The Condor 99~4346444 0 The Cooper Ornithological Society 1997 PATTERNS OF DOMINANCE AND AGGRESSIVE BEHAVIOR IN BLUE JAYS AT A FEEDER ’ KEITH A. TARVIN~ AND GLEN E. WOOLFENDEN Department of Biology, University of South Florida, Tampa, FL 33620 and Archbold Biological Station, P.O. Box 2057, Lake Placid, FL 33862 Abstract. We studiedinteractions among Blue Jays(Cyanocitta cristuta) visitinga feeder in south-centralFlorida over a 4-year period to examine the influence of sex, time of year, and body size on dominance and aggression,describe changesin dominance among indi- viduals over time, and test for the presence of linear dominance hierarchies. Males domi- nated females throughout the annual cycle, and in all 24 of the male-female significant dyads. We cannot reject the hypothesisthat male dominance over females results from the larger body size of males. We infer that males also were more aggressive than females becausethey were involved in more interactionsthan expected by chance. Females became more, and males became less, aggressive immediately prior to the breeding season, but fluctuationsin aggressiondid not lead to shifts in intersexual dominance. Dominance rela- tionships among a few high-ranking males were intransitive and changed over time. Dom- inance hierarchies, characterizedby reversals, circular triads, and unknown relationships, were not linear. Whereas linear hierarchies have been shown to exist in New World jays that live in small, stable social groups, we suspectthe variable constituencyand instability of Rocksprecludes the emergence of strictly linear hierarchiesin the genus Cyanocittu. Key words: aggression, Blue Jay, Cyanocittacristata, dominance hierarchies, New World jays, seasonal@, sex-biased dominance. INTRODUCTION or engage in social interactions (Hardy 1961). Although the Blue Jay (Cyanocittu cristatu) is a However, the flocks vary in membership and are common and familiar eastern North American spatially and temporally ephemeral (Cohen species,remarkably little is known about its ba- 1977). These social patterns hold for the non- sic ecology and social behavior, including dom- migratory population in south-central Florida, inance relations. Only Racine and Thompson which we have studied since 1990. Our present (1983) have reported on dominance in Blue study of dominance and aggression at a feeder Jays, and although they found that rank order of is relevant because members of different pairs jays visiting a feeder was constant over a period often have the opportunity to interact and do so of a few weeks, they did not provide information as they encounter each other on more or less on other features of dominance relationships in “neutral” ground when foraging or harvesting their study population. Here we report on Blue acorns. Because Blue Jays do not defend or Jay dominance and aggression as it relates to maintain exclusive territories, individuals en- sex, time of year, and body size, describe counter many other individuals from within and changes in dominance relationships among in- beyond the local neighborhood, and perhaps en- dividuals over time, and test for the presence of gage with them in competition for food, nest linear dominance hierarchies. Analysis of the in- sites, etc. Thus, although attracting jays to an fluence of site on dominance was outside the artificial food sourceto observe interactions may scope of our study. quantitatively enhance levels of aggression or The basic social unit of Blue Jays is the frequency of interactions, it probably does not breeding pair. These pairs often share spacewith change the quality of the interactions because it other pairs and individuals, and these pairs and mimics the social and spatial mileu within which individuals often coalesce into flocks to forage Blue Jays normally exist. For the congeneric Steller’s Jay Cyanocittu stelleri, Brown (1963) reported dominance as ’ ’ Received 3 July 1996. Accepted 12 December site-dependent, rather than dependent on fixed 1996. ZCorresponding author: Keith A. Tarvin, Archbold social relationships (sex excluded). In contrast, Biological Station, PO. Box 2057, Lake Placid, FL many other New World jays live in small stable 33862, e-mail: [email protected] groups apparently organized around families. [4341 DOMINANCE IN BLUE JAYS 435 Linear dominance hierarchies have been report- cealed position within a nearby house. By man- ed in three group-living jays for which domi- ually operating the trap, which was otherwise nance is related to sex, age, and breeding status locked open when baited, we were able to catch of group members (Florida Scrub-Jay Aphelo- focal jays, yet allow other jays to enter and leave coma coerulescens,Woolfenden and Fitzpatrick the trap. Because we rarely attempted to capture 1977, Mexican Jay A. ultramarina, Barkan et al. jays (especially marked ones), jays rarely re- 1986, Pinyon Jay Gymnorhinus cyanocephalus, sponded to the trap with apprehension. Further- Marzluff and Balda 1992). However, an analysis more, those jays that were captured usually fed of the degree of linearity of hierarchies has yet again from the feeder within a day or so of their to be performed on Cyanocitta using recently capture. proposed methods (Appleby 1983, deVries 1995). A color-marking program where numer- MORPHOLOGY AND SEX DETERMINATION ous individual Blue Jays were sexed and mea- For most captured jays, we measured bill depth sured provided the opportunity to examine cor- and width at the anterior margin of the nares, relates of dominance and aggression in a popu- bill length from nares to tip, culmen, head length lation of resident Blue Jays in south-centralFlor- from occiput to tip of upper mandible, tarsus, ida, and to compare patterns of dominance in primary 7, central rectrix, and mass. Feather this species with those found in other New measurements were estimated to the nearest World jays. mm; remaining linear measurementswere to the nearest 0.1 mm. Mass was estimated to the near- METHODS est 0.1 g. Multiple measurements of the same STUDY SITE individual taken after the jay had reachedits sec- ond calendar year were averaged. We studied dominance interactions among indi- We determined sex of many Blue Jays by vidually color-marked Blue Jays at a feeder at presence of a brood patch (females only), lapa- Archbold Biological Station, Highlands County, roscopy, or, in one case, by behavior at nests. Florida (27”lO’ N, 81”21’ W), from June 1991 Other jays were sexedusing a logistic regression through December 1994. The feeder was located analysis. The logistic regression procedure cor- in human-modified parkland with scatteredlarge rectly classified 88% of 124 Blue Jays of known slashpines (Pinus elliottii), surroundedby native sex, and calculated the probability that each bird scrubby flatwoods and sand-pine scrub habitats of unknown sex was a female based on length (Abrahamson et al. 1984). We determined that of the head and central rectrix. The procedure adult Blue Jays at Archbold are year-round res- made no classification errors within the 15% idents, and that birds in their first year tend to tails of the probability distribution when it was wander during autumn (unpubl. data). Virtually applied to the sample of Blue Jays of known sex. all the jays we observed at the feeder were band- When the probability of being female was 2 ed, and unbanded immigrants that became reg- 0.85 or I 0.15, we classified individuals of un- ular visitors were captured and marked within a known sex as female or male, respectively. In- few days of their arrival. Although many jays dividuals with intermediate probability scores marked at the feeder were not seen again, a core were categorized as unknown sex. Twelve males set of marked jays was seen in most months dur- and one female were sexed using this procedure. ing each seasonof each year, and many of these We were unable to assign sex to 13 jays. core individuals were present for more than one year. Most observations were of these frequent ASSESSMENTOF DOMINANCE visitors, and our use of significant dyads (see From within the house we observed Blue Jays below) ensured that we did not unduly weight as they interacted at the feeding platform placed interactions of drifters and stragglers.The num- about 15 m in front of a window. A Potter trap ber of jays visiting the feeder during an obser- centered on the platform and locked open con- vation sessionranged from 1 to 5 in autumn to tained about 30 g of commercial bird seed or a 10 to 20 in spring and summer. slice of bread that could be obtained by only a We capturedjays in a 4-cell Potter trap placed few jays simultaneously. Blue Jays quickly on the feeding platform by using a string to drop learned to associatethe presence of food with a the door from a remote and essentially con- characteristicwhistle that we emitted as we bait- 436 KEITH A. TARVIN AND GLEN E. WOOLFENDEN ed the feeder, and food (and concomitant obser- verts and presence or absence of bars on the vations of interactions) usually lasted no longer greater upper secondary coverts following Dater than 15 min. Limited accessto a small amount [ 19701 and Bancroft and Woolfenden of food and the whistle alerting jays to its pres- [1982]).We considered one bird dominant over ence combined to concentratejay activity within another only when the win:loss ratio within the a short time span, and thus promoted agonistic dyad (the summary of interactions between interactions used to assessdominance. members of a pair) differed significantly from Observation periods usually were conducted random as determined with a two-tailed bino- l-3 times per day, typically in the early morning mial test. A binomial probability I 0.05 reflect- and around noon. Observation periods were ini- ed a significant dominance relationship (hence- tiated on most days during the study period, ex- forth “significant dyads;” see Barkan et al.