Food patches and foraging group size in granivorous birds

Anita. Behav., 1989, 38, 665-674

Food patches and foraging group size in granivorous birds S C O T T M. P E A R S O N

Department of Zoology, University of Georgia, Athens, GA 30602, U.S.A.

Abstract. The effect of the size of food patches on the size of foraging groups was examined in field and aviary experiments. In the field, reducing the area of a high-quality patch significantly reduced the size of heterospecific foraging groups. Outdoor aviary experiments with three patch sizes measured aggression and inter-bird distance in monospecific groups. Group size in white-throated sparrows, Zonotrichia albicollis, was reduced on smaller patches. Group size in field sparrows, Spizella pusilla, was not affected by patch size, as it had been in the field where field sparrows were exposed to aggressive attacks by heterospecifics. Individual aggression rates did not increase with decreasing patch size in white-throated sparrows, but the overall incidence of aggression in white-throated sparrow groups did increase. Aggression in field sparrows was infrequent and unaffected by patch size. Individuals of both species fed closer together when monospecific flocks were larger. Additionally, field sparrows fed closer together on smaller patches. However, patch size did not significantly affect inter-bird distance in white-throated sparrows. Aggression may regulate group size even though aggression levels may not change relative to group size.

There are definite advantages to group foraging. Groups of predators such as lions are able to subdue larger prey animals when the lions cooperate (Caraco & Wolf 1975). Other animals can reduce their vulnerability to potential predators by feeding in groups. Group members sharing vigilance are likely to detect a predator more quickly (Pulliam 1973; Powell 1974; Lazarus 1979; Barnard 1980a; Bertram 1980; Elgar & Catterall 1981). While in a group, individuals can spend less time scanning and devote more time to feeding, social interference or other activities. When a group is attacked by a predator that takes only one animal, the individual's chance of being killed becomes smaller as group size increases (Hamilton 1971; Vine 1973; Pulliam & Caraco 1984). Furthermore, foraging groups can find cryptic patches of food more quickly (Krebs et al. 1972; Caraco 1981) and reduce the chance of finding no food at all (Baker et al. 1981). The most obvious cost to group foraging is that food resources must be shared by all members. Nearby foragers may interfere with the feeding efficiency of others (Goss-Custard 1976). As animals feed closer together, the probability of social interactions increases. Some species defend individual distances (Marler 1956) through aggressive acts or displays when other individuals approach too closely. This defence may interfere with the feeding of the aggressor as well as the victim of this 0003-3472/89/100665 + 10 $03.00/0

aggression. Furthermore, aggression levels tend to increase as group size increases (Silliman et al. 1977; Barnard 1980b; Wilkinson 1982; Barnard & Thompson 1985; Elgar 1987). Group Size and Food Patches Aggression is negatively correlated with distance between individuals (Feare & Inglis 1979; Caraco & Bayham 1982; Barnard & Thompson 1985; Monaghan & Metcalfe 1985). House sparrows, Passer domesticus, tend to aggregate around highquality clumps of food (Barnard 1980a, c). Barnard (1980c) found that flock size in house sparrows is strongly affected by the area of these high-quality food patches. When these food patches are too small to accommodate all the birds that attempt to feed there, subordinate birds are excluded. Elgar (1987) observed that the frequency of agonistic interactions was greater in house sparrow flocks at smaller feeders. Thus, the size of the food patch can influence the costs and benefits and, in turn, govern group size. Many granivorous bird species that spend the winter in south-eastern United States associate in mixed-species groups while feeding on the fruits and seeds in early successional habitats. Th~se food items commonly occur in clumps of differing size. Because these birds must meet high energy requirements in cold weather, they should be sensitive to

9 1989 The Association for the Study of Animal Behaviour 665

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factors that influence the benefits and costs of flocking, making them appropriate subjects for experimentation. By varying food dispersion at a high-quality patch in field and laboratory experiments, I observed the effects of patch size on foraging group size and measured the costs of social interference in these flocks. I hypothesized that increasing benefits relative to the costs of group foraging should increase group size, but increasing costs without increasing benefits should reduce group size. Larger groups were expected at the more attractive foraging situations in these experiments. Rates of aggression and individual distances, measured by nearest-neighbour distance, may be involved in regulating group size.

EXPERIMENTAL M E T H O D S Field Experiment During January 1987, I conducted a field experiment at Whitehall Experimental Forest near Athens, Georgia, (U.S.A.) to test the effects of food dispersion on the size of heterospecific groups of granivorous birds. Two feeding stations were established 20 m apart near the edge of a 2-ha field dominated by broomsedge, Andropogon spp., and scattered plants of blackberry, Rubus spp. The field was bordered by hardwood forest on three sides and mesic successional thicket on the fourth. Each feeding station was approximately 2 m from a narrow natural hedge of privet, Ligustrum sinense, which was approximately 2.5 m tall, at the edge of the field bordering the forest. The stations were located within the broomsedge that dominated the open field; however, the vegetation was 30% denser and 10% higher around the station designated as 'the left station'. In addition, there was a loose clump of blackberry plants, 2 m in diameter and 1 m high, beside the left station. An individual station consisted of a square platform measuring 30 • 30 cm raised 1.5 m above the ground on a small post and a square board measuring 120• 120 cm on the ground directly below the platform. I observed both feeding stations from an elevated blind located in the field approximately 12 m from each station. My field experiment simulated two antagonistic consequences of foraging in a group. By adding food to the raised feeder, I attracted additional birds to the feeding stations, possibly giving individuals on the ground greater anti-predator bene-

fits (increased vigilance, dilution effect) but not increasing the costs associated with social interference or food depletion. Decreasing the patch size on the ground forced individuals at that patch to feed closer together and, presumably, required them to interact more often. Thus, feeding on the smaller patch intensified any cost of being part of a group, but offered none of the benefits of a larger group. Work by Grubb (1977; Grubb & Greenwald 1982) has shown that birds use structural properties of the habitat to reduce thermoregulatory requirements in winter. These factors were considered when the feeding stations were established. Both stations were located south-east of the hedgerow and forest edge so that the morning sun first struck both stations at about the same time. The experiments on sunny mornings always began after the rising sun had begun to shine on both stations. Both stations offered equal shelter from wind. I varied food dispersion in two ways, vertically and horizontally. The vertical food dispersion treatment consisted of two levels: (1) food on the ground only, and (2) food on the ground and on the platform. Horizontal dispersion also contained two treatment levels: (1) large patch, food spread over the entire feeding board placed on the ground; and (2) small patch, food spread within a smaller area, measuring 60 x 60 cm, in the middle of that board. The food was a mixture of one part canary seed, Phalaris sp., to two parts proso millet, Panicum sp., for the ground board and oil-type sunflower seed, Helianthus sp., for the high platform. Sunflower seeds were used because they readily attracted birds to the raised feeders and did not spill to the ground as often as grass seed. At the beginning of each experimental run, 100 g of the seed mixture was spread evenly on the board, regardless of patch size, and 150 g of sunflower seed was placed on the high platform. There were four treatment combinations for the two factors at two levels in this experiment, but an observer in the blind could monitor only two stations at one time. I overcame this problem by using a confounded factorial design consisting of six half-replicates. This design also allowed me to gather information on each possible source of experimental variation (Cochran & Cox 1957). The ordering of individual half-replicates, as well as the pairing of daily treatments with stations, were determined randomly, and the entire series was completed twice.

Pearson: Food patches and foraging group size

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throated sparrows (all adults of unknown sex) and one northern cardinal, Cardinatis cardinalis, (adult female) were captured and placed in the aviary. Northern cardinals often join sparrow flocks as single individuals or pairs. To determine if sparrow interactions were different in the presence of a cardinal, I included this species in a portion of the aviary experiments. However, this individual was never involved in any aggressive interaction, and foraging groups that included the cardinal were not significantly different in either aggression rates or individual spacing from monospecific groups. Nevertheless, groups where the cardinal was preAviary Experiment sent were omitted from these analyses. The experiment began after 3 days of acclimaIn February and March 1987, I conducted an tion. I deprived the birds of food from sunset on the experiment at Horseshoe Bend Research Area, evening before each experiment until 30 min after Athens, Georgia to test the effect of varying food sunrise the next morning when the experiment patch size on rates of aggressive interactions and individual spacing in two species of birds: the began. Each day of the experiment, the birds were allowed to feed sequentially on each of the three white-throated sparrow, Zonotrichia albicollis, and the field sparrow, Spizella pusitla. These experi- patch sizes. Each size of patch was available for 30 ments were conducted in a large outdoor aviary rain, after which the feeding board was emptied of comprising three rooms. The largest room, measur- seed and the new patch size established. The ing 5 x 3 x 2.1 m, was constructed of a series of sequence of patch sizes for each day was deterhardware cloth panels supported on wooden mined randomly, conforming to a replicated latin square design (Cochran & Cox 1957). Activities of frames. Two smaller rooms, measuring 2 x 3 x 2.1 the birds on the board were videotaped with a m, were situated at either end of the large room. The large room contained a small brush pile and a camera inside the middle room. This experiment square feeding board on the ground measuring was performed on 6 days within a 9-day period, 120 x 120 cm. During the experiment, the birds then the white-throated sparrows were released. On 9 March 1987, five field sparrows (all adults of were allowed to feed in the large room or retreat through a doorway to one of the smaller end unknown sex) were captured and placed in the rooms. The opposite small room served as a blind aviary with the same cardinal, and the experiment for the observer who could view the foraging arena was repeated after 3 days of acclimation. through a one-way mirror. Three sizes of food patch were used in this experiment: large, 120 x 120 cm; medium, 60 x 60 DATA ANALYSIS cm; and small, 30 x 30 cm. I created these patches by evenly spreading 10 g of canary seed over the specified area on the feeding board. A smaller Field Experiment The daily mean group sizes for the food disperamount of seed was used on the aviary feeding board to facilitate the complete removal of uncon- sion treatments were calculated by summing the numbers of birds present at each station (ground sumed seed at the end of a 30-rain experiment (see feeder only), and then dividing that sum by the below). The seed concentrations chosen in the aviary and the field experiments were high enough number of records (2-min censuses) when at least one bird was present at that station. Effects on so that feeding rates would not vary over the group size were tested by analysis of variance different patch sizes. More seed was used in the field experiment because of the longer duration of appropriate to this experimental design (Cochran & Cox 1957). each experimental session and the larger group The group sizes of individual species were taken sizes. Each patch was centred in the middle of the from censuses of both monospecific and heterospeboard. From 23 to 26 February 1987, five white- cific groups. Monospecific groups were not

During each observational period, the numbers and species of birds feeding at each station were recorded every 2 min for 2 h. At each 2-min interval, only birds perched on the ground board or the elevated platform were recorded, though other individuals may have been present in nearby grass and shrubs. One half-replicate was performed per day, 3 days per week. Each experimental session began within 40 min after sunrise regardless of the weather, except that days of heavy rainfall were avoided.

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recorded often enough to permit a separate analysis. Cardinals, white-throated sparrows and field sparrows were the only species with sufficient data for these analyses. The frequency with which each group size occurred during the experiment was also examined for all species combined and these three individual species. G r o u p size frequencies were defined as the number of 2-min censuses that a group of size N was recorded over all days. A group of size zero occurred when there were no birds present at one station, but at least one bird was observed at the other station. The frequency distributions for the two patch sizes were compared by performing a contingency table analysis on the frequencies (total counts) for each group size, including the zero group sizes. This analysis was also performed excluding the zero groups.

Aviary Experiment Videotapes of the experimental runs were reviewed to determine aggression rates in the aviary experiment. A n aggressive event was defined as the directed movement of one bird (aggressor) toward another (recipient) that resulted in the displacement o f the recipient (aggressor wins) or repulsion of the aggressor (aggressor loses). The number of aggressive events was recorded for each group size. Aggression rates (aggressive events per min) were calculated by dividing the total number of aggression events for each group size by the total time that group size was present. This number was then corrected for group size (N) by dividing by N. There was a maximum of six possible data points for any group size treatment combination, but all group sizes did not occur during each treatment on each day. D a t a for group sizes above four birds were rare, so these groups were not included in any analysis. Since the number of data points varied for each treatment combination, the mean number of aggressive interactions was used in the analysis for simplicity. These means were used in a two-way analysis of variance to test the effects of group size and patch size on aggression rates. Using the videotapes, the mean nearest-neighbour distance was calculated for each bird present on the board at 5-s intervals. First, the relative positions of each bird were recorded from the video image by means of a sonic digitizer (Science Accessories Corporation, model GP-8) that generates coordinates on a plane Cartesian coordinate

system. The distortion of the video image due to the oblique angle of the camera view was corrected by performing a two-dimensional affine transformation (Burnside 1985) on the digitized coordinates. The distance between each bird and its nearest neighbour was calculated using the corrected coordinates. M e a n nearest-neighbour distances were obtained by averaging the nearest-neighbour distances for each treatment combination over all dates.

RESULTS

Field Experiment The mean +_SE group size was 2.25 _+0.19 birds; eight species of birds attended the feeding stations. Station, patch size and patch size by station interaction were significant sources of variation in group size (station, F1.14=12'74; patch size, F1,14=9'81; patch size • station, FI,14= 18.43; P < 0 - 0 1 for all F). However, no factor involving vertical dispersion treatments showed significant effects. The mean group sizes were greater for the left feeding station than the right (overall means: left, 2.62 birds/station; right, 1.89; t22-- 3.36, P < 0-01). Although the records with zero group size were not included when calculating mean group size, the percentage of such records was greater for the right station than the left (77__ SE: right, 61 "9 ___22-7; left, 40"8 ___21.3; two-sample t-test with pooled estimate of variance, t22=2-348, P<0"025). A goodness of fit test indicated that attendance at the two stations was independent (Z2=2.054, d f = l l , P=0.998). Mean group sizes for the large patch treatments were greater than for the small patches (Table I).

Table I. Mean (SE) group sizes of birds in the field experiment Patch size

All species Cardinals White-throated sparrow Field sparrow

Large

Small

T*

P

2.5 (0.17) 1.4 (0.08)

2.0 (0.27) 1.2 (0.12)

2.53 2.06

<0.01 <0-05

1.0 (0.I0) 2.1 (0.16)

0.8 (0.15) 1.7 (0.21)

1.73 2-17

<0.05 <0.03

* Means were compared with pooled variance t-test, df-- 22.

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Figure1. Percentage of observations of different group sizes feeding on small (ll) and large (121)food patches in the field experiment. Larger group sizes were observed more frequently on the large patch in (a) all species combined, (b) northern cardinals, (c) field sparrows and (d) white-throated sparrows.

The large patch treatment at the left station produced the largest groups of all. The above analysis was performed without regard to group composition. However, the same patterns held true when the three most common species (cardinals, field sparrows, white-throated sparrows) were examined individually. The mean group size of these individual species was greater on the larger patch (Table I). When examining the group size frequencies, different patterns of patch size use became apparent (Fig. 1). The larger group sizes were present more frequently on the large patches than the small patches when all species were combined and in field sparrows and cardinals (Fig. 1). Since data for the larger group sizes of white-throated sparrows were absent, it was not possible to make this comparison. Contingency table analysis indicated that the distributions, relative to patch size, were significantly different (significant results: all species, with N = 0 , ;(2=29.89, df=5, P<0"01; cardinals, with N = 0 , ;(2=13-35, df=5, P < 0 . 0 2 , without N = 0 , ;(2=11.03, d f = 4 ; field sparrows, with N = 0 , ;(2=20"82, df=5, P<0-01).

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Figure Z Mean (-I-SE) rates of aggression in whitethroated sparrows. Reducing patch size increased aggression rates, but aggression rates tended to decline in larger groups. Number of agonistic events for each group size was divided by the total time that group size was observed feeding and N (group size). The three patch sizes used were: large (ra), medium (I~) and small (11).

Aviary Experiment The incidences of aggression in field sparrows and white-throated sparrows were very different. The low rate of aggressive events in field sparrows did not suggest any significant variation due to patch size and group size treatments. Only three

Animal Behaviour, 38, 4

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Table IL Number of 5-s intervals that each group size of

white-throated sparrows and field sparrows occurred in aviary experiments

Table III. Analysis of variance of nearest-neighbour

distance in field sparrows and white-throated sparrows Source

df

SS

MS

F

2 2 4 8

177.15 63.22 8.24 248.61

88.575 31.610 2.06

43.00** 15.34"

Flock size Field sparrows

Patch size

1

2

3

4

5

530 414 296

257 163 29

122 195 108

White-throated sparrows

Large 731 721 Medium 916 688 Small 1509 824 Z2 = 541.49, df= 8, P < 0.001 Field sparrows

Large 861 348 Medium 642 510 Small 783 273 Z2= 310.96, df=8, P<0.001

209 351 177

101 35 130

81 24 0

Chi-squared scores from contingency table analysis indicate whether the distributions of these totals between the three patch sizes are significantly different.

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Group size Figure 3. Percentage of observations of different group

sizes feeding on three patch sizes in the aviary experiment. Smaller patches reduced the frequency of larger groups of white-throated sparrows (a), but field sparrows (b) did not show a consistent response to patch treatments. Patch sizes same as for Fig. 2.

Flock size Patch size Error Total

White-throated sparrows

Flock size Patch size Error Total

2 2 4 8

252.59 81.57 55-07 389.22

126.30 40.78 13-77

9.17" 2.96

Both increasing group size and decreasing patch size significantly reduced distance between individual field sparrows (also see Fig. 4). However, only group size produced significant effects in white-throated sparrows. * P < 0.05; ** P < 0.01. d a t a points were o b t a i n e d from four aggressive events in this species. In the w h i t e - t h r o a t e d sparrow experiment, 78 aggressive interactions were recorded (Fig. 2). These experiments were exactly the same length each day, a n d each species spent roughly the same a m o u n t of time feeding. W i t h i n g r o u p sizes of two, three a n d four whitet h r o a t e d sparrows, aggression rates increased m a r kedly as the food patches decreased in size (Fig. 2). However, aggression rates tended to decrease as g r o u p size increased for a given p a t c h size (simple linear regression; p a t c h size, slope__SE: large, -- 0.0425 _+0.1261, NS; medium, - 0-5347_+ 0-052, P < 0.05; small, - 0'8534___ 0.0955, P < 0-05). There was n o significant change in aggression rates over the course of the three treatments administered each day (order effect, F2.~=0"72, P > 0" 10). T h e g r o u p size frequencies were also e x a m i n e d in the aviary experiment (Fig. 3, Table II). Whitet h r o a t e d sparrows d e m o n s t r a t e d a p a t t e r n similar to t h a t observed in the field experiments where larger groups occurred o n larger patches (cf. Fig. 1). C o n t i n g e n c y table analysis confirmed t h a t the frequency distributions o f different g r o u p sizes feeding o n large, m e d i u m a n d small patches were dissimilar (Table II). However, in field sparrows, the distribution o f g r o u p sizes did n o t change consistently relative to the p a t c h size treatments, even t h o u g h the distributions were significantly different (Table II, b u t see Discussion below). W h e n the n e a r e s t - n e i g h b o u r distances between

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DISCUSSION 50

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Field Experiment

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Figure 4. Mean (• nearest-neighbour distance in the aviary experiment. Nearest-neighbour distance declines with increasing group size and decreasing patch size. Only changes in distances in different groups sizes were significant in white-throated sparrows (a), but both patch and group size treatments produced significant effects in field sparrows (b) (see Results). Patch sizes same as for Fig. 2.

field sparrows were examined, both the patch size treatments and group size significantly affected distance between group members (Table III). The mean distance between nearest neighbours declined when groups fed on smaller patches. Nearestneighbour distance also decreased with increasing group size (Fig. 4). There was little difference in inter-bird distances in groups of two birds when they fed on the large and medium patches. However, patch size strongly affected nearest-neighbour distance in groups of three and four birds. Nearestneighbour distances of white-throated sparrows also declined with increasing group size (Fig. 4). However, analysis of variance indicated that there was no significant effect due to changing patch size (Table III).

The factorial design of the field experiment permitted the measurement of the effects of food in the raised feeder and varying patch size on mean group size. The presence or absence of food in the high feeder had no effect on group size on the ground, suggesting that birds on the ground perceived no benefit from birds attending the high feeder. The birds on the raised feeder may have been too far away to be considered part of the foraging group on the ground. Alternatively, the antipredator benefits of this arrangement may have been too small to allow detection by these methods. Though these flocks risked attack by bird-eating hawks (Accipiter spp.) which frequent this habitat, the details of predation hazard, such as the probability of attacks on different group sizes, were not known. Caraco (1979) demonstrated theoretically how increases in social interference with increasing group size may offset any potential advantages of increased predator avoidance when predator attack rate is independent of group size. Altering patch size changed the mean group sizes significantly in the field. Forcing the birds to feed closer together by reducing patch size may have increased the probability of social interactions and any associated costs. If benefits and costs changed relative to group size, there may have been a preferred group size for which benefits relative to costs were maximized for individual foragers, resulting in different preferred group sizes for the different patch sizes. The reduction in mean group size with the smaller food patch and the result that larger groups fed more frequently on the large patch suggested this was true. Elgar (1987) found such a change in costs and benefits in house sparrows. While house sparrow feeding rates increased with flock size at large feeders, they decreased with flock size at small feeders where aggression rates were higher. Preferred group sizes can be different for subordinate and dominant birds because cost benefit schedules may vary for individuals of different social status (Pulliam & Caraco 1984). Dominant individuals might enjoy the benefits of larger groups without additional losses due to interference. Benefits gained by subordinate birds may be offset by disproportionate losses in feeding time and food access when the number of dominant birds increases (see Pulliam & Caraco 1984 for a

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more complete discussion). For example, dominant dark-eyed juncos, Junco hyemalis, obtained more seeds during feeding bouts on small patches than subordinate juncos; however, at large patches this difference was eliminated (Theimer 1987). Differences between the two feeding stations produced some of the strongest effects on group size. The left station seemed to be more attractive than the right station since birds were present more often on the left, and mean group sizes were larger on the left. Because sparrows prefer to feed near cover (Schneider 1984), the additional cover at the left station probably influenced the birds' preference. Caraco et al. (1980a) found that, given the choice between feeding sites near and far from cover, yellow-eyed juncos, Junco phaeonotus, choose sites close to cover despite higher aggression rates. They also noted that feeding sites without cover are unattended more often.

Aviary Experiment White-throated sparrows and field sparrows differed greatly in their rates of aggression. The pattern of group size frequencies for whitethroated sparrows feeding on the different patch sizes was similar to that observed in the field data. Larger groups occurred more frequently on the larger patches. However, the field sparrows did not conform to this pattern (Fig. 3). This difference between these two species was probably related to differences in their aggressive tendencies. In aggressive species like white-throated sparrows, the high costs of social interference may logically limit the persistence of large groups on small patches. The less aggressive field sparrows did not show the same patterns of group size frequencies with respect to patch size in the aviary experiments as they did in the field. In the aviary, they were observed in more or less monospecific groups, but in the field they co-occurred with a host of other, socially dominant, species. Song sparrows, Melospiza melodia, swamp sparrows, Melospiza georgiana, and white-throated sparrows often displaced the smaller field sparrows from the feeding boards. Therefore, the effects of aggression, as well as its absence, on field sparrow groups can be contrasted in the field and laboratory experiments. In the aviary, where aggressive events seldom occurred, smaller patches did not limit the occurrence of larger field sparrow groups. In the field, patch size apparently limits group size (compare Figs 1 and

4), since field sparrows are more likely to be displaced by heterospecifics at smaller patches. Though individual field sparrows fed closer together in smaller food patches (i.e. had smaller nearest-neighbour distances), they did not experience any increase in aggression rates. Nearestneighbour distance in field sparrows also decreased in larger groups. White-throated sparrows fed closer together in larger groups, but not in smaller patches (Fig. 4). In white-throated sparrows, reducing the size of the food patch increased aggression rates over all group sizes. The overall incidence of aggression in white-throated sparrows was greater in larger groups. However, the number of aggressive events per individual was actually lower in larger group sizes. Thus, individuals in more dense groups did not experience an increase in social interference, as has been posited by previous time-budget models (Caraco 1979, 1980) and several empirical studies (Pulliam et al. 1974; Silliman et al. 1977; Barnard 1980b; Caraco et al. 1980a, b; Wilkinson 1982). Other studies (Pulliam et al. 1974; Caraco et al. 1980a, b; Millikan et al. 1985) have based timebudget data on observations of focal individuals instead of overall group means. In these studies, which investigated the effect of temperature on social behaviour and interspecific dominance, the investigators observed randomly selected, focal individuals as samples of the population. In my experiment, data for each individual were not complete for all group sizes and experimental treatments, so I used the group means to simplify the analysis and to provide a general description of group dynamics. Both methods should have given the same result. The previous studies also compared group size-specific aggression rates over constant temperatures, which was not done in this analysis. This experimental design sought to randomize treatments over all environmental factors. Thus, if temperature was the factor controlling rates of aggression, this difference in experimental design might be responsible for the observed differences in the relationship between aggression rate and flock size. Aggression may have been controlling group size. Individual distances and/or aggression rates of white-throated sparrows may have been determined by some physiological or environmental factor (i.e. hunger or temperature) so that in any given experimental run individual distances were dependent on the physiological state of individual

Pearson: Food patches and foraging group size birds. When aggressive tendencies were high, individual distances were so great that large groups could not exist without conflict within the confined foraging space of a small patch. This type of group size regulation can explain why interbird distances were reduced in larger groups but not on smaller patches (Fig. 4, Table Ili). When aggression rates were low and individual distances were short, more birds could fit into the foraging space without conflict. Therefore, group size was negatively related to interbird distance. The mutual repulsion between white-throated sparrows would make them unwilling to tolerate violation of individual distances required for larger groups in limited space, so each patch size was characterized by an equilibrium group size where group members fed without conflict but other potential members were excluded or supplanted current members. Though patch size affected group size, it may not have significantly affected nearestneighbour distance. In time budget analysis, aggression is assumed to reduce the total amount of time left for foraging (Pulliam 1976). Social dominants can potentially gain by defending small ephemeral patches of food (Caraco 1979). Though agonistie encounters are not readily apparent in house sparrows, some birds are excluded from high-quality feeding sites when the initial group size is greater than could be accommodated at that site (Barnard 1980b, c). White-throated sparrows and field sparrows commonly feed at small clumps of grass seeds during the winter. If group size was regulated by aggression, then aggression rates would not necessarily have increased in larger groups. M o r e simply, when aggression was high, g r o u p size was limited, but when aggression was low, groups grew. This type of group size regulation could act to constrain social costs of group members.

ACKNOWLEDGMENTS Ideas for this research were cultivated in discussions with H. R. Pulliam, C. W. Benkman and K. Johnson. H. R. Pulliam, R. C. Taylor, E. S. Helfman, J. B. Dunning, B. Danielson, T. Caraco and an anonymous referee offered suggestions on earlier drafts. M. Kollock and G. E. Reynolds helped out in the field and the laboratory. C. P. Lo provided technical assistance. The research was funded by a grant from the Stoddard-Burleigh-

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Sutton Fund at the University o f Georgia, and partially supported by National Science Foundation Grant BSR 8415770 to H. R. Pulliam.

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(Received 28 September 1988; revised 6 December 1988; MS. number: A5238)