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1992 Predation Risk and Feeding Site Preferences in Winter Foraging Birds Yen-min Kuo This research is a product of the graduate program in Zoology at Eastern Illinois University. Find out more about the program.

Recommended Citation Kuo, Yen-min, "Predation Risk and Feeding Site Preferences in Winter Foraging Birds" (1992). Masters Theses. 2199. https://thekeep.eiu.edu/theses/2199

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m Predaton risk and feeding site preferences

in winter foraging oirds (Tllll)

BY

Yen-min Kuo

THESIS

SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF

Master of Science in Zoology

IN TH£ GRADUATE SCHOOL, EASTERN ILLINOIS UNIVfRSITY CHARLESTON, ILLINOIS

1992 YEAR

I HEREBY R£COMMEND THIS THESIS BE ACCEPTED AS FULFILLING THIS PART or THE GRADUATE DEGREE CITED ABOVE

2f vov. l'ft:lz, DAil ABSTRACT

A foraging animal's choice of feeding location may represent a trade­ off between maximizing its energy or nutrient intake and avoiding predation. In the present study, two hypotheses were investigated to test the influence of predation risk on feeding site preferences of birds: 1) there are differences among the preferences of feeding heights of birds, 2) the magnitude of preference increases with increasing predation risk found in different habitats. In my study site, three feeding stations (located in the woods, the woods/field edge, and an open field) each containing three feeders (0 m, 1.5 m, and 3 m from the ground) were established for attracting winter birds.

I found that birds apparently perceived different degrees of predation risk in the different habitats and responded accordingly. Birds visited the woods and edge stations significantly more than the open field station. In all three habitats, the birds preferred the higher feeders. As a probable consequence of the trade-off between foraging efficiency and predation risk, the preferences of feeder heights changed over the course of the experimental sessions in different patterns· in the different habitats. At the woods station, the birds tended to maximize their foraging efficiency. They changed their feeding heights more according to the seed densities on the feeding tray, and shifted to the lower feeders earlier. In contrast, birds preferred the high feeder throughout the experimental period at the open field station. At the edge station, birds shifted to the lower feeders later than at the woods station. In addition, the frequency of aggressive encounters between birds decreased with increasing openness of the habitat. These results supported the following conclusions: 1) the preferences of feeding height increased with increasing distance from the ground; 2) the preferences of feeding habitat decreased with increasing openness of habitats; 3) the intensity of the preferences for the higher feeding sites increase with increasing openness of habitats; and 4) these feeding site preferences could be adaptive behaviors to reduce the predation risk perceived by birds in different habitats and different feeding heights.

i i ACKNOWLEDGEMENTS

I thank my graduate committee members: Dr. Eric K. Bollinger, Dr.

Kipp C. Kruse, Dr. Clay L. Pierce, and Dr. Edward 0. Moll for their helpful

advice and critical reading of this paper. I especially wish to express my

gratitude to Dr. Eric K. Bollinger, my advisor, for his encouragement and support during my graduate study and research. I am indebted to him for his

careful revising of this thesis.

I am grateful to Fox Ridge State Park and the Illinois Department of

Conservation for giving me permission to conduct my research within the

park, to the Physical Plant of Eastern Illinois University and David Lee for providing the materials for the construction of the feeding stations, and to

Mr. Fred Decker for generous! y dona ting the sunflower seeds used in this study.

I would like to express my hearty gratitude to my friends in the United

States and in Taiwan for their moral support, especially to Amber Wu, Ya-Fu

Lee, Hsueh Niang Chen, Marcy Kuo, Tony Chen, Eric Linder, and Jerry Chen.

They always offered help enthusiastically whenever I needed it and made my research an unforgettable memory.

My deepest gratitude goes to Dr. Lucia L. Severinghaus who led me into this wonderful worlds of avian ecology and research and strengthened me with proper training and encouragement.

iii Finally, I want to express my heartfelt gratitude to my parents. I owe them the tears they have shed for me for these years while I was far away. I wish to thank them for their understanding, financial aid, and moral support of my graduate study.

iv TABLE OF CONTENTS

Abstract...... i

Acknowledgements...... iii

Table of contents ...... v

Literature review ...... 1

Introduction ...... 12

Methods ...... 15

Results...... 20

Discussion ...... 33

Literature cited ...... 46

Tables ...... 50

Figures ...... 71

v LITERATURE REVIEW

The risk of predation affects the foraging behavior of birds in many

ways. First of all, it is one of the fundamental forces of group formation.

Birds may forage in groups to reduce the risk of predation by reducing the

probability of being attacked (Hamilton, 1971; Baker, 1978; Caraco et al., 1980a;

Waite and Grubb, 1987). They also gain the advantage of spending less time

scanning for predators and more time feeding (Powell, 1974; Caraco, 1979a;

Caraco, 1979b; Caraco, 1980; Caraco et al., 1980b; Mangel, 1990). Within feeding

groups, predation pressure can affect both the interspecific or intraspecific

competition among group members (Dhondt, 1989). This often leads to social

organization associated with unequal costs and benefits among group

members within the feeding group. For example, high-ranking birds often

occupy the more profitable feeding sites, which provide either higher net

energy return or more effective protection from predators, or both. They

typically force the subordinate birds to feed in less profitable areas ( Alatalo,

1981; Alatalo and Moreno, 1987; Alatalo et al., 1987; Caraco et al., 1989), in

which the subordinates spend more time scanning for predators than the

dominants which feed at safer sites (Waite, 1987).

Predation risk and social foraging

A common answer to the question "why feed in a group?" has been to reduce the risk of both starvation and predation (Krebs et al., 1972; Powell,

1974; Mangel and Clark, 1986; Ekman, 1987; Mangel, 1990). A number of

1 studies designed to test these hypotheses about predation pressure and group

formation have employed a common design. Researchers have used "time"

as a currency to measure the costs and benefits of individuals feeding in

different group sizes, or under the presence or absence of predators. They

recorded and categorized a bird's behavior as: 1) scanning for predators; 2)

feeding; or 3) interference. These three categories are assumed to be mutually exclusive and to encompass all the activities of the birds at a feeding site. The time that the birds allocated in the different behaviors was then compared under different treatments. These "time budget" studies have provided evidence that birds in flocks are more effective than single birds in detecting predators (Powell, 1974). Furthermore, they have shown that both the time used in scanning for predators and the group size increase in the presence of predators (Caraco et al., 1980; Mangel, 1990). An optimal group size model was proposed based on these time budget studies. It predicts that the optimal group size would be reached when group members can spend the highest proportion of time feeding (Caraco 1979a; Caraco 1979b). However, there is no single group size which is optimal for all group members. Further studies have suggested that flock size is determined by a stable outcome to the conflict between dominants and subordinates rather than by a solution that is optimal to all members (Ekman, 1987). This conflict, originating from the skewed costs and benefits of group membership associated with social rank, has been studied and documented experimentally, both in field and aviary

2 studies.

For example, groups of Willow tits (Parus montances) were observed

in a coniferous forest habitat in Sweden. Willow tits form groups typically

composed of one pair of adults and another pair of first-year birds. Ekman

and Askenmo (1984) found that birds foraging in the same tree consistently

use the same relative height within the tree. Resource partitioning itself

cannot explain the fact that group members consistently assorted themselves

with adults higher than first-year birds. Furthermore, first-year birds took

over the spatial-foraging position of adults when the adult birds were

removed. The authors suggested that low- ranking birds pay a higher price

for flock membership by serving as "sitting ducks". They were excluded from

the higher canopy, which may provide better protection from predators. They

also suffered higher mortality than the dominant birds.

In mixed-species flocks of tits (Parus ~'the larger species forage in

the inner tree parts, whereas the smaller species exploit food items in the outer canopy. Using aviary experiments, Alatalo and Moreno (1987) found evidence to support the hypothesis that this pattern was the result of interference competition, with the socially-dominant larger species selecting the most profitable foraging sites, while forcing the smaller species into less rewarding foraging locations.

Regardless of whether a predator is present or absent, the risk of predation continuously affects a forager's decision whenever it is feeding.

3 Although predation pressure is difficult to measure directly, a forager's

feeding site selection could reveal the existence and intensity of the predation

risk.

Predation risk and feeding site selection

How does a forager select a feeding site? How many factors are taken

into consideration before a bird decides to feed at a certain habitat? What is

the role of predation risk in this decision making? Optimal foraging theory

predicts that the forager should maximize its net rate of energy intake by

occupying patches of high food abundance (MacArthur and Pianka, 1966).

However, Sih (1980) pointed out that at least one factor, the risk of being eaten while feeding, is important in altering this "optimal" behavior of foragers. If the demands of maximizing feeding rate and minimizing the risk of predation conflict, foragers must choose a strategy that will balance these conflicting demands in a way that maximizes fitness (Sih, 1982; Cerri and

Fraser, 1983; Dill, 1986).

Caraco (1980) found that when cover was available, Yellow-eyed

Juncos (Junco phaeonotus) spent less time scanning and more time in aggression. However, the reduction in scanning time was greater than the increase in aggression, so that the feeding time was still greater when cover was nearby. When Juncos were far from cover, scanning increased and aggression decreased. In small flocks, the decrease in aggression did not compensate for the increase in scanning, so that feeding time decreased far

4 from cover. Therefore, distance to cover influenced the quality of feeding

sites for these juncos.

If food densities among patches are equal but the patches differ in their

distances from cover, dominant birds should occupy the patch nearest to

cover because of its higher quality. Schneider (1984) studied a flock of White­

throated sparrows (Zonotrichia albicollis) for which he knew the dominance

rank. She presented the flock with feeding sites that varied in distance from

cover. Individual birds demonstrated a preference for sites near cover over

those in more open areas. Furthermore, whenever two or more birds fed

simultaneously at different distances from cover, a relative ranking was

assigned to the individuals involved; an individual closer to cover was

ranked higher than the bird further from cover. These observations were

used to construct a "distance hierarchy", which was significantly correlated to

the dominance hierarchy. Since high rank allowed access to preferred sites, evidence provided from the "distance hierarchy" supported the hypothesis that the protection from predators provided by a feeding site is also one of the important qualities which are competed for in birds.

In the same study, Schneider determined if the birds foraged in a way that maximized the rate of food intake. The food was presented in two distributions: uniform and patchy. In the uniform distribution, the food was sprinkled evenly on four strips that varied in distance from cover. This allowed more than one bird to feed at the same distance from cover. In the

5 patchy distribution, however, the food was distributed on four 10-cm squares

of matting, each of which could be dominated by a single bird. Results from

the uniform food distribution revealed that the areas close to cover tended to

be utilized first. Extensive feeding at more distant sites did not begin until

those sites close to cover were nearly exhausted. The tendency for birds to

deplete areas close to cover was still evident in the patchy distribution, but depletion of the more distant areas began earlier, because one patch could be occupied by only one bird. Subordinate birds were forced to feed further from cover much earlier in the experimental period. Birds did respond to changes in food density because they eventually shifted feeding areas, but they spent more time than expected at the low-density feeding site. These results have indicated that optimal foraging may involve more than simply maximizing the rate of energy intake.

One other similar study revealed that, for White-throated sparrows, regardless of dominance status, older birds fed closer to cover than younger birds and males fed in more dangerous situations than females. However, the reasons for these behaviors were not clear (Piper, 1990).

However, being close to cover does not always make a bird safe from predators. Lima et al. (1987) argued that cover can serve as a hiding place for mammalian predators. Therefore, a bird's use of space may reflect a trade-off between the perceived risk of feeding too close to cover versus that of feeding too far away.

6 Furthermore, Grubb and Greenwald (1982) tested how wintering

House Sparrows (Passer domesticus) balance the conflicting demands of energetic efficiency and predation protection by placing food at different distances from a brushpile and in positions differentially exposed to the wind. Results reinforced the hypotheses that: 1) where food patches are equal in net energy return, foragers use the one that provides the most protection from predators; 2) when food patches furnish equal protection from predation, foragers will feed in the area providing the largest net energy return; and 3) when birds have available a colder, safer feeding site and a warmer, riskier one, they will feed at both sites, and the extent of their foraging at the safer, colder site will be positively correlated with temperature and solar radiation, and negatively correlated with wind velocity. This study provided evidence that not only will animals trade-off their rate of food intake with predation risk (Schneider, 1984; Cerri and Fraser, 1983), they may also sacrifice foraging efficiency (by raising their energy expenditure) to reduce predation risk.

The trade-off between energy expenditure and predation risk has been tested in another study by Todd and Cowie (1990). A computer was programmed to control the food delivery rate of feeders. It became easier or more difficult to obtain food from individual feeders depending on how frequently they were used. Thus, the ease with which food could be obtained at each of the feeders, which were arranged at different distances from the

7 tree canopy, would reflect the energy cost of predation risk at that locality.

The results, again, supported the hypothesis that birds prefer to feed close to

cover even if it costs them more energy to obtain food items. Another

significant outcome of this study was that birds showed high sensitivity to a

feeder's distances from the tree canopy, even when the distance between the

nearest and furthest feeder was only three meters.

Despite considerable interest in the relationships between vigilance,

group size, food depletion, and distance from cover, few studies have

examined how birds relate the height of a potential feeding site to predation

risk. Lendrem (1983) demonstrated that feeder height is an important

determinant of tit feeding behavior. Not only did the birds spend less time

on the feeder, but scanning rates increased as tits fed closer to the ground.

Combining this result with results of studies considering distance from cover, we can derive a general pattern for the spatial distribution of avian foraging in response to predation pressure: birds prefer feeding close to cover and prefer higher feeding sites to lower ones. It is likely that this pattern will vary with species, habitat type, season, weather condition, and other factors.

The studies mentioned above were generally focused on fine-scale distributions of foraging birds and have shown that birds perceive fine differences in predation risk which constrain their foraging distributions.

How do these fine-scale patterns of foraging behavior in relation to predation risk affect broad-scale distribution patterns of foraging? Barnard (1980)

8 observed a winter population of House Sparrows feeding in two distinct

habitat types: cattlesheds and open fields. The risk of predation was higher in

the field where birds scanned more frequently than in the cattlesheds.

Individual time budgets were more influenced by flock size than by seed

density in the field but more influenced by seed density than by flock size in

the cattlesheds. He concluded that, the relative importance of feeding

efficiency and predator avoidance changed from one type of habitat to

another. These changes appeared to be adaptive in view of the selective

pressures the birds faced.

Strategies of feeding

The studies reviewed in the preceeding sections have demonstrated that areas or habitats with higher predation risk may be partially or completely avoided by foraging birds even when they contain higher food densities. However, after an animal has decided where to gather food, the next decision it has to make is where to consume the food.

Lima (1985) performed a field experiment to illustrate a way in which predation can influence the within-habitat decisions of foraging Black-capped

Chickadees (Parus atricapillus). A trade-off model was established. It considered that under some circumstances carrying prey items to protective cover before they are consumed will minimize the time spent exposed to predators. On the other hand, feeding at the place where the food items are found will maximize foraging efficiency. Whether or not there is a conflict

9 between maximizing foraging efficiency and minimizing exposure time

depends upon the handling time of the food item and the traveling time to

cover. This model predicts that the tendency to carry food from the site will

decrease as the distance of the patch from cover (traveling time) increases,

and will increase as the handling time of the food item increases. Short

handling times and/or long travel times typically lead to the birds

consuming the food where it is found. However, conflicts arise when there

are long handling times and/or short travel times to cover.

Lima used nine combinations of food item size (large, medium, and

small) and distance to cover (2, 10, and 18 m from the edge of cover) to test

his model. The results clearly showed that Black-capped Chickadees will

trade-off energetic considerations against the risk of being preyed upon under

conflict situations.

Conclusions

Roman and Cant (1984), in a discussion of foraging adaptations of

nonhuman primates, described three comprehensive adaptations of animals:

avoiding predation, acquiring food, and reproducing (reviewed by Mangel

and Clark, 1986). When prey supplies are virtually unlimited and predictable

in time and space, scanning for predators takes priority over feeding and

feeding takes priority over initiating fights (reviewed by Barnard, 1980).

Predation pressure, although generally very difficult to detect, is a strong selection force which constantly affects an animal's foraging decision-

1 0 making. A variety of adaptive behaviors associated with predation pressure have been demonstrated by researchers. First, birds congregate to reduce their predation risk. Within the feeding group, dominant individuals can further decrease their probability of being attacked by occupying the safer feeding sites and forcing the subordinates to feed at sites further from cover. In addition, birds must consider both their net energy intake and their risk of predation when deciding where to search for food, where to consume food, and when to move to the next food patch. However, most of the previous studies which dealt with birds' feeding site preferences associated with predation risk were designed to demonstrate their horizontal distribution relative to cover, and were conducted in a single habitat type. What remains less well understood is the vertical distribution of foraging site preferences and the foraging patterns in different types of habitats. These are the questions I attempted to answer with this study.

1 1 INTRODUCTION

Predation pressure is a major selection force which constantly affects an animal's decision-making when foraging (Lima and Dill, 1990). Various adaptive behaviors of foraging birds resulting from predation pressure have been demonstrated. First, it is one of the fundamental forces behind flock formation. Birds forage in flocks to reduce the risk of predation by reducing the probability of being attacked (Hamilton, 1971; Pulliam, 1973; Powell, 1974;

Baker, 1978). They also benefit by spending less time watching for predators and more time feeding (Powell, 1974; Caraco, 1979a; Caraco, 1979b; Barnard,

1980; Coraco 1980; Caraco et al., 1980a; Mangel, 1990). Within feeding groups, predation pressure can affect both interspecific and intraspecific competition among group members (Dhondt, 1989). This often leads to a social organization within the feeding group. High ranking birds often occupy the more profitable feeding sites, which provide either higher net energy return or more effective protection from predators. Subordinates are forced to feed in less profitable areas (Alatalo, 1981; Ekman and Askenmo, 1984; Alatalo and

Moreno, 1987; Alatalo et al., 1987; Ekman, 1987; Caraco, et al. 1989).

Birds appear to perceive predation risk differently among different feeding sites and show preferences for the safer sites. For example, White­ throated sparrows (Zontrichia albicollis) preferred feeding sites near cover, and extensive feeding did not begin at more distant sites until those close to

12 cover were nearly exhausted (Schneider, 1984; Piper, 1990). In addition, birds

may sacrifice foraging efficiency to reduce predation risk (Grubb and

Greenwald, 1982; Dill, 1986; Todd and Cowie, 1990). Birds will also increase

their inspection time before feeding and decrease the time spent on a feeder as

the distance of the feeder from the ground decreases (Lendrem, 1983).

Furthermore, individual foragers of some species will trade-off foraging

efficiency and predation risk by either consuming the food item where it was

found, or by carrying food items to protective cover before they are consumed

(Lima, 1985; Lima, et al., 1985; Valone and Lima, 1987).

Thus, to maximize fitness, foraging behavior should balance

energy /nutrient acquisition and predator avoidance simultaneously (Sih,

1980; Cerri and Fraser, 1983; Dill, 1986; Mangel and Clark, 1986, Lima and Dill,

1990). Optimal foraging theory (MacArthur and Pianka, 1966) alone cannot

explain the foraging behavior of some animals when the demands of

maximizing feeding rate and minimizing the risk of predation conflict.

However, predation pressure is difficult to measure directly.

Researchers have used "time" or "energy" as a currency to demonstrate the existence of predation risk in various habitats (e.g., Caraco, 1980; Barnard,

1980; Grubb and Greenwald, 1982; Todd and Cowie, 1990). Although the basic foraging patterns associated with predation pressure have been demonstrated

(e. g., Schneider, 1984; Lima et al. 1987), what remain less well understood are the vertical distribution of foraging site preferences and the foraging patterns

1 3 in different types of habitats. In the present study, a mixed-species wintering flock of birds was observed in three habitat types. For each habitat, three feeders were constructed at different heights to test the hypotheses that: 1) there are differences among the preferences of feeding heights of birds; and 2) the magnitude of this preference increases with increasing predation risk found in different habitats.

14 METHODS

Study site:

The field work was completed at Fox Ridge State Park, five miles (8 km) southeast of Charleston, Illinois (39° 25' N, 88°10' W). Three feeding stations were constructed, each in a different habitat. The first was located in a small, second-growth woodlot dominated by American Elm (Ulmus americana L.). The second feeding station was at the edge of the woods just beyond the canopy of the woodlot to the east, 5.8 m from the woods to the north, and 10.7 m from a dense shrub and grass thicket to the west. The third station was established in an open field; the nearest tree was 60 m away to the west, and a dense shrub and grass area was 19 m away to the north (Fig. 1).

Feeding Stations:

The feeding stations at the edge of the woods and in the field were both supported by three steel poles extending approximately 4 m above the ground. Each pole was 1.2 m from the other two. I used aluminum poles to connect the steel poles at the top. Each aluminum pole had a pulley fastened to the center and a 72 x 58 cm wooden feeding tray was hung from the pulley by a nylon rope (Fig. 2). The feeding station in the woods was supported by three tree trunks connected by aluminum poles (Fig. 3). At each feeding station, the feeding trays were hung at three different heights above the ground: 0 m (on the ground), 1.5 m, and 3 m. Because the feeders were hung in a triangular pattern, no feeder was in the center. Furthermore, the height

1 5 of each feeding tray was adjustable, so that no feeder was always hung at the

same height on a given side. Each combination of feeding heights/feeding

station sides was used during a total of three experimental sessions. Only one

combination was used per session.

Banding:

To determine if the same individuals were utilizing all three feeding

stations, I set up mist nets around the edge feeding station between 21 Nov

and 26 Dec 1991. I color-banded 45 Carolina Chickadees (Parus carolinensis),

14 Tufted Titmice (Parus bicolor), 22 Dark-eyed Juncos (Junco hyemalis), 3

White-breasted Nuthatches (Sitta carolinensis), and 2 Northern Cardinals

(Cardinalis cardinalis).

Data Collection:

Feeding stations were established one month prior to the start of

experiments (9 Nov 1991). After the birds became regular visitors to the

feeding stations, two experiments A & B (see below), were conducted from 1

Dec 1991to24Feb1992.

Experiment A. There were a total of 36 experimental sessions in experiment

A, 12 sessions per feeding station. Experimental sessions were conducted in each of the three habitats on consecutive days except when heavy rain or snow precluded observation. Sessions started at about 0830 CST and lasted for

90 min after the first bird visited any one of the three feeders. One thousand sunflower seeds (500 striped and 500 oil-type) were presented to the birds on

1 6 each feeder. Seeds in the feeders were covered with a thin layer of yellow

sand to increase the searching time for the birds (Lendrem, 1983; Lima et al.,

1985; Lima, 1985). While one station was undergoing observation, the other

two stations were closed by lowering all feeders to the ground and covering

them with cloth. Activities of birds on feeders were recorded with a video

recorder placed in a blind approximately 10 meters from the station. At the

end of a session, any seeds left on the feeders were counted to calculate seed

consumption on each feeder. The following data were recorded during

sessions of experiment A: 1) weather conditions: including local temperature,

wind speed, (0: tree leaves barely move; 1: leaves and twigs move; 2: small

branches move; 3: large branches move; 4: whole trees in motion; adopted

from Cole, 1980), and cloud cover (sunny, cloudy); 2) numbers (visit

frequency) and durations (feeder time) of birds' visitation to the feeders; and

3) aggressive behavior. I recorded when and what species of bird visited each

feeder. The time between the arrival and the departure of a bird was defined

as its feeder time. When bird visitation rate increased to the point where I

could not record the feeder time of each bird exactly, I estimated the mean

feeder time for all the birds visiting the feeder in one minute intervals.

Finally, I recorded the species involved in any agonistic encounters including

chasing, pecking, and fighting on each level of the feeders.

Experiment B. Trials for experiment Blasted for approximately two afternoons. Ninety g of striped sunflower seeds were provided in all nine

1 7 feeders at the three feeding stations. Seeds were put on the feeders at about

1230 CST, and left until the next morning. The seed remaining on each of the feeders was weighed at about 0800 CST and then returned to the feeder from which it had been collected. All nine feeders were opened again at 1230 CST with the amount of seed left from the previous day remaining on each feeder. Usually after two days of consumption, at least one of the nine feeders was nearly depleted of seed. Seed left on feeders was measured again on the third day. After this, 90 g of seed was made available for birds on all 9 feeders for the next experimental session. In 4 of the 12 sessions, seed consumption was measured at 1730, (after sunset), and again at 0730 on the next morning, before birds began their daily activity. This was done to determine if seeds were being eaten by rodents or other animals during the night. In each of these instances, little or no seed was consumed overnight. Furthermore, I never observed squirrels on my feeding stations during the day. Therefore, I am quite confident that all or nearly all the seeds on my feeders were consumed by birds.

Data Analysis:

One-way and/or two-way ANOVAs were used to test for differences among habitats and among feeder heights for all variables. Extended Tukey tests (p<0.05) were used to determine if there were significant differences between each pair of treatments. I tested for changes in the intensity of preferences for feeder heights among habitats by a Chi-square contingency

18 table test. Aggressive encounter rates and the proportions of birds consumed seeds on the feeders were subjected to an arc-sine transformation before

ANOVA tests were performed because most of the proportions were lower than 30 percent (Sokal and Rolf, 1981). Paired t-tests were used to test for preferences of seed type at each feeder. In addition, I used Spearman rank correlation coefficients to determine linear relationships between feeding variables (visit frequency and total feeder time) and weather conditions

(temperature and wind speed). Chi-square contingency table tests were also used to determine if the intensity of preferences for feeder heights changed with temperature and wind speed. T-tests were used to test for differences in visit frequency and mean feeder time between sunny and cloudy days.

1 9 RESULTS

The feeding group

Although not all of the birds in my study area were color banded, repeated observations enabled me to determine the approximate number of individuals of each species visiting my feeders. There were 30-40 Carolina

Chickadees, 5-10 Tufted Titmice, 2 White-breasted Nuthatches, 2 Downy

Woodpeckers and 1 Red-bellied Woodpecker regularly visiting my feeding stations. Occasional visitors included Dark-eyed Juncos, American

Goldfinches, and Northern Cardinals. Banded individuals present on the feeders were recorded whenever possible (Table 1). From repeated observations of these banded birds, it is reasonable to conclude that the three feeding stations were all located within the winter feeding territory of a large mixed-species flock. They presumably divided into several sub-groups when they foraged outside of the feeding stations, but there were no noticeable behaviors to indicate the division of the large feeding group while at the stations.

Carolina Chickadees, Tufted Titmice, and White-breasted Nuthatches accounted for 99 percent of the total number of visits (Table 2). Carolina

Chickadees were the most constant and frequent visitors on all nine feeders.

In fact, Carolina Chickadees determined the overall foraging pattern for the entire feeding group. Data taken from all 8 species were pooled together to examine the foraging patterns of the entire feeding group. In addition, the

20 three most abundant species were each analyzed separately.

Visit frequency and total feeder time

Comparisons among habitats

Table 2 shows the visit frequency and total feeder time for each of the

eight species in all 36 experimental sessions (12 sessions for each habitat).

Habitat had a significant effect on visit frequency of all species combined (one­

way ANOVA, F(2,105)=92.08, P<0.0001 ). The visit frequency in the open field

was significantly less than the visit frequency of the woods and edge stations

(Tukey test, P<0.01), however, there was no difference between the woods

and edge stations. A comparison of total feeder time among habitats showed

the same trend (Table 3). Birds spent significantly less time in the open field

than at the other two stations. The mean feeder time (total feeder time

divided by visit frequency for each feeder) was longer in the open field

station than at the other two stations (13.13 sec, versus 6.48 sec and 5.86 sec,

respectively for open field, edge and woods station, one-way ANOVA,

F(2,102)=55.11, P<0.01). For all species combined, birds visited the woods

station more, spent more time at it, but the average time spent per visit was

less. The open field station was visited less frequently but birds stayed longer

per visit. The edge station had intermediate values for both visit frequency

and mean feeder time (Table 2).

For each of the three most abundant species, the visit frequency was strongly affected by habitat (Table 3). Extended Tukey tests indicated that the

21 visit frequency in the open field for both Carolina Chickadees and White­ breasted Nuthatches was significantly less than that in the other two habitats.

In fact, White-breasted Nuthatches visited the open field station only once

(Table 2). The Tufted Titmouse, the second most abundant species, had visit frequencies that did not differ between the edge and woods stations or between the woods and open stations. The difference between the edge and open field stations, however, was significant (Table 3).

The total feeder times of each of these three species were also dissimilar among the three habitats (Table 3). Carolina Chickadees and Tufted Titmice spent more time at the edge station than at the woods station. However, this difference was not significant. Both species spent significantly less time in the open field station (Table 3). Therefore, the lower total feeder time in the field was due to lower visit frequency and not a lower mean feeder time per visit.

White-breasted Nuthatches had the highest feeder time in the woods, but this was not significantly different from that in the edge station. The single visit in the open field did not bring the total feeder time close to the other two stations.

Comparisons among feeder heights within each habitat

For all birds visiting the feeding stations combined, the differences in visit frequencies among the feeder heights were significant in the edge and open field stations, but not in the woods station (Table 4). In all three habitats, high feeders had the highest visit frequency, middle feeders were

22 intermediate, and the low feeders had the lowest visit frequency.

Looking at the three most abundant species individually, Carolina

Chickadees had the same pattern as the whole feeding group except that the visit frequency for middle feeders exceeded the high feeders in the woods and the edge station (Table 2, 4). Tufted Titmice and White-breasted Nuthatches showed a different pattern from Carolina Chickadees. There were no significant differences in visit frequencies among feeders in the open field station for either of the two species. However, the feeder heights significantly affected the visit frequency at the other two stations. The lack of significant differences in the open field station can be explained by the fact that the visit frequencies of Tufted Titmice and White-breasted Nuthatches in open field station were quite small. This is especially true for nuthatches. In addition, nuthatches visited the middle feeder more frequently than the high feeder in the woods station, whereas they visited the high feeder more frequently than the middle feeder at the edge station (Table 2, 4).

The comparisons of total feeder times among feeder heights in each habitat showed a similar pattern to that of visit frequency (Table 4).

Differences of mean feeder time within habitat (among feeders) are not significant at the edge and woods stations (one-way ANOV A, edge:

F(2,33)=0.32, woods: F(2,33)= 0.85, P>0.05), whereas the mean feeder time at the low feeder of the open field station was significantly less than the other two higher feeders (one-way ANOV A, F(2,30)=7.02, P<0.01). In general, birds

23 did show preferences for the heights of the feeding sites, although the pattern

varied somewhat among the species of birds.

Table 5 and Table 6 show the results from two-way ANOVAs for

testing how the two treatments, habitat and feeder height, affected the visit

frequency and total feeder time for the whole feeding group (all 8 species),

and for Carolina Chickadees, Tufted Titmice, and White-breasted Nuthatches

separately. In all cases, both the effects of habitats and feeder heights were

significant, the interaction between habitats and feeder heights are all not

significant except for the White-breasted Nuthatch. This significant

interaction was probably caused by the extremely low visit frequency in the

open field station. Except for Tufted Titmice, the intensity of the preference

for high feeders increased with increasing openness of habitats (Table 7).

Temporal feeder visiting patterns in different habitats

In all three habitats, the highest visit frequencies in the first 5 minutes

occurred at the high feeders, middle feeders had intermediate values, and

low feeders had the lowest visit frequencies (Fig. 4a-c). Eventually, the birds

visited the middle and low feeders more frequently, with rates sometimes

exceeding those of the high feeder in the woods station. In the edge station,

the same pattern occurred, but the time needed to increase the visit

frequencies of the middle and the low feeder to the same level as the high

feeder was longer. In addition, the peaks of the three curves were closer to each other in the woods station than in the edge station. In the open station,

24 the three curves never crossed one another. The high feeder was always the

favorite feeder throughout the 90-minute trial period. The visit frequencies

remained significantly different among feeders until 20 minutes, 30 minutes,

and 75 minutes in the woods, edge, and open field station, respectively (Table

Sa). Comparisons of feeder times among feeders in 5 minutes intervals

showed a similar pattern (Fig 5a-c, Table Sb).

I also examined the temporal feeder visiting pattern for species

separately. The patterns for Carolina Chickadees were very similar to the

patterns for the feeding group as a whole (Fig. 6a-c, 7a-c). Tufted Titmice,

however, almost always preferred the high feeder throughout the entire

observation period in all three habitats. The curve for the middle feeder was

closer to the the high feeder's curve in the edge station than in the woods

(Fig Sa-c & Fig 9a-c). The visit frequencies for all three feeders combined in

the open field was dramatically lower, and very few Titmice visited the low

feeder.

The pattern for White-breasted Nuthatches was also somewhat different. First, a single individual visited the open field only once. In the woods station, curves for the high and middle feeder almost overlap in the first 55 minutes (Fig. 10a, Fig. lla). In the edge station, Nuthatches showed a clear preference for higher feeders, and this pattern was maintained throughout the 90 minutes (Fig. 10b & Fig llb).

25 Seed consumption of experiment A

Overall, 57,298 seeds were consumed by birds in the 36 sessions of

experiment A. The birds preferred the oil-type seeds over striped seeds in all

three habitats and at all three feeder heights (Table 9).

Habitat type and feeder height both significantly influenced the

amount of seed consumed by birds (P<0.0001), but the interaction between

these two variable was not significant. There was no statistical difference in

seed consumption between the woods and edge stations. However,

significantly less seed was comsumed in the open field (one-way ANOVA,

F(2,105)=75.3, P<0.0001, Tukey test, P<0.01). Comparisons of seed

consumption among feeder heights in each habitat showed no difference in

the woods station, but significant differences in the edge station and open

stations (one-way ANOVA, woods: F(2,33)=1.09, P>0.05; edge: F(2,33)=5.84,

P<0.01; open: F(2,33)=7.41, P<0.01).

Feeding efficiency of experiment A

Two types of feeding efficiency were calculated: 1) average time

required for birds to consume or take one seed (sec/seed), and 2) the average

number of visits required for birds to eat or pick up one seed (visits/seed)

(Table 10). It took birds an average of 6.69 sec, 7.19 sec, and 10.07 sec to remove one seed from the woods, the edge, and the open field station, respectively

(one-way ANOVA, F(2,101)=5.49, P<0.01). In addition, one seed was removed every 1.14, 1.11, and 0.767 visits in the woods, the edge, and the open field

26 stations, respectively (one-way ANOVA, F(2,101)=46.42, P<0.0001). Thus,

birds spent less time per visit on feeders but visited them more times to pick

up a single seed in the woods station. On the other hand, they spent more

time per visit on feeders but visited them fewer times to get a seed in the open field station. The edge station had intermediate efficiency values.

In all three habitats, the low feeder was the "most efficient" feeder.

Birds spent less time/visit and removed more seeds/visit at this feeder. The least efficient feeder in the woods and open field was the high feeder, and the least efficient feeder in the edge was the middle feeder (Table 10). However, one-way ANOVA tests showed no statistical differences for both kinds of feeding efficiencies among feeder heights in the woods and the edge stations. In the open field, the time taken by birds to remove one seed (sec. I seed) from the low feeder was significantly less than the other two feeders (F(2,29)=10.91, P<0.01).

Seed consumption of experiment B

After one day, the high feeder in the woods had the highest seed consumption, whereas seed on the low feeder in the open field station was consumed the least (Fig. 12). After the second day, the high feeder in the woods had only 5.5% (on average) of the seeds remaining, whereas the low feeder in the open field still had 85.6% of the original amount (Fig. 12). In each habitat, the amount of seed consumed by birds decreased with decreasing feeder height. For each feeder height, the amount of seed

27 consumed by birds decreased with decreasing coverage of the habitat. A

comparison among habitats for the amount of seed left on feeders indicated

that the differences among habitats were significant (one-way ANOVA

F(2,105)=94.69, P<0.0001). Extended Tukey test shows no difference between

the woods and the edge stations, but significantly more seed was left at the

open field station after the first day (P<0.01). Seed left after the second day showed the same trend as the first day (oneway ANOVA, F(2,105)=158.00,

P<0.0001; Tukey test, P<0.01). The total seed consumed after two days' feeding showed no difference between woods and edge stations, but the open field had significantly less seed removed than the other two stations (one-way

ANOVA, F(2,105)=158.00, P<0.0001. Tukey test, P< 0.01).

Aggressive Behavior

Intraspecific agonistic encounters occurred more often than interspecific agonistic events at every feeder (Table 12, 13). White-breasted

Nuthatches had a higher social rank than Tufted Titmice, which were dominant to Carolina Chickadees.

The rate of aggressive encounters (no. of encounters/minute) was calculated by dividing the number of aggressive encounters by the total feeder time (min) for each feeder. This rate increased significantly with increasing cover of the habitat (one-way ANOVA, F(2,102)=6.4, P<0.01). In the woods and edge stations, aggression rate increased from the high feeder to the low feeder (one-way ANOVA, woods: F(2,33)=1.20, P>0.05; edge:

28 F(2,33)=3.89, P<0.05, Tukey test, P<0.05). In contrast, the highest aggression rate occurred at the high feeder in the open field station, and decreased with decreasing height above the ground. However, the differences were not statistically significant (one-way ANOV A, F(2,30)=3.24, P>0.05).

Strategies of feeding

This study was not designed to directly test the model which predicts that the tendency to carry a food item away from its origin should decrease with the distance of food from cover (Lima, 1985). Therefore, I did not collect data comparing the number of times the birds consumed a seed on the feeder, and the number of times the birds left the feeder with the seed.

However, if I arbitrarily choose a visit duration of 10 sec or more to indicate that an individual bird ate the seed at the feeder (less time indicating that the bird left the feeder with the seed), I can address this hypothesis. I had to omit six of the experimental sessions because of factors such as very low bird visitaton rate or predators present in the vicinity. I choose 10 sec because it seems long enough for a bird to find a seed on a feeder even when the seed density was very low.

Based on this classification, the proportions of visits in which the birds stayed on the feeders to eat the seeds is shown in Table 14. A comparison among habitats indicated that the proportion of birds staying in the open field was significantly greater than that in the woods and edge (one-way

ANOVA, F(2, 258)=91.03, P<0.0001. Tukey test, P< 0.01). Within each habitat,

29 there were no differences among feeders in the edge and open stations.

However, in the woods station, more birds stayed on the high feeder than on

the low feeder (one-way ANOVA, F(2,87)=3.14, P<0.05, Tukey test P< 0.05).

Except for the low feeder at the open field station, there was a consistent

trend for the proportion of staying to increase towards the end of the

experiment period.

Season and weather

There were significant differences among the three months of this

study with respect to the birds' visit frequency when data from all three

habitats were pooled (one-way ANOVA, F(2,105)=3.99, P<0.0214, Table 15a).

As expected, birds had their highest visit frequency in January, but only the difference in visit frequency between January and February was significant

(Tukey test, P<0.05). Results from the comparison of the total feeder time showed the same trends (one-way ANOVA, F(2,105)=5.91, P<0.0037; Tukey test, P<0.01, Table 15b).

In all habitats, visit frequency and total feeder time were both negatively correlated with temperature (Figure 13a-b) and wind speed (Figure

14a-b, Spearman rank correlations coefficients, Table 16). Temperature was negatively correlated with visit frequency and total feeder time in all three habitats. However, mean feeder time (per visit) was significantly correlated with temperature only in the edge (P<0.01). Strength of the wind had no effect on all three variables in the woods, but significantly affected both visit

30 frequency and total feeder time at the edge (negatively), and visit frequency

in the open field. Cloud cover affected visit frequency in the woods station,

total feeder time and mean feeder time at the edge station, and mean feeder

time in the open field station (t-tests, P<0.05, Table 17). In general, birds spent

less time per visit at the feeders on cloudy days than on sunny days.

Birds appeared to move to higher feeders in all three habitats on cloudy

days, although the difference was only statistically significantat at the edge

habitat (X"' =18.086, P<0.01, Table 18). Birds tended to increase their visit

frequency (proportionally) on higher feeders with increasing temperature

and wind speed (Table 19a-c, table 20a-c).

Presence of predators

The responses of birds to potential predators observed in this study

were somewhat different in the different habitats. I observed only one case in which a predator actually attacked the feeding birds at the woods station.

Unfortunately, I was unable to identify the predator, because I did not notice that there was an avian predator near the station. Neither did the feeding birds, however. Before the attack, no alarm calls were given, and the birds were feeding normally. After the attack, the birds gave alarm calls for a few seconds, some of them flew up to the canopy, and some stayed on the feeders or perched on the branches beside the feeders. No freeze behavior was noticed (Ficken & Witkin, 1977; Ficken, 1990). In less than 10 seconds, they resumed their normal feeding activity.

31 At the edge station there were no observed attacks by predators. All predators that the feeding birds responded to were flying above or near the station. Their responses usually included alarm calls which resulted in birds flying to cover nearby. In several cases, two or three Chickadees would remained motionless on the feeders.

In the open field, I usually did not suspect that there were predators near by until after the birds had stopped feeding for a relatively long time.

There were never alarm calls given at this station. On several occassions, before they resumed feeding, many calls would be given from the cover, usually by Chickadees. These calls would last for a relatively long time

(usually more than one minute).

32 DISCUSSION

Visit frequency and feeder time

The lower visit frequency and total feeder time at the open field station

indicated that birds were more reluctant to feed in this habitat. Similar

results have been found in previous studies, which have used distance to

cover as an indication of predation risk in wintering passerines (e.g.,

Schneider, 1984; Lima et al. 1987; Hogstad, 1988; Piper, 1990). Alternatively,

because the nearest cover from the open field station was 19 m away, the

energy costs to fly this distance, especially in the winter, may not be

negligible. It may have been a factor that would discourage birds from

visiting the open field station. In addition, the open field station provided no

protection from the wind. Therefore, birds feeding at this station

undoubtedly experienced a higher energy expenditure than birds at the more

protected feeding stations. However, if energy costs were the only concern,

birds could reduce these costs by staying and consuming seeds on the feeders

(Lima, 1985; Lima et al., 1985). In addition, birds should feed at the lowest feeder in more exposed areas because there were differences in wind speed and temperature among feeder heights (see also Grubb and Greenwald, 1982).

From the low to the high feeder, temperature dropped 0.5 °C for every 1.5 m above the ground. Although I did not measure the wind velocity at each feeder, it is reasonable to assume that the wind speed increased with increasing feeder heights. Considering these factors together, birds should

33 consume seed while remaining on the low feeder at the open field station to

have the highest energy return. Given that I observed the opposite pattern

suggests that birds had to trade-off their foraging efficiency with some other

factor(s) (e.g., predation risk).

Data from those birds that did visit the open field station supported the

model which predicts that animals will stay and consume the food at the

place where they found it (and incur the increased risk of predation) if the

energy required to carry the food to cover is relatively high (Lima, 1985;

Valone and Lima, 1987). Birds stayed on higher feeders at the open field

station longer than they did at the other two stations. However, even here

more than 60% of the birds did not stay on the feeders for more than 10 sec.

In other words, many birds that visited the open field station increased their

energy expenditure to decrease their exposure to predators. In addition, they

visited the low feeder the least (9.5% of the visits). This further supported the

idea that birds will feed at a safer site at the expense of energy intake (Todd

and Cowie, 1990). Unfortunately, the degree to which the higher energy costs

of the open field station reduced the willingness of birds to visit this station

could not be distinguished from the effects of predation risk in the open field.

The energy costs of birds visiting the edge and the woods stations should be approximately equal because trees bordered both stations. Many birds flew back and forth to the feeders from trees more than 10 m from the woods station even though there were trees available at closer distances.

34 However, greater cover in the woods should provide better protection from

avian predators than in the edge habitat. Although the edge station had

intermediate values for visit frequencies and mean feeder times, no

significant differences existed between the woods and the edge station.

Previous studies have documented that in a foraging group, dominant

individuals or species will force subordinates to feed in the sites more

exposed to predators. These sites are typically the lower or outer parts of the

canopy, or areas further from cover (Alatalo, 1981; Ekman and Askenmo,

1984; Schneider, 1984; Ekman, 1987; Alatalo and Moreno, 1987; Alatalo et al.,

1987). Preferences for safer feeders should be stronger in a habitat with higher

predation risk.

For all eight species combined and for Carolina Chickadees, both visit

frequencies and total feeder time had no significant difference among feeder

heights in the woods station, whereas at the edge and open stations, the low feeder was visited less frequently by birds and they spent less time on it than

the other two feeders. The preference for higher feeders increased with the openness of the habitat. For both nuthatches and Tufted Titmice, the lack of significant differences among feeder heights at the open field station was caused by the fact that there were simply too few visits to show significant differences. White-breasted Nuthatches showed a stronger preference for higher feeders at the edge than at the woods station, whereas the Tufted

Titmice showed a stronger preference for higher feeders in the woods. Thus,

35 Tufted Titmice were the only exception to the pattern of incresing preference for higher feeders in more exposed habitats. One possible explanation may relate to interactions between White-breasted Nuthatches and Tufted

Titmice. White-breasted Nuthatches appeared to have a higher social status than the Tufted Titmice in my study, and they had a strong preference for higher feeders at the edge station. Therefore, in order to avoid aggressive attacks by the White-breasted Nuthatches, the Tufted Titmice may have been forced to feed more at the lower feeders at the edge station (Ekman and

Askenmo, 1984; Ekman, 1987; Waite, 1987; Todd and Cowie, 1990). This hypothesis was supported by the fact that all 12 aggressive encounters at the edge station occurred between White-breasted Nuthatches and Tufted

Titmice at the high feeder. The overall feeding preferences of the Carolina

Chickadees were probably not affected by these interactions because they were so much more abundant than the other species.

In addition, mean feeder time (time spent on a feeder/visit) increased with increasing openness of the habitats. This could have resulted from an increase in the proportion of birds staying on the feeders to consume seeds with an increase in the openness of the habitats. Alternatively, birds may have increased the time they spent scanning for predators from high coverage to low coverage habitats (Barnard, 1980). Within each habitat, mean feeder time was greatest at the middle feeders and least at the low feeders.

Although these differences were not statistically significant, the trend was

36 consistent in all three habitats. This increase in mean feeder time from high

feeders to the middle feeders may have resulted from an increase in scanning

time (Lendrem, 1983). This is likely because the seed density of the high

feeder was generally lower than the density at middle feeders.

Temporal feeder visiting pattern

There appeared to be two main factors, seed density and predation risk,

which influenced feeder preference (Mangel and Clark, 1986; Dill, 1986). If

predation risk was constant birds should feed in areas of the highest seed

density. If birds feed in a high risk habitat, however, they might be more

likely to continue visiting the safer sites, even when the seed densities are

much lower than the riskier sites (Schneider, 1984).

In all three habitats, high feeders were always visited first. Thus, when

seed densities did not differ, high feeders were preferred, probably because

they were the safest. After seed density was decreased to a certain level at the

high feeders, however, birds increased their visits to the middle feeders.

Eventually, more birds would venture to the low feeder to compensate for

the decreasing feeding efficiency at the other two safer heights. Although the

time needed for birds to start or increase their visits to lower feeders varied among habitats and among species of birds, the hypothesis that the preference for feeding height increased with increasing distance from the ground was strongly supported by data from all three habitats.

The time required to increase the visit frequency and total feeder time

37 of the two lower feeders to the same level as the high feeder at the edge

station was longer than that in the woods station. In the open field station,

the visit frequency and total feeder time of the lower two feeders never

equaled the high feeder throughout the observation period. This suggests

that predation risk was highest in the open field, lowest in the woods, and

intermediate at the edge.

It is also possible that birds never changed their preferred feeder at the open field station simply because the seed density at the high feeder never decreased to a level as low as it was at the high feeders of the other two stations, and not because of any difference in predation risk between habitats.

Unfortunately, I could not record the seed density without disturbing the feeding activities of birds on the feeders. However, if the seed density decreased linearly with increases in the total visit frequency, we can estimate the relative seed density at the high feeders when the visit frequencies of three feeders were no longer significantly different from each other.

Using this approach, it seems unlikely that the birds changed their feeder-height preference based only on seed density. At the time when visit frequency showed no significant difference among feeders, the seed density at the high feeder in the woods was higher than that at the edge (woods: 2745 visits; edge: 5043 visits; open: 2427 visits). The higher predation risk at the edge was the most probable explanation for this pattern. In the open field station, the three curves never crossed, and the high feeder was always the

38 preferred feeder throughout the observation period. The lack of statistical

difference beyond the 75th minute was likely caused by the very few visits to

the open field station.

Seed consumption

Experiment A

The two kinds of feeding efficiencies calculated from experiment A

showed only the effectiveness of removing seeds from the feeders, and not

necessarily the energetic efficiencies per unit time or visit. This is important

because there appeared to be two strategies of consuming seeds used by birds

according to the distance of the food resource from cover (Lima, 1985). Birds rarely consumed seeds on feeders at the woods station, consequently, time spent on the feeders was mostly used for searching. They spent time off the feeders to handle the seeds. In contrast, more than 303 of birds consumed seeds while on feeders at the open field station. Therefore feeder time of birds at the open field station not only included searching time, but also scanning and handling times. The time budgets of birds on feeders at the edge station were more like those in the woods, but contained more scanning and handling time since the proportion of birds staying on the feeders was higher. One of the ways that predation risk can affect feeding efficiency directly is by increasing scanning rate (Lendrem, 1983). The higher the predation risk in a habitat, the more time birds spend in scanning for predators (Barnard, 1980; Caraco et al., 1980b). However, I did not record the

39 time budgets of birds on feeders. Therefore, I cannot address how the predation risk affected the feeding efficiency of birds from my data.

At the woods and edge stations, birds spent less time per visit and needed fewer trips to remove a seed from the low feeders than from the higher feeders. This may have resulted from the higher seed densities there.

On the other hand, birds still tried to find seed on the higher feeders even when the seed densities were very low. Occasionally, after searching for seed on higher feeders, birds left or moved to the lower feeders without getting any seed on the higher feeders. This increased the total visit frequency and feeder time for the higher feeders, while decreasing the feeding efficiencies.

This may also explain why the high feeder at the woods station had a significantly higher proportion of birds staying. The search time might have exceeded 10 sec if the birds still tried to find seed under the low seed densities present near the end of the observation periods. Rarely did birds handle seed at the feeders of the woods station.

Birds also rarely consumed seed on the low feeder at the open field station. If this feeder had the highest predation risk, the best strategy of feeding at this most dangerous feeder would probably be to stay as short a time as possible. Behavioral observations confirm this prediction. Generally, birds picked up seeds and either flew back to the cover or flew up to the higher feeders for handling the seeds. Birds visiting the low feeder had the advantage of high seed density, so that the searching time was the least at the

40 low feeder of the open field station among all nine feeders.

Experiment B

Since all nine feeders were simultaneously available to birds,

preferences for feeders could be an indication of the quality of the feeders.

Since the initial seed densities were equal, birds should visit the safer feeder first. In the same habitat, birds preferred higher feeders, and for a given feeder height, birds preferred the habitat with the greater cover. These results were consistent with the other results of this study.

Aggressive encounters

Aggression rate is determined by whether or not benefits increase with higher social rank and if there is adequate time to obtain and defend that status aggressively (Caraco, 1979a). In this study, food resources were renewed rapidly, and individuals may not have needed to aggressively protect long­ term access to those resources (Gill and Wolf, 1975). The large group size could be a result of the low aggression rate. From behavioral observations, the social ranking of the three most abundant species was quite clear, but the ranks within species could not be determined by the aggressive encounters that occurred on the feeders. I can only conclude that the dominance hierarchies within species were maintained among smaller "subgroups", but that the "composite" group visiting the feeding stations was not arranged into a clear dominance hierarchy (Hartzler, 1970).

The fact that aggression rate increased with increasing coverage of the

41 habitats could be an indication of decreased levels of aggression in the riskier habitats. This may occur because the pressure for birds to decrease the time they are exposed to the predators is higher, and the time needed for scanning is increased (e.g., Caraco, 1980b). Alternatively, the higher aggression rate at the woods and edge stations could be a consequence of the larger group sizes at these habitats (Caraco, et al. 1980a, but see Pearson, 1989). It may also have been caused by birds competing for seeds especially toward the later parts of the experimental sessions when seed densities were low. This explanation was supported by the higher aggression rate toward the end of the experimental periods at the woods and edge stations.

The delayed aggressive encounters at the woods and the edge stations could also result from the high energy requirements of birds at the beginning of the sessions. The energy requirements at this time may have been so great that time for aggression was severely constrained (Caraco,

1979a). This may also explain why the highest aggression rate occurred at the low feeders at these two stations because toward the end of the sessions the low feeders had the highest seeds densities.

At the open field station (where seed densities remained high throughout the sessions), birds should compete for a safer space, so they can stay longer instead of spending the time and energy to fly back and forth to cover. The greater need for safer feeding sites could be the force that increased the aggression rate at the high feeder in the open field station. The aggression

42 rate here even exceeded the rate detected at the edge station's high feeder in spite of the higher predation risk in the open habitat.

Weather conditions

Weather conditions must be considered in this study because previous studies have documented that this factor influences the spatial distributions of birds, especially when temperatures are low (Grubb, 1975; Grubb, 1977). In all three habitats, visit frequencies were significantly higher when temperatures were lower. This was possibly a consequence of higher energy demands of birds on cold days (Caraco, 1979b; Elgar, 1986). Wind speed had no significant effect on visit frequency and feeder time of birds at the woods station, probably becausee the woods provided protection from the wind.

However, wind speed significantly decreased the visit frequencies at both the edge and the open field stations, and also decreased the total feeder time at the edge. The higher energy costs of feeding at these sites which provided little protection from the wind may have decreased the birds' visitation rate

(Grubb and Greenwald, 1982).

The reasons why the effects of cloud cover on visit frequencies were different among habitats was not clear. However, a previous study suggested that the density of feeding birds was more affected by cloud cover at feeding sites with less cover than feeding sites with greater cover (Watts, 1991).

Comparison of the feeding-height distributions under sunny and cloudy days revealed no significant differences at the woods and the open

43 field stations (see also Grubb, 1975). The edge station was an exception

(greater preference for higher feeders on cloudy days) possibly resulting from the decreased visibility on cloudy days (Metcalfe, 1984; Watts, 1991). Under these conditions, birds may prefer to feed at higher, safer feeders. The visibility would not be increased much in the woods by moving higher, and the higher feeders were always strongly preferred at the open field station.

Birds increased their use of the higher feeders with both increasing wind speed and temperature. The tendency of the birds to use the lower feeders when temperatures were lower was consistent with the energy conserving needs for birds on colder days (Grubb, 1975). The lower feeders were warmer and more protected from exposure to wind. However, the tendency of the birds to use higher feeders with increasing wind speed was not consistent with an energy conserving strategy. What caused this result was not clear.

Conclusions

In conclusion, birds appeared to perceive predation risk differently in the three habitats and at the three feeder heights, and responded accordingly.

In general, birds were more likely to visit the more protected woods and edge stations and preferred the higher to the lower feeders in all three habitats.

Furthermore, the intensity of the preferences for higher feeders increased with increasing openness of the habitat. These feeding site preferences could be indications of different intensities of predation risk perceived by the birds

44 in the different feeding sites. However, how the time budgets changed for birds feeding under different predation pressures, how the feeding efficiencies changed with time, and how the bird's social rank affected predator avoidance remained unsolved by this study. Further studies are needed to answer these questions concerning how predation risk affects a bird's foraging decisions.

45 LITERATURE CITED

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___, and J. Moreno. 1987. Body size, interspecific interactions, and use of foraging sites in tits (Paridae). Ecology 68:1773-1777.

___, R. V., D. Eriksson, L. Gustafsson, and K. Larsson. 1987. Exploitation competition influences the use of foraging sites by tits: experimental evidence. Ecology 68:284-290.

Baker, M. C. 1978. Flocking and feeding in the great tit Parus major -an important consideration. American Naturalist 112:779-781.

Barnard, C. J. 1980. Flock feeding and time budgets in the house sparrow Passer domesticus L.. Animal Behavior 28:295-309.

Caraco, T. 1979a. Time budgeting and group size: a theory. Ecology 60:611-617.

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49 Table 1: Numbers and percentages (in parentheses) of banded birds observed feeding at the feeding stations.

Species Number of Number of banded Number of banded banded birds seen feed­ birds seen individuals ing on at 'least feeding in all one station three habitats

Carolina 45 37 (82%) 17 (38%) Chickdee

Tufted 14 7 (50%) 2 (14%) Titmouse

White­ 3 2 (67%) 0 (0%) breasted Nuthatch*

Dark-eyed 22 0 (0%) 0 (0%) Junco

Northern 2 0 (0%) 0 (0%) Cardinal

*: White-breasted Nuthatches only visited the open station once, and it was not determined if it was one of the three banded individuals. Two nuthatches that visited the woods and edge stations were always the same two individuals.

50 Table 2: Total number of vlalta (upper row) and total feeder time (In aeconda, lower row) t>r each of the .aght bird &J>9clea which f9c:i at each ieeder (3 t.. der heights at 3 habitats).

Edge Open Total Speclea wooca. High Middle Low Subtotal High Middle low Subtotal High Middle Low Subtotal /1 'II Carolina 1112 1503 7350 24021 8067 1723 80l4 28404 2575 1151 461 4181 52370 84 .s. Chlcudae 48241 52ao5 42555 144400 50145 51345 39147 154337 34278 27343 400I 86830 304403

Tuftad 1528 l5CS 095 3078 10HS 1174 no 3508 250 130 55 444 7091 11.45 Titmouse 7231 4253 3475 14861 1705 0732 3882 18428 2182 1117 367 8731 31124

WhH•breaatad 403 4n 222 1102 524 220 31 781 0 0 1 1884 3.04 Nut hatoh 1827 2118 875 4802 2808 10451 130 4088 84 0 0 84 8005

Dow~ 123 55 10 181 41 20 3 04 28 11 0 41 281 0.48 U'1...... Woodpeckar 620 247 05 131 300 II . 13 388 1105 113 0 13"48 2685

Rad· bell lad 57 42 100 27 31 0 05 3 13 1 17 112 0.28 Woodpeclcar 202 117 5 454 114 320 0 504 45 128 4 172 1130

Dark-eyed 4 1 0 5 12 3 47 12 I 1 1 I 75 0.12 Junco 13 3 0 11 51 13 fi67 121 38 7 8 fi6 0820

American 22 1 0 23 18 7 0 2CS 0 0 0 0 48 o.oa Goldflnch 1552 30 0 1512 1eoa 071 0 2247 0 0 0 0 3151

Northam 1 0 0 1 0 0 0 0 0 0 1 1 2 0.003 Card Ina I 4 0 0 4 0 0 0 0 0 0 2 2 0

Total 10300 ·9935 1284 21518 1808CS 10181 1843 28030 2172 2020 511 54045 CS1967 100 eooe1 58533 47057 1CS7251 08111 07228 44539 111653 37114 21643 4380 71037 419841

Mean faeder 5.18 6.88 5.18 s.eo 1.41 1.80 1.41 1.4& 13.17 14.21 ... 13.13 1.71 tlma (per vlalt) Table 3: Results of one-way ANOVAs for comparing visit frequencies and total feeder time among habitats.

Visit frequency Total feeder time Species F- value F- value

All 8 species F=92.08 * F=30.00 *

Carolina Chickadees F=86.87 * F=25.64 *

Tufted­ T i tmice F=24.65 * F=16.81 *

White­ breasted F=30.3 * F=25.50 * Nuthatches df= 2, 105 *: P<0.0001

52 Table 4: Results of one-way ANOVAs for comparing visit frequencies and total feeder time among feeder heights within each habitat.

Visit frequency Total feeder time Species Habitat F- value F- value

All 8 Woods F= 1. 77 F= 2.38 species Edge F= 6.11 ** F= 5.11 * Open F= 6.94 ** F=l0.15 ** Carolina Woods F= 0.61 F= 1.11 Chickadee Edge F= 4.63 * F= 3.95 * Open F= 7.22 ** F=l0.10 ** Tufted Woods F= 4.20 * F= 3.1 Titmouse Edge F= 3.76 * F= 3.07 Open F= 2.10 F= 2.11

White- Woods F= 3.95 * F= 5.58 ** breasted' Edge ~ F=29.59 F=30.73 ** Nuthatch Open @ ** @ df= 2, 33 *: P<0.05 **: P<0.01 @: too few data to test

53 Table 5: F-values from two-way ANOVAs for comparing visit frequencies of habitats and feeder heights.

Source of Variation Species Habitat Feeder heights Hab. x Feeder df(2,99) df(2,99) df{4,99)

All 8 110.09 ** 11. 98 ** 0.64 species

Carolina 97.00 ** 7.80 * 0.73 Chickadee

Tufted- 28.71 ** 9.25 * 1.19 Titmouse

White­ 49.75 ** 18.33 ** 9.19 ** breasted Nuthatch *: P<0.001 **: P<0.0001 Table 6: F-values from two-way ANOVAs for comparing total feeder times of habitats and feeder heights.

Source of Variation Species Habitat Feeder heights Hab. x Feeder df(2,99) df(2,99) df(4,99)

All 8 38.48 ** 16.21 ** 0.81 species

Carolina 31. 63 ** I 12.35 ** 1.45 Chickadee

Tufted- 18.70 ** 7.66 * 0.63 Titmouse

White­ 49.82 ** 26.32 ** 13.37 ** breasted Nuthatch

*: P<0.001 **: P<0.0001

SS Table 7: Visit frequencies of birds at each feeder in each habitat. Chi-square contingency table tests were used to determine if the intensity of preferences for higher feeders changed with habitats. Percentages are in parentheses.

Woods Edge Open Chi-square Species High Middle Low High Middle Low High Middle Low value

All 8 858 828 690 908 849 578 239 168 43 85.88* species (36.2) (34.8) (29.0) (38.9) (36.4) (24.7) (53.1) (37.3) (9.6)

Carolina 680 709 613 721 727 507 215 155 38 91. 64* Chickadee (34.0) (35.4) (30.6) (32.9) (37.2) (25.9) (52.7) (38.0) (9.3)

Tufted 127 71 58 135 98 65 22 11 5 3.98 01 Titmouse (49.6) (27.7) (22.7) (45.3) (32.9) ( 21. 8) (57.9) (28.9) (13.2) 0\ White- 34 40 19 44 19 3 @ @ @ 16.90* breasted (36.6) (43.0) (20.4) ( 66. 7) (28.8) ( 4.5) Nuthatch *: P<0.005 @: too few data to test. Table Ba: F- values from one-way ANOVAs for comparing visit frequencies among feeder heights in each habitat at 5 min intervals.

Time (minutes into session)

Habitat 5 10 15 20 25 30 35 40 45

Woods 5.78 ** 4.80 * 6.87 ** 5.43 ** 2.10 0.32 0.84 0.95 0.36 Edge· ll.68 ** 12.54 ** 16.21 ** 13.37 ** 9.37 ** 7.75 ** 3.16 l.30 1.04 Open 4.67 * 6.38 ** 3.93 * 4.88 * 4.18 * 5.53 ** 4.8 * 3.33 * 4.14 *

(continued) (.J1 'l Time (minutes into session) Habitat 50 55 60 65 70 75 80 85 90 . .... ·- Woods 0.77 1.36 2.88 2.47 3.56 1.03 0.52 0.13 0.12 Edge 0.42 0.11 0.02 0.24 0.30 1.57 0.64 2.93 1.54 Open 3.1 2.37 3.52 * 3.05 2.87 4.00 * 1.53 1.30 3.14 *= P<0.05, **: P<0.01. Note that P~ values are relative (and not absolute) measures of significance as tests are interdependent. Table Sb: F- values from one-way ANOVAs for comparing total feeder times among feeder heights in each habitat at 5 min intervals.

Time (minutes into session) Habitat 5 10 15 20 25 30 35 40 45 Woods 6.34 ** 5.84 ** 6.76 ** 6.32 ** 2.18 0.59 0.94 1.44 0.68 Edge 19.36 ** 16.70 ** 12.98 ** 13.68 ** 10.13 ** 9.88 ** 3.97 * 3.19 2.11 Open 6.66 ** 7. 37 ** 5.89 ** 6.45 ** 4.48 * 7.45 ** 8.07 ** 7.18 ** 6.84 **

(continued) (JI 00 Time (minutes into session) Habitat. 50 -55 60 65 70 75 80 85 90 Woods 0.45 1.24 0.83 2.13 0.83 1.82 0.54 0.44 0. 74 Edge 1.27 0.85 0.34 0.13 0.06 0.97 0.68 0.88 0.45 Ope:n 3.76 * 2.74 4.34 * 4 .. 43 * 4.64 * 5.96 ** 4.47 * 4.58 * 6.07 ** *: P<0.05. **: P<0.01. Note that P- values are relative (and not absolute) measures of significance as ·tests are interdependent. Table 9: Mean numbers (standard deviations in parentheses) of striped-type and oil-type sunflower seeds consumed by birds during 90 min sessions at each feeder. (paired t-test, df=ll)

Habitat Feeder Striped-type Oil-type t- value

Woods High 273.08(106.64) 461. 33 ( 53. 53) 5.818 ** Middle 273.58(155.26) 449.58(63.25) 4.881 ** Low 211.17(160.15) 414.67(125.79) 4.731 **

01 Edge High 333.42(141.37) 484.17(16.13) 3.576 ** \0 Middle 257.67(163.18) 453.50(52.23) 4.821 ** Low 168.58(136.98) 389.17(110.92) 6.036 ** Open High 41. 50 ( 63. 43) 276.75(179.25) 5.150 ** Middle 38.00(47.59) 165.92(113.34) 5.292 ** Low 11. 58 ( 20. 53) 53.92(66.19) 3.069 * *: P<0.05 **: P<0.01 Table 10: Feeding efficiencies of birds at each feeder

Woods Edge Open All three habitats High Middle Low Combined High Middle Low Combined High Middle Low Combined combined

No. of Seeds 8813 8678 7510 25001 9811 8750 6693 25254 3819 2447 786 7052 57298 consumed

Feeding efficiency: 6.88 6.86 6.26 6.69 7.12 7.68 6.65 7.19 9.90 11. 79 5.57 10.07 7.33 sec/seed removed °'0 Feeding efficiency: 1.17 1.14 1.10 1.14 1.11 1.16 1.04 1.11 0.75 0.82 0.66 0.767 1.08 # of visit/seed removed Table 11: Rate of aggressive encounters between birds at each feeder.

Woode Edge Open All three habitats High Middle Low Combined High Middle Low Combined High Middle Low Combined combined

No. of 109 141 132 382 74 110 156 340 68 19 1 BB BlO aggressive events

Aggression 0.108 0.142 0.169 0.137 0.064 0.098 0.210 0.112 0.108 0.040 0.014 0.047 0.116 rate (# of events/min)* °'...... *= # of aggressive events divided by the total feeder time of birds Table 12: Total number of aggressive encounters between various species occurring at each feeder.

Woods Edge Open

Species* High Middle Low High Middle Low High Middle Low

C-+C 62 90 70 21 58 105 56 18 1 T-+T 5 5 9 16 13 7 4 0 0 W-+W 8 3 2 2 1 0 0 0 0 0\ W-+C 10 23 7 11 4 0 0 0 0 N T-+C 20 14 43 12 34 44 7 1 0 W-+T 1 5 0 12 0 0 0 0 0 T~W 3 0 1 0 0 0 0 0 0 D--D 0 0 0 0 0 0 0 0 0 W-+D 0 1 0 0 0 0 0 0 0 D-+C 0 0 0 0 0 0 1 0 0

Total 109 141 132 74 110 156 68 19 1 Total number of aggressive encounters: 810 *: C: Carolina Chickadee T: Tufted Titmouse W: White-breasted Nuthatch D: Downy Woodpecker Table 13: Numbers of interapecific and intraspecific aggressive encounters occurring at each feeder. Feeder Woods Edge Open heights Inter-* Intra-* combined Inter- Intra- combined Inter- Intra- combined High 34 75 109 35 39 74 8 60 68 Middle 43 98 ·141 38 72 110 1 18 19 a- Low 51 81 132 44 112 156 0 1 1 V) Total 128 254 382 117 223 340 9 79 88 * Inter-: Interspecific encounters Intra-: Intraepecif ic encounters Table 14: Percentages of all visits in which birds stayed for more than 10 sec for all 8 species combined at each feeder. The upper, middle, and lower rows are the first, second and last 30 min of the experimental period, respectively.

Feeder Woods Edge Open

High 3.06 2.37 2.37 7.49 7.97 31.23 13.76 17.76 42.39

Middle 3.82 4.40 30.93 5.12 5.91 42.03 9.91 12.89 41.59

Low 2.04 5.82 28.21 4.78 4.41 23.60 7.19 14.54 18.45

64 Table 15: Means (standard deviations in parentheses) of: A) visit frequency, and B) total feeder time at 9 feeders in each month {Upper, middle, and lower rows are for December, January, and February, respectively.

A: Visit fequency

Feeder Woods Edge Open

High 822.25(124.36) 1085.00(149.75) 243.50(139.58) 947.00(156.89) 925.50(134.60) 310.00(197.59) 798.25(287.58) 712.00(163.72) 164.50(173.80)

Middle 773.75(251.95) 924. 75( 151.18) 205.00(127.34) 1028.50( 93.30) 989.75(330.53) 213.75(180.76) 686.75(264.57) 612.00(223.12) 85. 50 ( 91. 65 )

Low 645.50(291.95) 620.75(168.15) 69.75( 78.10) 887.50(152.04) 758.25(193.08) 50. 75( 71.65) 534.50(193.27) 345.50(186.53) 8.50( 10.97)

B: Total feeder time-

Feeder Woods Edge Open

High 5120.50(1243.74) 7086.50(1079.18) 3545.75(1480.97) 5724.50( 467.84) 5751.75(1299.48) 3657.50(2355.67) 4345.00(1413.11) 4580.50(1420.78) 2250.25(2033.71)

Middle 4562.25(1715.61) 6582.00( 998.56) 2802.00(1507.81) 6312.00( 620.28) 6334.25(2457.65) 2802.75(2505.74) 4029. 25 { 1571. 06) 3892.75(1968.24) 1559.50(1560.29)

Low 3469.75(1540.86) 4073.50(1035.95) 669.75{ 789.39) 5323.50( 761.89) 4640.75(1202.57) 359.25{ 511.64) 2871.25(1300.57) 2352.00(1755.33) 60.00{ 74.05)

65 Table 16: Spearman rank correlation coefficients for the relationships between weather conditions (temperature and wind speed) and visit frequency, total feeder time, and average feeder time.

Weather Visit Total Average Habitat condition f reguency feeder time feeder time Woods Temperature -0.642 ** -0.671 ** -0.258 Wind speed -0.315 -0.169 0.133 Edge Temperature -0.492 ** -0.542 ** -0.352 ** Wind speed -0.458 ** -0.398 * -0.153 Open Temperature -0.626 ** -0.570 ** 0.207 Wind speed -0.388 * -0.279 0.317 *= P<0.05 **= P<0.01

66 Table 17: Comparisons (t-tests) of foraging patterns in each habitat on sunny and cloudy days.

Woods Edge Open

Variables Sunny Cloudy t-value Sunny Cloudy t-value Sunny Cloudy t-value

Visit 671. 42 581.63 -2.260 * 834.75 658.00 1.85 138.10 167.00 -0.562 frequency

°''l Total 4039.58 4939.88 -1.718 5567.54 3962.92 2.44 * 1695.57 1970.00 -0.007 feeder time. Mean 5.86 5.82 0.134 6.62 5.91 2.30 * 14.32 10.25 2.53* feeder time *: P<0.05 Table 18: Comparisons of the number of visits at each feeder height in each habitat on sunny versus cloudy days. (x~ contingency table test for increasing preferences for higher feeders on cloudy days; df=2.) (percentages in parentheses)

Feeder Woods Edge Open height sunny cloudy sunny cloudy sunny cloudy

High 719 924 932 858 211 279 (35.7) (36.2) (37.2) (43.5) (50.4) (56.0)

Middle 719 885 934 657 159 181 (35.7) (34.6) (37.3) (33.3) (39.1) (35.5)

Low 576 746 638 458 44 42 (28.6) (29.2) (25.5) (23.2) (10.5) (8.4) xa-value 0.5594 18.086 ** 2.466 **: P<0.01

68 Table 19: Comparison of the number of visits at feeders in a: woods; b: edge; and c: open habitat under different wind speeds. Chi-square contingency table tests show that the preferences of feeder heights are not independent of wind speed in all three habitats. (percentages in parentheses)

a: woods

Feeder Strength of the wind heights 0 1 2 3 4

High 820 912 712 1056 784 (35.7) (34.1) (40.4) (37.9) (37.8)

Middle 745 957 568 1042 728 (32.4) (35.8) (32.2) (37.4) (34.9)

Low 731 806 483 690 567 ( 31. 8) (30.1) (27.4) (24.7) (27.3) x 1 = 53.63, df=8, p<0.01. b: edge

Feeder Strength of the wind heights 1 2 3 4 High 973 997 534 740 (35.9) (40.2) (43.6) {42.6)

Middle 1040 852 466 612 (38.4) (34.4) (38.0) {35.2)

Low 698 629 226 386 (25.7) (25.4) (18.4) (22.2) x 1 = 49.073, df=6, p<0.01. c: open field

Feeder Strength of the wind Heights 2 3 4

High 360 182 155 (49.5) (57.8) (52.5)

Middle 274 118 99 (37.7) {37.5) (33.6)

Low 93 15 41 {12.8) (4.8) ( 13:9) xi =22.804, df=4, P

69 Table 20: Visit frequencies of birds at feeders in a: woods; b: edge; and c: open field stations under different temperatures. Chi-square contingency table tests show that the preferences for high feeders increased with increasing temperature in all three habitats. (percentages in parentheses)

a: woods Temperature c· c > Feeder heights -5 - 0 1 - 5 > 5

High 1013 (31.9) 833 (35.9) 773 (42.6) Middle 1114 (35.0) 830 (35.8) 537 (29.6) Low 1052 (23.1) 655 (28.3) 505 (27.8)

x,,=65.88, df=4, P<0.005

b: edge

Temperature ( 0 c) Feeder heights < -5 -5 - 0 1 - 5 >6

High 1068 (36.4) 990 (39.6) 942 (36.7} 64 (43.5) Middle 1177 ( 40: 1} 829 (33.2} 905 (35.3) 54 (37.2) Low 688 (23.5) 679 (27.2) 719 (28.0) 28 ( 19·. 3)

x~=71.79, df=6, P<0.005 c: open

Temperature c·c > Feeder heights < -5 -5 - 0 1 - 5 >6

High 379 (40.8) 445 (48.6) 141 (55.5) 147 (64.8) Middle 394 (42.4} 360 (39.3} 95 (37.4) 72 ( 31. 7) Low 156 (16.8) 111 (12.1) 18 ( 7.1) 8 ( 3.5) x 1 :;65. 57, df=6, P<0.005

70 ,. fl' ;,, i' f :f ''f' :f J~~·' F :r .,,, ~ __..,._ ~- -- "-"'"'~ .,. ~ f ' 1 &J ,- ' ' 'f p ' f 1• " ~~ f ., f ~ ~ J:. ,. f :r ~~~~~ '1--~ . . . " ~ ~- ...... " ii!. " 'f ~ 1 T Y.·• . . • • . ... , . . ' 'f ' :p ' l"'..,. • • • • • ~ ~ l' ~__...... _ . • • 'f ...... " . . "' '.f :f ...... ~ I 1 ' • ' 'f :f ' ' f ' • ~ N ' 1 t ? ' --..... ' Sm l' 'f T f ' f ' w : Woods Station lf' t ' f f f • E : Edge Station " ' r) 0 : Open Station 't • 'f ' ~ ' f f/J: Dense Shrubs ' ' • I: "J: Woods . :f ' ' • " Figure 1: Habitats of three feeding stations at Fox Ridge State Park. Colee County. Illinois. USA. --

Figure 2: Feeding stations at the edge and the open field.

72 Figure 3: Feeding atation in the woods.

73 A: Woods 70 --HIGH 60 +MIDDLE ;, LOW 50

40

30

2

o:-::-:--:-:-~~~~-:-~~~~~~~~~~~~- 5 10 15 20 25 30 351 40 45 50 55 60 65 70 75 80 85 90 e: Edge 80

tz ~ 0 UJ fE ~ Ci5 s: 20

0'--~~~~~~~~~~~~~~~~~~~~~ 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

C: Open 20

O~*-~~~~~~~~~~~~~~~~~~ 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 TIME (MIN) Figure 4: Visit frequencies (total # of visits) for all B species combined in the a: woods. b: edge. and c: open field stations in 5 min intervals ...High ..... Middle"'., and "Low"' refer to feeders 3 m. 1.5 m, and 0 m off the ground., respectively. 74 A: Woods 400 -HIGH +MIDDLE

300

o:-----~~~~~~-.--~~~~~~~~~~~~~ 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 B: Edge 500

'U II ..!!!.. w ~ t= a: w 0 w w LL

c(-' I- 0 100 I-

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

C: Open 250

o. 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 TIME (MIN) Figure 5: Total feeder time (sum of the time that birds spent on feeders) for all 8 species combined in the a: woods. b: edge. and c: open field stations in 5 min intervals. "High". "Middle". and "Low" refer to feeders 3 m. 1.5 m. and 0 m off the ground., respectively.

75 A: Woods 60 -High

o.__~~~~~~~~~~~~~~~~~~~~~ 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 I

B: Edge 70

zt w :l 0 w fE 1- (ij > 10

1 0 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

C. Open 16

o~. --"""-~~~~~~~~~~~~~~~~~~~- 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 TIME (MIN) Figure 6: Visit frequencies (total # of visits) for Carolina Chickadees in the a: woods, b: edge, and c: open field stations in 5 min intervals ...High .....Middle'', and "Low .. refer to feeders 3 m, 1.5 m, and 0 m off the ground, respectively. 76 A: Woods 350 .. HIGH -t MIDDLE 300

250

100

50

oL-~~~~~~~~~~~~~~~~~~~~~ 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 8: Edge 400

w ~ i= a: w 0 w gt:e ...

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

C: Open 250

o:J-~======~=--~~~_:_:..=:__~~~~~~~ s 10 15 20 2s 30 35 40 45 50 55 60 65 10 75 eo es 90 TIME (MIN) Figure 7: Total feeder time (sum of the time that birds epent on feeders) for Carolina Chickadees in the a: woods. b: edge. and c: open field stations in 5 min intervals. °'High"'. "Middle°'. and °'Low"' refer to feeders 3 111. 1.5 m. and 0 m off the ground. respectively.

77 A: Woods 12 -HIGH +MIDDLE

o::--.:-:--:-=--:-:--~~~~~~~~~~~~~~- 5 10 15 20 25 30 35 . 40 45 50 55 60 65 70 75 80 85 90 8: Edge 14

12

10--- ~ aUJ 8 UJ ff us~ s:

o;-;~::--'.::-=-=-=--=-=---=-=-----~~~~~~~~ s 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 C:Open 3.5

3

2.5

2

1.5

40 4S 50 55 60 65 70 75 80 BS 90 TIME (MIN) Figure 8: Visit frequencies (total • of visits) for Tufted Titmice in the a: woods, b: edge, and c: open field stations in 5 min intervals. "'High"', "'Middle"', and "'Low" refer to feeders 3 m, 1.5 m, and 0 m off the ground, respectively.

78 A: Woods 50 HIGH

o~~~~~~~~-:--~~~~~~~~~~~~~ 5 10 15 20 25 30 35 ; 40 45 50 55 60 65 70 75 80 85 90 B:Edge 70

U'., .!!. I.LI ~ t= a: I.LI 0 I.LI ~ 20 ...J j! 10 0 t-

oL--~~~~~~~~~~~~~~~~~~~~~ 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 C: Open 25

20

15

10

5

o: 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 TIME (MIN) Figure 9: Total feeder time (sum of the time that birds spent on feeders) for Tufted Titmice in the a: woods. b: edge. c: open field stations in 5 min intervals. ··High''. "'Middle··. and ••Low•• refer to feeders 3 m. 1.5 m. and 0 m off the ground. respectively.

79 A: Woods 5 --HIGH +MIDDLE

15 20 30 35 40 45 50 55 60 65 70 75 80 85 90 zti w :::::> 0 w a: LI...... Ci) > B: Edge 6

5

4

35 40 45 50 55 TIME (MIN}

Figure 10: Visit frequencies (total # of visits) for White­ breasted Nuthatches in the a: woods and b: edge stations in 5 min intervals. "High'". "Middle". and "Low"' refer to feeders 3 m. 1.5 m~ and 0 m off the ground. respectively.

80 A. Woods 25 -HIGH +MIDDLE 20 ~:Low

15

10

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

B: Edge 40

35

30

25

20

15

5

30 35 40 45 50 55 60 65 70 75 80 85 90 TIME (MIN)

Figure 11: Total feeder time (sum of the time that birds spent on feeders) for White-breasted Nuthatches in the a: woods and b: edge stations in 5 min intervals. "High".,

"Middle" 11 and "Low"" refer to feeders 3 m. 1.5 m. and 0 ID off the ground. respectively.

81 DDAY1 ...... OcAv. -2

...... -~ I...... z~ 100 :i eo·· a:L&J ~ 0 L&J 60 L&Jen f-z 401 . L&J 0 I. .. a: 20 L&Ja.. OK < < < / / / / / ./ ./ / / / I FEEDER: H M L H M L H M L · HABITAT: WOODS EDGE OPEN Figure 1 2 : Percentages of seeds left at feeders in experiment B (see text). •H" refers to high feeders (3 m off ground). •M" to middle feeders (1 m off ground), and •L• to low feeders (on the ground). •... •Q w ~ ~ ~ ~ ..... i •+* ....N ... ! ...0 ca i .§: • ~• .... :g 0 .2 ~ • w c a: ~ ..• :CJ' ... w~ •• Q. ">- 0 ::i c ... w 0 . t- Cit .,qi ~ •! ".,, i.. • !. ":- e ~ 'I' qi I 0 - ~ .. •:I ... Q o• u:: I I I I ....~ .... (uo1sses/JepeeJfs11s1A- so #) AON3n031:1.:t !ISi/\

83 :ZS -wooDS +eooe

i=- 20 Ci5

~fl) -1s w :E i= ~ a: w 0 10 w w LL I z I I ±==- •I + ~::e 5

O'--~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ·11 ·10 -0 .a ·7 -e -& ~ -G ·2 ·1 0 1 2 S 4 5 S 7 I I 10 11 12 11 11 TEMPERATURE ct)

Figure 13b : Temperature effects on mean feeder time for birds in each habitat f ,0001 . -wooos /'-....._ fl) -='----- +eooe CD fl) ";::- 800 CD i .! ..:t::: eoo >"' 0 00 !let 01 - 400 ~z w :::> aw a: 200 "- ~ > 0 0 1 2 8 4 INDEX OF WINO SPEED

Figure 14 a: Wind effects on visit frequency for birds in each habitat (See text for description of wind index) 20 I -wooos +eooe I ~ '* ~• 15 •~ -w ::i ~ 10 co w °' 0w w L&. z U) 5 ::i

O'--~~~~~~~~~~~~~~~~~~~~~~~~~ 0 1 2 3 4 INDEX OF WINO SPEED

Figure 14b: Wind effects on mean feeder time for birds in each habitat. (See text for description of wind speed index.)