremont because there
was only a month of data Fall substrate use differed significantly when ponderosa
pine seeds were available All fall data were combined for Kriley where fall
substrate use did not differ significantly Use of the ground substrate (not including
ground seed foraging) among Kriley Knott and Gordon differed significantly among
sites during the fall (Kruskal Wallis p = 0 0002) with ground substrate use much
lower at Gordon Ground substrate use at Kriley and Knott in 1998 did not differ
significantly but ground substrate use at either alone was significantly higher than at
16


Gordon (Mann Whitney p < 0 05) However when ground seed foraging was
included in ground substrate use at Gordon fall ground substrate use among all sites
did not differ significantly When ground foraging and ground seed foraging were
combined they remained relatively equal to tree foraging until December at Gordon
Use of trees at Gordon did not differ significantly from either fall at Kriley or from
fall 1998 at Knott
Fall 1999 at Knott also differed significantly from the previous fall at that site
and fall at the other sites Ground foraging was more frequent than tree foraging as
at Kriley However ground foraging was significantly higher at Knott in 1999 than
at Knott in 1998 Gordon in 1999 or either fall at Kriley Fall tree foraging at Knott
in 1999 followed the reverse pattern and is significantly lower than at Knott in 1998
either fall at Kriley and at Gordon in 1999
17


Table 3 1 Comparisons of Steller s Jay substrate preferences between like seasons
in 1998 and 1999 using the Mann Whitney test (*p < 0 05) P values below 0 05
indicate a significant change in substrate frequency between the two years
Summer Summer Fall Fall
Site Tree Ground Tree ground
Kriley Pond 1 183 1 922 0 309 0 180
Knott Creek 1 779 2 124* 2 802* 3 067*
18


Table 3 2 Comparison of ground versus tree substrate preference of Steller s Jays
within a season at Kriley Pond and Knott Creek using the Mann Whitney test
(*p<0 05) P vales below 0 05 indicate a significant difference between ground and
tree substrate frequency
Site Season z score p value
Kriley Pond Summer 1998 3 731 0 000*
Fall 1999 0 927 0 354
Winter 1998 99 3 546 0 000*
Spring 1999 0 447 0 655
Summer 1999 2 101 0 036*
Fall 1999 0 506 0613
Knott Creek Summer 1998 2 941 0 003*
Fall 1998 1 998 0 046*
Winter 1998 99 0218 0 827
Spring 1999 0 187 0 852
Summer 1999 0 248 0 804
Fall 1999 4 103 0 000*
19


Table 3 3 Overall significant differences among seasons in each substrate used by
Steller s Jays at each study site using the Kruskal Wallis one way analysis of
variance *P values below 0 05 indicate a significant change in overall substrate
frequency
Site and Substrate H p value
Kriley tree 9 843 0 080
Kriley ground 19 232 0 002*
Knott tree 14 560 0012*
Knott ground 18 689 0 002*
Gordon tree 7 393 0 061
Gordon ground 8 404 0 038*
Gordon seed 2 239 0 507
Tremont tree 5 877 0 118
Tremont ground 6 485 0 090
20


Table 3 4 Comparisons of Steller s Jay substrate preferences between paired
seasons using the Mann Whitney test (*p < 0 05) P values below 0 05 indicate a
significant change in substrate frequency between the two seasons
Site and Substrate Mann Whitney z score
Summ 98/ Fall 98 Fall 98/ Winter Winter/ Spring Spring/ Summ 99 Summ 99/ Fall 99
Kriley tree 1 868 2 165* 1 721 0917 0911
Kriley ground 2 887* 2 545* 2 183* 1 351 1 400
Knott tree 1 072 0 112 0 355 0 000 1 529
Knott ground 1 960* 0 447 0 355 0 173 1 477
Table 3 5 Comparisons of Steller s Jay substrate use between paired months at
Gordon Creek using the Mann Whitney test (*p<0 05) P values below 0 05 indicate
a significant change in substrate frequency between the two months
Site and Substrate Sept/Oct Oct/Nov Nov/Dec
Gordon tree 0 574 0 441 2 548*
Gordon ground 0 888 0 649 2 002*
Gordon seed 0418 1 066 1 219
21


@ tree
1 ______________________________________________________ ground
other
Summer 98 Fall 98 Winter 98 99 Spring 99 Summer 99 Fall 99
Season
Figure 3 1 Seasonal substrate preferences of Steller s Jays at Kriley Pond based on
mean frequency of use Category other includes substrates besides tree and ground
e g fallen logs rock surfaces and shrubs
22


Figure 3 2 Seasonal substrate preferences of Steller s Jays at Knott Creek based on
mean frequency of use
23


Frequency
1
08
06
04
02
0
M tree ground
0 seed other
September October November December
Month
Figure 3 3 Monthly use of substrates of Steller s Jays at Gordon Creek based on
mean frequency of use
24


Figure 3 4 Ground seed foraging by Steller s Jays ground seed foraging rate of
Steller s Jays and ground seed density at Gordon Creek in 1999 Seed foraging
represents the mean frequency of foraging time allocated by the jays to harvesting
seeds from the ground
25


4 Discussion
As expected based on their sedentary existence in a variable environment
Steller s Jays have foraging substrate preferences that differ by season These
preferences also vary among habitats with different vegetation structure The
availability of pine seeds alters fall foraging patterns in that the jays switch from
other foraging behaviors to seed harvesting when pine seeds are abundant This
suggests that the jays prefer pine seeds as a food source The pattern of the jays pine
seed use also suggests that they forage on pine seeds only when the seeds are
abundant and readily available
Sedentary lifestyles and opportunistic flexible foraging habits are common in
corvids Blue Jays (C cristata) (the only congener of Steller s Jays) Gray Jays
(Perisoreous canadensis) and Florida Scrub jays (Aphlecoma coerulescens) are all
monogamous and sedentary and exhibit omnivorous opportunistic feeding behavior
(Strickland and Ouelette 1993 Woolfenden and Fitzpatrick 1996 Tarvin and
Woolfenden 1999) Resident sedentary species tend to show more flexibility in
foraging repertoires (Miles 1990) Furthermore species occupying harsher more
variable environments also tend to show more variation in foraging behavior (Miles
1990) Steller s Jays fit both of these characterizations
The foraging patterns of Steller s Jays in this study were complex and
flexible They used a wide vertical range of substrates foraging from the ground up
to the tops of the trees Their use of substrates varied however from year to year and
among different habitats At each site jays foraged in different parts of their habitat
during different seasons and substrate preferences usually differed significantly
overall among seasons During spring and summer there were consistent substrate
preferences at the two sites which did not produce pine seeds In summer tree
26


substrates were predominantly used in both years at Kriley and Knott In spring jays
at both sites made mixed use of the two major substrate types ground and tree
However in fall and winter substrate preferences differed both between sites and
between years
Variation in substrate preferences may be affected by both the availability of
alternative resources and by habitat structure Ground substrates appear to be
important year round in jays foraging especially in fall However ground is also a
more seasonally limited substrate than trees because it can be covered by snow
during winter Winter at Kriley was characterized by almost complete use of trees for
foraging (over 80%) tree foraging was significantly higher than ground foraging
during this season Jays foraging in winter at Knott made more equal use of ground
and tree foraging with tree use slightly higher Kriley and Knott differ in landscape
structure and the jays at Knott may have had more opportunity to forage on the
ground in winter Knott has more south and east facing slopes and much more open
grassland During February of 1999 there was a period when daily temperatures
reached above 15C and snow melted exposing ground in open areas During this
same period there was a spike in ground foraging at Knott suggesting that snow
cover prevented ground foraging In general there were fewer foraging observations
in winter and spring months than in other seasons However tree foraging at Knott in
January accounted for 100% of all foraging whereas it averaged only 28% of all
foraging in February Tree foraging increased again to 67% in March when colder
temperatures and more snow returned and so ground foraging declined It is possible
that jays focused on trees in the winter at Kriley and during some months at Knott
mainly because the ground was covered by snow When snow melted and the ground
was exposed at Knott jays took advantage of the opportunity to use this substrate
Weather combined with differences in vegetation structure might help explain
other patterns in substrate use Spring in Colorado is characterized by a mix of very
27


warm weather and occasional heavy wet snow If snow cover is a major factor
leading to foraging in trees in the early part of the year this mix would lead to mixed
substrate use patterns Foraging substrates available in spring could shift from day to
day especially early in spring This shifting would lead to a more equivalent overall
seasonal use of substrates as the data show
Landscape structure also might lead to additional differences in foraging
patterns Ground foraging in general was higher at Knott than at Kriley (on average
51% of observed jay foraging versus 32%) with one exception that will be discussed
later Landscape structure might affect resources Knott has more open grassland
and this might offer better foraging resources than ground under tree cover Another
possibility is that Kriley s more diverse forest offers better tree foraging resources
than options on the tree covered ground Factors not related to food may be
important For instance watching for potential predators could make tree foraging a
better strategy in less open areas These possibilities deserve further research
Regardless of the cause it appears that foraging patterns differ among habitats as
well as seasons
Substrate use also might be affected by the potential availability of alternative
resources such as pine seeds In fall of 1998 at Knott there was a failed ponderosa
pine cone crop Trees in several ponderosa pine stands at the site produced from
dozens to hundreds of cones I monitored cone ripening phenology at ten trees but
not one cone opened and in fact it appeared that the vast majority of cones
throughout the study area did not open However before cones ripened during the
late months of summer and early months of fall jays probed cones without attempting
to remove seeds Similarly Steller s Jays also probed and harvested limber pine
seeds from cones at Tremont Mountain in fall 2000 They have been seen inspecting
cones and taking seeds of other pine species in other studies (eg P strobiformus
pers comm S Samano 1999) During fall 1998 at Knott the frequency of tree
28


foraging was higher than ground foraging unlike both falls at Kriley and the second
fall at Knott The other three falls did not differ significantly from each other in
ground foraging but were significantly higher in ground foraging than at Knott in fall
1998 During fall 1999 at Knott ground foraging was at its highest point for any
season at any site during the study It appears that when a cone crop was potentially
available the jays spent more time in the trees monitoring the cone crop leading to
increased tree foraging as compared to the same fall at Kriley and the subsequent fall
at both sites
Whereas ponderosa pine seeds were unavailable because cones failed to open
at Knott limber pine seeds at Tremont were unavailable because they were removed
quickly by more specialized seed foragers Jays at Tremont had only a small window
of opportunity to harvest limber pine seeds During that window many cones were
still partly or completely closed making them inaccessible to Steller s Jays which
unlike nutcrackers cannot break into closed cones (Vander Wall and Baida 1981)
At the same time about half of the cones were rapidly removed from the trees by red
squirrels and nutcrackers (Benkman 1984 Vander Wall 1988) The nutcrackers were
also at this time rapidly removing seeds from cones at all stages of ripeness By the
time the last cones opened probably very few seeds were left Jays even though they
spent time in limber pine trees initially and even inspected cones did not increase
tree foraging as they did during the failed ponderosa pine seed crop at Knott If the
reason behind increased tree foraging during early cone ripening is to monitor seed
availability that purpose disappeared very quickly at Tremont because cones were
very quickly removed Foraging patterns were therefore similar to fall seasons
without cone crops with use of ground substrates predominating The jays may have
had no more chance at getting limber pine seeds at Tremont than they had at
ponderosa pine seeds from the failed cones at Knott Additional research is needed to
29


see if limber pine seeds play a larger role in the fall diet of Steller s Jays when cone
production is greater potentially swamping the more specialized seed predators
When seeds were produced abundant and accessible the jays fall foraging
patterns changed The increase in fall tree foraging at Knott in 1998 is consistent
with foraging patterns at Gordon Creek in fall 1999 which produced a crop of
ponderosa pine seeds Use of trees was slightly higher than that of ground during the
fall months at Gordon Jays again inspected cones as they were ripening on the
ponderosa pine trees but did not take seeds from cones Clark s Nutcrackers and
other birds including Hairy Woodpeckers (Picoides villosus) and Mountain
Chickadees removed seeds from cones Once seeds were blown from the cones to
the ground jays foraging patterns diverged from those at Knott in 1998 with an
increase in ground seed foraging
Most cones opened by the third week in October at which time the first
ground seed harvesting was observed Seed harvest from the ground replaced other
ground foraging but did not affect tree substrate use When fall seasons with and
without seeds were compared among sites the frequency of tree foraging did not
differ significantly among sites with and without seeds but the frequency of ground
foraging was significantly lower at the site with seeds However when ground seed
foraging and other ground foraging at Gordon were combined they did not
significantly different from ground foraging at the other sites Also whereas a higher
and statistically similar frequency of tree foraging occurred at both Knott in fall 1998
and Gordon in 1999 ground foraging (not including seeds) was significantly lower at
Gordon than at Knott in fall 1998 This presumably occurred because at Gordon seed
foraging replaced other ground foraging in the fall
Pine seeds played a prominent role in the jays diet for a short period when
the seeds were at their peak density and foraging was most efficient For thirteen
days at the beginning of November ground seed foraging occurred more frequently
30


than all other foraging behaviors This increase in ground seed foraging
corresponded to a peak in ground seed density which then declined at the same time
seed foraging began to decrease (Fig 3 4) Seed foraging rate also increased at the
same time seed foraging decreased and ground seed density decreased The jays
appeared to focus primarily on seed foraging only when it was most efficient even
though they did forage for seeds before and after this point Jays still occasionally
foraged for seeds in small open patches in the snow around the base of ponderosa
pine trees as late as the end of December when observations ended In these cases
they might have been removing seeds from cones in squirrel middens which were at
the base of many ponderosa pine trees
When available pine seeds appear to be an important food for jays Pine
seeds are a high energy resource consumed and cached by corvids in temperate
habitats where food resources diminish and temperatures decrease in winter Cached
seeds provide a baseline level of food in times of winter scarcity Food caching in
general is more common in harsher and colder environments where climate is more
variable and cold winter temperatures help preserve stored food (Smith and Reichman
1984) The jays at Gordon Creek carried seeds in their esophagus and flew off site
presumably to cache them The dietary importance of the seeds to jays is underscored
by the switch from other food resources to ponderosa pine seeds as the seeds became
increasingly abundant
Food storage is a common attribute of corvid lifestyles and another way these
birds deal with variable food resources This behavior is best exemplified by Clark s
Nutcracker which depends upon conifer seeds year round (Vander Wall and Baida
1977 Tomback 1978 Tomback 1982 Tomback 1998) Other more sedentary and
omnivorous corvids including Steller s Jays Blue Jays Gray Jays Florida Scrub
Jays Mexican Jays (Aphlecoma ultramarina) and Black billed Magpies {Pica pica)
also cache mast and conifer seeds to supplement their diet (Smith and Reichman
31


1984 Strickland and Ouelette 1993 Brown 1994 Woolfenden and Fitzpatrick 1996
Greene et al 1998 Tarvin and Woolfenden 1999 Trost 1999) These species do not
solely subsist on caches during any part of the year However whereas these species
do not specialize on seeds they can be highly affected by seed availability For
instance lack of mast seed production may be associated with the occasional east
west movement of Blue Jays in the eastern United States (Smith 1986) Similarly
rare irruptions of Steller s Jays usually occur at the same time as irruptions of corvids
dependent on conifer seeds during times of exceptionally poor cone crops (Vander
Wall and Baida 1981 Greene et al 1998) This suggests an important role for conifer
seeds in the jays fall diet and potential for jays to act as pine seed dispersers
Ponderosa pine seeds and cones are adapted for wind dispersal but may be
secondarily dispersed by animals The seeds are small and winged and the cones on
the branch are oriented in all directions so that the seeds can be blown out of cones
by the wind (Tomback and Linhart 1990) Steller s Jays may act as secondary seed
dispersers a phenomenon already described for Jeffrey pine (P jeffreyi) another pine
primarily dispersed by wind (Vander Wall 1992) In the case of Jeffrey pine and
ponderosa pine secondary dispersal by animals may lead to seedling establishment
(Tomback 1978 Vander Wall 1992) During fall 1999 at Gordon Creek a point was
reached when the jays preferred the seeds over the other available resources With
this preference for seeds when they are abundant the jays potential as seed
dispersers increases with increasing seed production
Pine seeds do play a significant role in the jays fall diet because jays
abandon other resources for them However the jays are far from dependent on pine
seeds Previous observations suggest that the jays diet might consist largely of mast
seeds (Bent 1946 Greene et al 1998) However the jays at two of the sites in this
study did not have seeds available to them for two years and were likely relying on
insect food resources to a much larger extent than previously reported Whereas
32


determining the exact make up of the jays diet was a goal of this study the jays were
observed taking moths (Lepidoptera) grasshoppers (Orthopetera) and bees
(Hymenoptera) In areas where jays had been foraging ants (Hymenoptera) and
beetles (Coleoptera) were commonly found It was often not possible to discern what
insect the birds were taking during foraging observations However with no pine
seeds available the jays could not rely on cached seeds during the winter or any other
time of the year
The results of this study suggest that Steller s Jays are not just generalists but
efficient foragers that make use of high energy pine seeds in a way that maximizes
energy gain Although jays are not dependent on pine seeds ponderosa pine seeds
were preferred over other resources when available Jays foraged for ponderosa pine
seeds in a pattern corresponding with optimizing energetic efficiency such that the
frequency of seed foraging corresponded with foraging rate Jays appeared to harvest
seeds only when seeds were abundant and foraging rates were fast From this pattern
of resource use it appears that jays may base their foraging choices at least partly on
energy return
Other corvids are efficient foragers that can both evaluate the quality of
alternative resources and choose the more energetically profitable food item Clark s
Nutcrackers forage efficiently for pine seeds changing foraging behaviors to
maximize energy return (Vander Wall 1990 Torick 1995) Blue Jays in laboratory
studies were able to evaluate the quality of food patches and maximize the number of
prey taken improving with increased experience (Kamil and Yoerg 1985) Blue Jays
also recognized changes in patch quality and varied patch residence time with travel
distance so that increasing patch residence correlated with increasing travel time
between patches (Kamil et al 1985 Kamil et al 1988) Gray Jays maximized caloric
return by selecting prey based on multiple factors including caloric value handling
time and degree of crypsis (extent to which object is concealed or made hard to
33


recognize) (Maccarone and Montevecchi 1986) Gray Jays also recognized and
adjusted foraging efforts to food abundance by decreasing food storage and storing
food closer to the source as food resources increased (Stirling 1968) Steller s Jays
have many of the same habitat and lifestyle characteristics as other caching corvids
and may also be foraging so as to optimize energy return
Foraging substrate use appears to be affected by numerous factors across
small scale differences in habitat and resource availability leading to a complex
pattern of foraging Steller s Jay foraging behavior was sensitive to differences in
season habitat type and resource abundance Foraging varied significantly between
two sites with different vegetation and landscape structure that were only a few
kilometers apart The jays took advantage of snow melt altering foraging patterns
when a substrate became temporarily available The jays were sensitive to increases
in seed density on the ground quickly altering foraging patterns to take advantage of
increases and decreases in seed numbers Their switch to ponderosa pine seeds when
seeds were abundant suggests a potential role as secondary ponderosa seed dispersers
Pine seeds were the focus of foraging efforts when the seeds were abundant The
dynamics of both foraging substrate use by the jays and potential seed dispersal by
jays appear to be affected by a range of factors including season landscape structure
available resource alternatives and foraging efficiency The jays may be
opportunists but they are not random opportunists clearly foraging efficiently as
resources allow
34


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