Behavioral Ecology Vol. 15 No. 3: 412-418
Behavioral Ecology vol. 15 no. 3 © International Society for Behavioral Ecology 2004; all rights reserved
The effect of patch size and competitor number on aggression among foraging house sparrows
a Department of Biology, Concordia University, 1455 de Maisonneuve Blvd W, Montréal, Québec H3G 1M8, Canada, and b Département des Sciences biologiques, Université du Québec à Montréal, Case postale 8888, succursale Centre-Ville, Montréal, Québec H3C 3P8, Canada
Address correspondence to C. A. Johnson, who is now at the Department of Zoology, University of Guelph, Guelph, Ontario, Canada, N1G 2W1. E-mail: johnsonc{at}uoguelph.ca.
Received 6 December 2002; revised 14 May 2003; accepted 27 June 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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We examined the effect of patch size and competitor number on aggression among house sparrows, Passer domesticus, foraging at patches of seven different sizes in a doubling series (0.014, 0.029, 0.058, 0.116, 0.230, 0.462, and 0.922 m2). Contrary to our expectations, the birds did not defend an entire patch, even when it was small as 0.014 m2. The frequency of aggression among the birds decreased gradually with increasing patch size, in contrast to the step decline predicted by resource defense theory. Moreover, the birds fought more frequently and more intensely as competitor density increased. Both results are consistent with the predictions of a modified hawk-dove model for shared patches. Females were more aggressive and fed at a higher rate than did males. The proportion of females increased as patch size decreased, and aggression became more frequent and intense. Even when patches are shared, patch size has an important effect on the frequency and intensity of foraging competition and the size and composition of foraging groups.
Key words: aggression, competitor density, hawk-dove, interference, patch size, Passer domesticus, resource defense, truncated phenotypic distribution.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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On encountering a food patch, group foraging animals typically defend the entire patch or share it with others (Giraldeau and Caraco, 2000
). Whether competition occurs primarily via interference or exploitation (sensu Keddy, 1989) will influence the type of individuals foraging in a patch. If defendable patches promote aggressive behavior, then individuals that are successful interference competitors may be attracted to these patches, whereas subordinate individuals may be excluded or seek alternative foraging opportunities. Resource defense theory, a body of literature originally developed to explain the occurrence of territoriality (see Brown, 1964; Davies and Houston, 1984; Myers et al., 1981), has been remarkably successful at predicting the transition from exploitation to interference competition among social foragers competing for ephemeral patches of food (Grant, 1993; Grant et al., 2002; Isbell, 1991), perhaps because territoriality is just an extreme form of interference competition. Resource defense theory identifies two key variables that likely influence the decision of whether individuals will use aggression during social foraging interactions: patch size and competitor number (Grant, 1993).
A small patch is thought to be economically defendable (sensu Brown, 1964) because the benefits of monopolizing food outweigh the energetic costs of excluding competitors from the small area. Because the costs of aggression increase rapidly with increasing patch size (see Praw and Grant, 1999), resource defense theory predicts a step decline in aggression beyond a threshold for patch size for which interference competition is uneconomical (Myers et al., 1981). Indeed, many fish (see Robb and Grant, 1998; Rubenstein, 1981), birds (see Goldberg et al., 2001; Zahavi, 1971), and mammals (see Monaghan and Metcalfe, 1985) become nonaggressive when patches are large. However, many animals do not defend ephemeral patches in an all-or-none fashion (Craig and Douglas, 1986; Wolf, 1978). Instead, aggressive behavior often decreases in a continuous way from the frequent vigorous defense of an exclusive area to the infrequent use of low-intensity aggression in shared areas (see Carpenter and MacMillen, 1976). But, there has been no quantitative description of changes in an organism's behavior over a wide range of patch sizes to test between a step and a gradual decline in aggressiveness with increasing patch size.
If small patches favor interference competition more than large patches, then subordinate individuals may be relatively more abundant in large patches, where aggressive behavior is less frequent. Such a truncated phenotype distribution (Parker and Sutherland, 1986; Sutherland and Parker, 1992) predicts that the proportion of subordinate individuals from a population will increase in foraging groups as patch size increases, assuming subordinates initiate fewer attacks and receive more aggression from dominants (but see Hein et al., 2003).
The number of competitors attracted to a patch also influences the economics of aggression. Resource defense theory predicts that aggression will be highest when a moderate number of competitors are attracted to a patch. When few competitors are at a patch, the relative abundance of food per competitor is high, so that any time spent on aggression is time away from exploiting the patch. Because relative food abundance decreases as the number of competitors increase, individuals benefit from defending a patch by securing a larger proportion of the resources until the increased cost associated with attempting to chase an increasing number of intruders (see Hixon, 1980) makes aggression uneconomical (Chapman and Kramer, 1996; Grant et al., 2000). Unlike the eventual decline in aggression predicted by resource defense theory, Sirot's (2000) hawk-dove game theory model predicts that aggression will increase monotonically with increasing competitor number. However, Dubois et al. (2003) suggest that Sirot's predictions depend crucially on the assumption of pairwise interactions between competitors in hawk-dove models. When competitors compete only in pairwise interactions, then aggression increases with competitor density, as in Sirot's model. When this assumption is relaxed so that multiple intruders can simultaneously challenge potential defenders, Dubois et al.'s (2003) game theory model predict that aggression declines at high competitor densities, consistent with resource defense theory.
The goal of the present study is to explore the effect of patch size and competitor number on the aggressive behavior of foraging house sparrows, Passer domesticus. We used seven patch sizes to describe changes in aggressive behavior over a broad range (65-fold) of patch sizes. House sparrows are a good test species because of their flexible behavior. The birds often forage in large groups with little aggression (i.e., forage socially), but can aggressively defend and exclude competitors from food (see Elgar, 1986b, 1987). House sparrows also exhibit dominance behavior (Summer-Smith, 1988), but conflicting evidence exists about which sex is dominant. Some studies suggest that females are dominant over males (Hegner and Wingfield, 1987), others report no difference between the sexes (Hein et al., 2003; Liker and Barta, 2001), and others suggest that females rarely participate in aggressive interactions (Møller, 1987). Specifically, we tested whether (1) aggressive behavior decreased gradually or in a step-fashion with increasing patch size, (2) the frequency of aggressive behavior increased continuously with competitor density or declined at high competitor densities, and 3) the relative abundance of the more aggressive sex decreased with increasing patch size.
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METHODS |
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Study site and experimental design
The house sparrows were observed on 35 days from October 16 to December 17th, 1998, in a residential backyard of Montréal, Canada (40°30' N, 73°40' W). Fences and cedar hedges enclosed the study site. Each feeding table (with a 1.5-cm plastic rim; see below) was elevated 25 cm from the ground and placed in the north-westerly corner of the site. The number of house sparrows visiting the site varied daily from 20 to 50 birds.
The experimenter (C.J.) manipulated patch size by using square feeding tables of seven different sizes (0.014, 0.029, 0.058, 0.116, 0.230, 0.462, and 0.922 m2), each table doubling in surface area from the smallest to the largest patch. Seed density was kept constant among the patch size treatments at 870 ml/m2 of white millet seed spread evenly over each patch surface. This seed density corresponds to about 3500 seeds/m2, below that required for maximum feeding rates in house sparrows (see Barnard, 1980). Seed was replenished when about one-third of the surface of the table was bare (determined visually) and no birds were present at a patch. This method minimized depletion such that small patches (e.g., 0.029 m2) were replenished when about 67% of the initial seed weight remained at the patch. Birds were presented with a single patch per trial. All seven patch sizes were presented to the birds in random sequence before repeating the procedure five times per patch size treatment for a total of 35 trials (seven patch sizes x five replicates).
Because time of day affects aggression among foraging house sparrows (Barnard, 1981), one trial was conducted per day, between 0700 and 1000 h. The ambient temperature was recorded, to the nearest 1°C, at the onset of each trial to test for possible effects of temperature on the number of birds and rate of aggression at a patch (Barnard, 1981; Caraco, 1979). A trial consisted of several short bouts of foraging interspersed with periods with no birds at the patch. On average, the number of birds at the patch changed every 3.0 ± 0.2 s (mean ± SE) during a foraging bout. The duration of the bouts was recorded by using a stopwatch. A trial was ended when the cumulative foraging time reached 10 min or when birds were absent from the patch for more than 25 min (mean ± SE foraging time per trial = 9 min 56.5 s ± 19.4 s).
The trials were filmed using a super-VHS video-recorder mounted on a tripod. The following data were transcribed from the video tapes: the number and duration of all aggressive encounters, defined as any aggressive act including threat displays such as wing flapping (see Elgar, 1987), as well as lunging and pecking (Lowther and Cink, 1992); the sex of the initiator and the recipient of aggression; and the times corresponding to changes in the number of birds at the patch. The number of females and males at a patch were counted during scan samples taken every 15 s for each trial. At 2-min intervals during a trial, the experimenter (C.J.) haphazardly chose one male and female bird from those foraging at a patch; each bird was observed until it left the patch. The number of seeds eaten and the changes in the number of competitors at a patch were recorded for each bird.
Data analysis
Because individual birds were not identifiable, we used a trial as a datum in our analyses to deal with the possibility that not all behavioral observations were independent (Lima, 1992). Because of the daily fluctuations in the number of birds visiting the site and the frequent changes in the number of birds at the patch during a trial, we believe that this method allowed us to meet the statistical assumption of independence.
We weighted the number of birds observed at a patch during a trial by the cumulative time spent foraging at that number (minimum = 3 s) before calculating the mean weighted number of birds per trial. Rate of aggression was quantified as the total rate of aggression (total interactions per minute). To control for the opportunity for aggression (i.e., number of individuals actually competing at a patch), we also calculated the per capita rate of aggression (interactions per minute per bird; see Robb and Grant, 1998) for each number of birds observed at a patch, weighted by time (see above). Data for solitary birds were omitted when calculating the weighted mean per capita rate of aggression per trial to avoid underestimating aggression.
Following the method of Lowther and Cink (1992), aggressive encounters were ranked according to intensity; escalated fights, defined as fights most costly in terms of energy expenditure and risk of injury (Archer, 1988), received a score of five (Table 1). Our measure of aggressive intensity, referring only to instances when aggression was observed, was the mean aggressive score per trial. Although our measure of aggressive intensity was a ranked variable, we assumed its distribution approximated a continuous variable (sensu Sokal and Rohlf, 1995) to facilitate analysis of covariance and multiple regression. All univariate analyses on the ranked data were nonparametric.
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To estimate sex ratio for each trial, we calculated the mean proportion of females at a patch from the total birds counted during scan samples taken every 15 s. To determine whether aggressiveness differed between the sexes, we calculated the weighted mean per capita rate and intensity of aggression initiated by females and males. We also calculated the weighted mean per capita rates of aggression received by each sex. Two-way ANOVAs were used to determine whether aggression differed between the sexes among the patch size treatments.
We defined foraging rate as the number of seeds eaten per unit time a focal bird was observed at a patch at a given group size (range = 1–25 birds). Three to five foraging females and males (n = 6–10 birds) were observed for an average of 18 s (range = 5–48 s) per trial. For each sex, we calculated the mean foraging rate for the different group sizes by pooling the foraging data per trial.
We log10 transformed patch area for all analyses because it was a geometric series. The aggressive intensity data required no transformation, but the proportion of females was arcsine-square-root-transformed, and the remaining dependent variables were log10 transformed to satisfy the assumptions for parametric tests. Values reported in the text have been back-transformed. We report second-order polynomial regressions only when the partial t value for the quadratic term was significant after first entering the linear term into the model. For all ANCOVA models, we only mention interactions with covariates when statistically significant (p <.05); nonsignificant interactions were removed from the models (Freud and Littell, 1981). Temperature (range = –10°C to 3°C) was omitted from the analyses because it had no detectable effect on the number of birds at a patch, the rate of total aggression, or per capita aggression (ANCOVA: p >.24) in our study. Also, no temporal trends in the number of birds, or total or per capita rate of aggression were detected over the duration of each trial (MANCOVA: p >.40).
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RESULTS |
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General results
House sparrows perched on the fence before feeding and normally arrived at the patch 5–10 min after it was filled with seeds. Birds on the fence sometimes called conspecifics (i.e., attracted, Elgar, 1986a), but rarely called when at the patch, particularly for small patch sizes. Birds returned to the fence when fleeing from aggression. Although more birds were observed fleeing from the smallest (e.g., mean ± SE number of birds fleeing from a patch per trial = 4.1 ± 0.96) versus the largest (0.40 ± 0.22) patch, birds did not attempt to defend the entire patch. Rather, birds attacked their closest neighbors. Moreover, aggression never involved more than two birds at a time.
The mean number of birds per patch increased with patch size, from about 2.5 on the smallest patch to 7.5 on the largest (linear regression: r2 =.74, F1,33 = 92.4, p <<.00001) (Figure 1). In contrast, the mean density of birds on a patch decreased with increasing patch size (quadratic regression: r2 =.95, F2,32 = 335.2, p <<.00001, partial t value for the quadratic term = 4.06, p =.0003) (Figure 1). The percentage of each trial that a bird foraged alone did not differ among the patch size treatments (27% on the smallest patch versus 16% on the largest; F6,28 = 2.28, p =.065). However, competitor numbers changed more frequently at large versus small patches (ANCOVA: F6,508 = 4.24, p =.0004), presumably because birds were more willing to join conspecifics feeding at large patches owing to low rates of aggression. More birds foraged on the ground near the patch when presented with small patches.
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Effect of patch size
House sparrows were less frequently aggressive with increasing patch size. The total rate of aggression remained relatively constant at four interactions per minute for the three smallest patches, but decreased to 0.7 interactions per minute at the largest patch (quadratic regression: r2 =.41, F2,32 = 11.33, p =.0002, partial t value for the quadratic term = 2.02, p =.035) (Figure 2a). The per capita rate of aggression did not show the step decline predicted by resource defense theory but instead decreased continuously with increasing patch size (quadratic regression: r2 =.75, F2,32 = 46.71, p <<.0001, partial t value for the quadratic term = 2.22, p =.034) (Figure 2b). This continuous decrease in aggression was apparent whether or not the per capita rate of aggression was log10 transformed.
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Overall, 69% (774/1115) of all observed aggression involved physical contact between birds, with 76% (586/774) of these interactions receiving a score of three (Table 1). In contrast to the frequency of aggression, there was no overall decrease in the intensity of aggression (Spearman rank correlation: rs = –.27, n = 35, p =.11) or escalated fighting (i.e., score = five) as patch size increased (Spearman rank correlation: rs = –.33, n = 35, p =.062). Escalated fights represented only 6% of all interactions.
Effect of bird number and density
To examine the effect of competitor number on aggression, we calculated the mean per capita rates and intensity of aggression (n = 5 replicates) for each observed group size of two or more birds for the seven patch size treatments. To avoid zero values (see Stewart-Oaten, 1996), we removed data from the analyses when the cumulative time that a given number of birds spent at each patch (n = 5 replicates combined) was less than 30 s. The few remaining zero values for per capita rates of aggression were averaged with the nonzero value for the next largest group size. For example, a per capita aggression rate of zero for three birds and two for four birds averaged to one interaction per minute per bird for 3.5 birds at a particular patch. We tested for trends in the rate and intensity of aggression versus competitor number among the patch size treatments by using an ANCOVA with competitor number as a covariate.
The effect of competitor number on the per capita rate of aggression differed among the patch size treatments (Fig. 3a), as indicated by a significant interaction between patch size and competitor number (F6,65 = 4.38, p =.001). Hence, we used regression models to determine the effect of competitor number on per capita rate of aggression for each patch size separately. Although there was no detectable effect of competitor number on the per capita rate of aggression at the smallest patch (p =.070), per capita rate of aggression increased significantly with competitor number for patch sizes 0.029 through 0.230 m2 (linear regressions: p values 
.016) (Figure 3a). In fact, per capita rate of aggression increased at an accelerating rate at the 0.029-m2 patch (partial t value for the quadratic term = 3.75, p =.033), but at a decelerating rate at the 0.230-m2 patch (t = –2.80, p =.015) (Figure 3a). Number of birds had no effect on aggression at the two largest patches (p values 
.3). In general, the strength of the relationship between per capita rate of aggression and competitor number decreased with increasing patch size (Figure 3a).
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To examine whether crowding was responsible for the higher per capita rate of aggression at the small patches, we used an ANCOVA with log10 bird density as the covariate. Per capita rate of aggression was still higher at small compared with large patches after controlling for changes in bird density (F6,71 = 6.97, p <<.0001). Contrary to the dome-shaped relationship predicted by resource defense theory, per capita rate of aggression increased with bird density with no sign of a decrease, even after controlling for differences in patch size (F1,71 = 49.93, p <<.00001) (Figure 3b).
House sparrows also fought more intensely as the number of foragers increased at a patch (ANCOVA: F6,71 = 2.95, p =.013) (Figure 4) and as the size of a patch decreased (F1,71 = 9.38, p =.003; linear polynomial contrast: ß ± SE = –0.921 ± 0.264, p =.001) (Figure 4). To test whether the patch size effect was really caused by bird density, we included bird density and patch size in a multiple regression. The analysis (data not shown) revealed that the increase in intensity of aggression was primarily caused by crowding at the smaller patches; there was no effect of patch size after controlling for differences in bird density (linear regression: F1,77 = 0.52, p =.594), whereas the intensity of aggression increased with competitor density after controlling for patch size (quadratic regression: F2,76 = 5.16, p =.008).
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Effect of sex
Both males and females became less aggressive as patch size increased (two-way ANOVA: F6,56 = 17.19, p <<.0001) (Figure 5a). Females were more aggressive than were males, judging by their higher rate of initiating attacks (F1,56 = 34.68, p <.00001) (Figure 5a). In contrast, the per capita rate of receiving aggression did not differ between females and males (F1,56 = 1.55, p =.22); both sexes received aggression less frequently as patch size increased (F6,56 = 21.09, p <<.00001) (Figure 5b). Fights were also more intense when initiated by females than by males (F1,56 = 4.52, p =.038); there was no overall effect of patch size (F6,56 = 1.44, p =.22) or a significant patch size by sex interaction (F6,56 = 0.84, p =.55) on the intensity of aggression.
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As predicted by a truncated phenotype distribution, the proportion of females decreased with increasing patch size (linear regression: r2 =.14, F1,33 = 5.39, p =.027) (Figure 6).
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Patch size had no significant effect on intake rates (ANCOVA: F6,55 = 1.19, p =.12) (Figure 7). However, intake rates increased with competitor density (F1,55 = 6.17, p =.016). Females fed faster than males, consuming on average 1.3 seeds/s compared with 1.1 seeds/s for males (F1,55 = 12.03, p =.001); there was no significant patch by sex interaction (ANCOVA: F6,55 = 0.39, p =.89).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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The gradual decline in frequency of aggression with increasing patch size did not support the predictions of resource defense theory. Moreover, the intensity of these aggressive interactions did not decrease significantly as patch size increased. These results were likely related to house sparrows being unwilling or unable to defend the entire patch, even when as small as 0.014 m2. Rather than attempting to chase all intruders from the patch, house sparrows only fought with their immediate neighbors.
There are at least two reasons why house sparrows may have preferred to share rather than defend small patches. First, a single house sparrow defending an elevated patch far from cover was likely vulnerable to predators. Hence, sharing a small but relatively rich patch may have been preferable to exclusive defense. Solitary house sparrows will often use a chirrup call to attract conspecifics to a patch to reduce the amount of time spent alone and the risk of predation (see Elgar 1986a). Although we cannot refute the predation-risk hypothesis, two lines of evidence suggest that it may not be a sufficient explanation for the lack of defense in our study. Despite fewer conspecifics, birds (1) did not spend less time foraging alone at small versus large patches, and (2) rarely called conspecifics at small patches. Our observations are more consistent with Elgar's (1986b) findings that house sparrows prefer to feed alone when the benefits of securing a larger proportion of food outweigh the antipredator benefits of feeding in a group. Second, house sparrows may be unable to defend against an intruder flying onto the patch, or may be unwilling because of the risk of injury. This asymmetry, such that intruders almost always win (also see Hansen, 1986), may make the exclusive defense of a patch against flying intruders unfeasible. Despite the lack of defense, birds were more reluctant to join conspecifics and fled from small patches more frequently, whereas they were more likely to aggregate at large patches. Interestingly, bird densities were low at large patches despite reduced aggression, presumably because too few birds visited our site to maintain the high bird densities observed at small patches.
In contrast to the dome-shaped relationship predicted by resource defense theory, aggression among the house sparrows increased with competitor number within a patch and with overall competitor density, with no sign of a decrease at high numbers or densities. Despite our efforts to control for an increased opportunity of aggression, the results may be influenced by higher aggression at high densities because the birds encountered conspecifics more frequently. Nevertheless, our data did not indicate that individual birds became less aggressive at high local densities. Rather, the results are more consistent with the prediction of Sirot's (2000) game theory model. In the shared patches, house sparrows searched for single food items and engaged in aggression only when encountering another individual. The hawk-dove model predicts that hawklike behavior will be favored in these pairwise contests as long as the value of the food item exceeds the costs of injury. However, even after controlling for the effects of competitor density, the frequency of aggression was inversely related to patch size, perhaps because the benefit of aggression was inversely related to patch size. Aggressive individuals may secure a larger proportion of the resources by displacing birds that may be more likely to leave smaller patches because of the space constraints. The fact that a departing bird has a greater effect on competitor density on smaller patches than on larger patches may explain why competitor number had a greater effect on the frequency of aggression in smaller patches.
Females initiated attacks more frequently and fought more intensely than did males. The female-biased sex ratio at small patches suggests that males sought foraging opportunities elsewhere, presumably on the ground rather than at small patches. Young oystercatchers spend more time than do older birds in poor-quality habitats where the risk of predation is high (Caldow et al., 1999; also see Goss-Custard and Durell, 1988), perhaps because they avoid areas where high competitor densities result in high aggression (Ens and Goss-Custard, 1984). Similarly, the resultant female-biased sex ratio appears to be consistent with that predicted by a truncated phenotype distribution (Parker and Sutherland, 1986; Sutherland and Parker, 1992). However, aggression may not be a good indicator of dominance when examining differences between the sexes in house sparrows. Hein et al. (2003) show that aggression by female house sparrows correlates with dominance only when directed toward males, and suggest this increased aggressiveness toward males helps females assess male quality and, later, mate choice. We cannot refute this possibility given that the cedar hedges surrounding the study site may have represented potential nest sites.
Despite allocating more time to aggression, females fed at higher rates than did males. Results from another study conducted at the same study site suggest that house sparrow increase the time taken to handle seeds to monitor aggressive conspecifics, resulting in low feeding rates (Johnson et al., 2001). Thus, females may have maintained high feeding rates by investing less time in conspecific vigilance than did males. However, why females and males in the present study might have different handling times remains unclear given that both sexes received similar rates of aggression. Interestingly, feeding rates did not appear to be adversely affected by increased aggression at small patches, perhaps because only individuals less susceptible to the interference caused by aggression foraged at small patches (see Gray, 1994). This explanation would also account for increased intake rates with competitor density.
In summary, the changes in aggressive behavior among the house sparrows in response to patch size and competitor density were more consistent with the modified hawk-dove model than with resource defense theory, likely because they shared the patch and fought over food items as assumed by the modified hawk-dove model. Even though patches were shared, patch size had important effects on the frequency of aggression, and the size and composition of foraging groups. Patch size will likely have an even greater effect on social foraging groups when patches are not divisible.
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ACKNOWLEDGEMENTS |
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We are indebted to Carol MacDonald for allowing us to run the experiments in her backyard. We thank Daphne Fairbairn, Don Kramer, Grant Brown, David Westneat, and two anonymous reviewers for commenting on earlier versions of this manuscript. This project was supported by an
quipe grant from Fonds pour la Formation de Chercheurs et l'Aide à la Recherche (FCAR) to L-AG, JWAG and Louis Lefebvre. The manuscript was written while C.A.J. held an FCAR scholarship. |
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
|---|
Archer J, 1988. The behavioural biology of aggression. Cambridge: Cambridge University Press.
Barnard CJ, 1980. Flock feeding and time budgets in the house sparrow (Passer domesticus L.). Anim Behav 28:295-309.[CrossRef]
Barnard CJ, 1981. Factors affecting flock size mean and variance in a winter population of house sparrows (Passer domesticus L.). Behaviour 74:114-127.
Brown JL, 1964. The evolution of diversity in avian territorial systems. Wilson Bull 76:160-169.
Caldow RWG, Goss-Custard JD, Stillman RA, Durell SEA le V dit, Swifen R, Bregnballe T, 1999. Individual variation in the competitive ability of interference-prone foragers: the relative importance of foraging efficiency and susceptibility to interference. J Anim Ecol 68:869-878.[CrossRef]
Caraco T, 1979. Time budgeting and group size: a test of theory. Ecology 60:618-627.[CrossRef]
Carpenter FL, MacMillen RE, 1976. Threshold model of feeding territoriality and test with Hawaiian honeycreeper. Science 194:639-642.
Chapman MR, Kramer DL, 1996. Guarded resources: the effect of intruder number on the tactics and success of defenders and intruders. Anim Behav 52:83-94.[CrossRef]
Craig JL, Douglas ME, 1986. Resource distribution, aggressive asymmetries and variable access to resources in the nectar feeding bellbird. Behav Ecol Sociobiol 18:231-240.[CrossRef]
Davies NB, Houston AI, 1984. Territory economics. In: Behavioural ecology: an evolutionary approach, 2nd ed (Krebs JR, Davies NB, eds). Sunderland, Massachusetts: Sinauer Associates; 148–169.
Dubois F, Giraldeau L-A, Grant JWA, 2003. Resource defense in a group foraging context. Behav Ecol 14:2-9.
Elgar MA, 1986a. The establishment of foraging flocks in house sparrows: risk of predation and daily temperature. Behav Ecol Sociobiol 19:433-438.[CrossRef]
Elgar MA, 1986b. House sparrows establish foraging flocks by giving chirrup calls if the resources are divisible. Anim Behav 34:169-174.[CrossRef]
Elgar MA, 1987. Food intake rate and resource availability: flocking decisions in house sparrows. Anim Behav 35:1168-1176.[CrossRef]
Ens BJ, Goss-Custard JD, 1984. Interference among oystercatchers, Haematopus ostralegus, feeding on mussels, Mytilus edulis, on the Exe estuary. J Anim Ecol 53:217-231.[CrossRef]
Freud RJ, Littell RC, 1981. SAS for linear models. Cary, North Carolina: SAS Institute; 187–205.
Giraldeau L-A, Caraco T, 2000. Social foraging theory. Princeton, New Jersey: Princeton University Press.
Goldberg JL, Grant JWA, Lefebvre L, 2001. Effects of the temporal predictability and spatial clumping of food on the intensity of competitive aggression in the Zenaida dove. Behav Ecol 12:490-495.
Goss-Custard JD, Durell SEA le V dit, 1988. The effect of dominance and feeding method on the intake rates of oystercatchers, Haematopus ostralegus, feeding on mussels. J Anim Ecol 57:827-844.[CrossRef]
Grant JWA, 1993. Whether or not to defend? the influence of resource distribution. Mar Behav Physiol 23:137-153.
Grant JWA, Gaboury CL, Levitt HL, 2000. Competitor-to-resource ratio, a general formulation of operational sex ratio, as a predictor of competitive aggression in Japanese medaka (Pisces: Oryziidae). Behav Ecol 11:670-675.
Grant JWA, Girard IL, Breau C, Weir LK, 2002. Influence of food abundance on competitive aggression in juvenile convict cichlids. Anim Behav 63:323-330.[CrossRef]
Gray RD, 1994. Sparrows, matching and the ideal free distribution: can biological and psychological approaches be synthesized? Anim Behav 48:411-423.[CrossRef]
Hansen AJ, 1986. Fighting behavior in bald eagles: a test of game theory. Ecology 67:787-797.[CrossRef][ISI]
Hein WK, Westneat DF, Poston JP, 2003. Sex of opponent influences response to a potential status signal in house sparrows. Anim Behav 65:1211–1221.
Hegner RE, Wingfield, JC, 1987. Social status and circulating levels of hormones in flocks of house sparrows, Passer domesticus. Ethology 76:1-14.
Hixon MA, 1980. Food production and competitor density as determinants of feeding territory size. Am Nat 115:510-530.[CrossRef]
Isbell LA, 1991. Contest and scramble competition: patterns of female aggression and ranging behavior among primates. Behav Ecol 2:143-155.
Johnson CA, Giraldeau L-A, Grant JWA, 2001. The effect of handling time on interference among house sparrows foraging at different seed densities. Behaviour 138:597-614.[CrossRef]
Keddy PA, 1989. Competition. London: Chapman and Hall.
Liker A, Barta Z, 2001. Male badge size predicts dominance against females in house sparrows. Condor 103:151-157.[CrossRef]
Lima SL, 1992. Vigilance and foraging substrate: anti-predatory considerations in a non-standard environment. Behav Ecol Sociobiol 30:283-289.[CrossRef]
Lowther PE, Cink CL, 1992. House sparrow. In: Birds of North America. Washington, DC: American Ornithologists' Union and Academy of Natural Sciences of Philadelphia; 7–9.
Møller AP, 1987. Variation in badge size in male house sparrows in male house sparrows Passer domesticus: evidence for status signaling. Anim Behav 35:1637-1644.[CrossRef]
Monaghan P, Metcalfe NB, 1985. Group foraging in wild brown hares: effects of resource distribution and social status. Anim Behav 33:993-999.[CrossRef]
Myers JP, Connors PG, Pitelka FA, 1981. Optimal territory size and the sanderling: compromises in a variable environment. In: Foraging behavior: ecological, ethological and psychological approaches (Kamil AC, Sargent TD, eds). New York: Garland Press; 135–158.
Parker GA, Sutherland WJ, 1986. Ideal free distributions when competitors differ in competitive ability: phenotype-limited ideal models. Anim Behav 34:1222-1242.[CrossRef]
Praw JC, Grant JWA, 1999. Optimal territory size in the convict cichlid. Behaviour 136:1347-1363.[CrossRef]
Robb SE, Grant JWA, 1998. Interactions between the spatial and temporal clumping of food affect the intensity of aggression in Japanese medaka. Anim Behav 56:29-34.[CrossRef][ISI][Medline]
Rubenstein DI, 1981. Population density, resource patterning, and territoriality in the everglades pygmy sunfish. Anim Behav 29:155-172.
Sirot E, 2000. An evolutionarily stable strategy for aggressiveness in feeding groups. Behav Ecol 11:351-356.
Sokal RR, Rohlf FJ, 1995. Biometry, 3rd ed. New York: Freeman.
Stewart-Oaten A, 1996. Sequential estimation of log (abundance). Biometrics 52:38-49.[CrossRef]
Summer-Smith JD, 1988. The sparrows: a study of the genus Passer. Calton: Poyser.
Sutherland WJ, Parker GA, 1992. The relationship between continuous-input and interference models of ideal free distributions with unequal competitors. Anim Behav 44:345-355.[CrossRef]
Wolf LL, 1978. Aggressive social organization in nectarivorous birds. Am Zool 18:765-778.
Zahavi A, 1971. The social behaviour of the white wagtail Motacilla alba alba wintering in Israel. Ibis 113:203-211.[ISI]
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