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Home , Aphelocoma, Cyanocorax, Cyanolyca, Dwarf jay, Florida scrub jay, Unicolored jay

The Auk 110(2):207-214, 1993

BIOLOGYOF COOPERATIVE-BREEDINGSCRUB JAYS (APHELOCOMACOERULESCENS) OF OAXACA,

D. BRENT BURT1'3 AND A. TOWNSEND PETERSON24 'Museum of Natural History, and Department of Systematics and Ecology, University of Kansas, Lawrence,Kansas 66045, USA; and 2Departmentof Zoology, Field Museum of Natural History, Roosevelt Road at Lake Shore Drive, Chicago, Illinois 60605, USA

ABSTRACT.-We studied a cooperative-breeding population of Scrub Jays (Aphelocomacoe- rulescens) in Oaxaca, Mexico. The jays occupied territories 1.5 ha in average size located in a wide range of and were found in temporally stable groups of two to six adult individuals. No yearling was observed breeding. The jays nested at least from early April to late July. Clutch size was three, with subsequent brood reduction to two fledglings typical. Only breeding females incubated . Breeding males fed nestlings more frequently than did breeding females. Ectoparasites were uncommon on adults, but nestlings commonly carried botfly larvae. Extra individuals commonly "helped" in predator defense, territorial defense, and feeding fledglings. Helpers attempted to assist in feeding nestlings, but usually were driven away by breeding adults. Singular breeding seems to be the rule, but plural breeding also may occur. Certain factors associated with the maintenance of cooperative breeding in the Scrub do not appear important in this Oaxacan population: usable habitats do not appear to be saturated with territories, and other jay do not present an aggressive barrier to Scrub Jay use. Received 3 October 1991, accepted 6 September 1992.

COMPARATIVE STUDIES of variation in social son 1989, 1991, Brown and Brown 1990, Emlen systems, ecology, and life-history traits have et al. 1991, Ligon and Stacey 1991, White et al. been very important in the development of cur- 1991). rent theory concerning the of social The jays, and the Aphelo- systems in (Stacey and Koenig 1984, 1990, coma in particular, have served as a test group Woolfenden and Fitzpatrick 1984, Brown 1987). for many ideas on avian social evolution (Brown Such comparative studies require that infor- 1974, 1987, Atwood 1980, Woolfenden and Fitz- mation on the social system, demography, and patrick 1984, Fitzpatrick and Woolfenden 1986, ecology be available for all representatives of Carmen 1988, Brown and Brown 1990, Wool- the groups under study, or at least that taxa in- fenden and Fitzpatrick 1990). Cooperative- cluded in the analyses be an unbiased sample breeding behavior is widespread but not uni- of the study group (Krebs and Davies 1987). versal in the genera , Calocitta, and Hence, documentation of the diversity of social , and absent or rare in the genera systems is important to advancing the under- and Gymnorhinus(Brown 1987). The standing of social evolution in birds. breeding behavior of the genus Cyanolycais lit- Most studies of cooperative-breeding birds tle known; however, flocking behavior suggests have documented ecological limitations that ap- that some form of cooperation takes place (Har- parently cause delayed dispersal and breeding dy 1964, Peterson pers. observ.). Within the ge- (Brown 1987, Smith 1990). Opinions differ, nus Aphelocoma,the (A. unicolor; however, on why these individuals display al- T. Webber pers. comm., Peterson pers. observ.), loparental or helping behaviors (Woolfenden the Gray-breasted Jay (A. ultramarina;Brown and and Fitzpatrick 1984, 1990, Brown 1987, Jamie- Brown 1981, 1990, Brown 1987), and the Florida populations of the Scrub Jay (A. coerulescens; Woolfenden and Fitzpatrick 1984, 1990) breed 3 Current address: Department of Ecology and Evo- cooperatively. lutionary Biology, University of , Tucson, Ar- of the izona 85721, USA. Comparisons cooperative-breeding 4Current address: Museum of Natural History, and Florida populations with the noncooperative Department of Systematics and Ecology, University western North American populations of Scrub of Kansas, Lawrence, Kansas 66045, USA. Jays have been particularly instructive. The 207

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t X 4 1 1 '~~~~~~~~~~~~~-4Sludy Localion

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Fig. 1. Map showing (A) western, (B) Florida, and (C) detailed southern Mexico distributions of Scrub Jays (Pitelka 1951, Peterson 1991).

Florida Scrub Jay lives in a rare, geographically to open, badly eroded grazed lands, cultivated fields, restricted, relict -scrub habitat that is lim- and bare rock. ited on all sides by either unusable habitat or We worked in the area on three occasions: (1) 7-13 habitat occupied by Blue Jays (Cyanocitta cris- May 1989, when Peterson made observations at sites tata; Woolfenden and Fitzpatrick 1984). West- throughout the Sierra'de los Mixes; (2) 30 December 1989-13 January 1990, when we banded jays and sur- ern Scrub Jays generally live in more contin- veyed habitat use and group sizes; and (3) 29 May- uous habitats, and probably can use a wider 16 June 1990, when we banded additional individu- range of habitats (Pitelka 1951, Peterson and als, mapped territories, and conducted intensive ob- Vargas 1992). This ecological difference, partic- servations at two nests. ularly the absence of marginal habitats in a sat- Working in pairs, we established survey points ev- urated environment in Florida, has been in- ery 260 m (400 paces) along each of five trails leading voked to explain the diversity of social systems out of camp, for a total of 34 points. At each point, in Scrub Jay populations (Atwood 1980, Wool- we played a 3-min recording of an intense territorial fenden and Fitzpatrick 1984, Fitzpatrick and interaction (A. c. californica vocalizations), recorded Woolfenden 1986). the number of individuals responding, and observed subsequent interactions to establish group member- Many Scrub Jay populations remain unstud- ships. Then, at each point, to measure aspects of hab- ied (e.g. , all of Mexico). This study itat structure, we established 50-m transects in each provides preliminary data on a cooperative- of the four cardinal directions. We counted the num- breeding population of Scrub Jays in southern ber and estimated the average heights of , oak, Mexico and discusses its implications for un- and other tree species of greater than 5 cm dbh within derstanding the evolution of social systems in I m of the transect. These figures were then averaged Aphelocomajays. across the four transects. The maximum elevation change along the four transects also was estimated. Both observers independently assessed the percent METHODS coverage by woodland (to nearest 5%), as well as the abundance of on the ground (0 = none, 1 = This study was conducted in the Sierra de los Mixes some, 2 = many), in the 100-m diameter circle; the (elevation 2,200 m), state of Oaxaca, Mexico, in a val- estimates of the two observers were averaged. To avoid ley 4 km NW of Santa Maria de Albarradas on the observer effects, the two observers (out of four total road to Santo Domingo de Albarradas (Fig. 1). The workers) and the trail to be surveyed were chosen at original habitat in this area probably included both random each day. Statistical analysis of these data was humid pine-oak and arid pine-oak forest (habitat ter- performed using the SAS statistical package (SAS In- minology follows Binford 1989). Now, through cen- stitute 1988). turies of human presence and greatly increased de- Jays were caught in mist nets and banded with forestation recently, the area contains a mosaic of unique four-band combinations of five colors of alu- habitats ranging from dense humid pine-oak forests minum bands. For each jay captured, we recorded: and more open arid pine-oak forests and oak scrub age (based on characters; Pitelka 1945), pres-

This content downloaded from 129.237.46.100 on Mon, 01 Jun 2015 14:07:13 UTC All use subject to JSTOR Terms and Conditions April 1993] CooperativeMexican Scrub Jays 209 ence of brood patch (during breeding), mass (to near- 8 est 1 g), wing chord and tail length (to nearest 0.5 E3 May1989 mm), tarsus length, exposed culmen length, and bill * Dec. 1989-Jan.1990 length, width, and depth (the latter three at anterior i. 6 I May-June1990 0 nostril edge; all five to nearest 0.1 mm). 0

The sex of individuals was determined by identi- UI. fication of a brood patch (breeding females only) for 04 captured birds and a female-specific vocalization (rattle or hiccup call; Woolfenden and Fitzpatrick 1984). The socially dominant male and female individuals were assumed to be the breeding individuals, and the re- 2 maining subordinate individuals of a group were as- sumed to be helpers. 2 3 4 5 6 Territorial boundaries were located by observing NUMBEROF JAYS territorial conflicts and individuals' home ranges. Fig. 2. Distributions of Scrub Jay group sizes en- Boundaries were plotted onto a map constructed from countered on three visits to study area. surveys of the study area. Average territory size was calculated by measuring the total area of usable land not just pairs with newly fledged young or wan- (i.e. excluding areas of bare rock) with a planimeter dering, nonterritorial floaters is indicated by and dividing by the number of territorial groups. detailed observations of group composition and We established blinds at two nests. The nest of temporal stability. We observed little fighting A-BYBm (m = male), B-AGAf (f = female), and two unbanded helpers was observed for 53 h (6-11 June), within groups and nothing indicating spatial and that of BA-BYm and YA-RAf (without helpers) separation within group territories. When re- for 24 h (11-13 June). Detailed notes on behavior and sponding to recordings of territorial vocaliza- the providing of food to nestlings were taken by sev- tions, all individuals usually arrived from and en observers during 2-h shifts from 0600 to 1800 CST departed in the same general direction. In nu- each day as weather conditions permitted. merous revisits to territories on different days or in different months, we found a high re- RESULTS peatability in group-size counts, indicating that group membership was stable. Habitat use. -Jays seemed to be most abun- Our observations of individually marked birds dant in cut-over, open oak woodland, using this (Fig. 3) support these results. Four of six groups habitat for most foraging and territorial inter- with banded individuals included three to four actions. They were rarely detected in tall, closed full-grown (based on plumage characters; Pi- forests. However, Mann-Whitney statistical tests telka 1945) individuals. All members of each of the individual habitat characters showed no group stayed within that group's territory and significant differences among habitats in which could be found at almost any hour of the day. jays were present or absent. Only abun- The only examples of changes in group mem- dance had a probability value (0.07) that ap- bership that we observed were in a group at proached significance. the northwestern end of the study area (not Using presence/absence of jays in each sam- shown in Fig. 3) in which a second-year indi- pling location as the dependent variable, we vidual banded in January 1990 disappeared by attempted to develop a predictive model for May 1990, and an unbanded individual took its habitat use by logistic regression, and forward- place. It is possible that the simply lost its backward stepping and best-subsets methods of four bands, although loss of aluminum bands selection for a multiple-linear-regression mod- is unusual in Scrub Jays (Burt unpubl. data, G. el. Residuals for each independent variable were E. Woolfenden pers. comm.). examined for any pattern, but none was de- Timingof breeding.-In May-June 1990 we en- tected. No statistically significant model could countered considerable variation in timing of be developed that predicted the presence of jays nest-cycle stages. In adjacent territories we si- at sites. These results suggest that the Oaxaca multaneously encountered a nest with a female Scrub Jays use the available habitats as a gen- just beginning incubation and a pair with in- eralist, particularly when compared with the dependent juveniles. For Oaxaca, Binford (1989) habitat use patterns of Jays. reported a temporal range from a nest with three Groups.-Group size ranged from two to six eggs on 7 April to a nest with two small young individuals (Fig. 2). That these "groups" were and one on 21 July. Assuming an incubation

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A-BYBm

RY-GAmYfdut B-AGAf

~~~~~~~10m adults

Wig >~~~a ul 2 adults17

group a2 UNB group?

I Pine-Oak Forest Road

El DryWash TerritoryBoundaries 50rn

P Unused Habitat --- Main Study Area Boundary NJ Fig. 3. Map of Scrub Jay territories and group composition 4 km northwest of Santa Maria de Albarradas, Oaxaca, Mexico. UNBD = unbanded; other abbreviations refer to color-band combinations and sex (m = male; f = female). period of 17 days and one egg laid per day, as as did two nests reported by Binford (1989). We in the (Woolfenden and Fitz- found no groups attending more than three patrick 1984), clutch initiation dates should young, either before or after fledging. range at least from 5 April to 2 July. Incubation is apparently by the breeding fe- Territoriesand nesting.-Each breeding terri- male only. We observed two groups during the tory contained both very open and relatively incubation stage, and at no point saw convinc- closed habitat types. Typical features that might ing evidence of more than one female incubat- be important for formation of territories in- ing, even though both groups contained helper cluded open areas for foraging, dense thickets, females. Females were fed on the nest frequent- a tall tree or two which might be important in ly by the breeding male, although the capture predator vigilance, and open arid pine-oak for- of a second-year or older helper in a net close est used for nesting. Average size of the 13 ter- to the nest of an incubating female suggests this ritories on the main portion of the study area helper also may have been carrying food to the (Fig. 3), -excluding habitat that appeared un- female. used, was 1.5 ha (density of 0.67 groups/ha). Two nests were observed during the nestling An important demographic parameter is the stage. At both, the male fed the young more age at which individuals first obtain breeding frequently than did the female (A-BYBm 58 territories. Although firm conclusions are not feedings [55.8%]vs. B-AGAf 45 feedings [43.3%]; possible without larger sample sizes and lon- BA-BYm 31 feedings [77.5%]vs. YA-RAf 9 feed- gitudinal data, we found no indication that these ings [22.5%]). Only one of two helpers at the jays breed when one year old, as all breeders first nest fed nestlings and then only once dur- handled were in full adult plumage. ing the total observation time. The second nest Eight nests were found at an average height was that of a pair only (no helpers). On average, of 2.8 m, all in oak trees averaging 5.1 m, gen- adults fed young 2.13 and 2.14 times/h at the erally within 1 m of the main trunk. Four nests two nests, respectively. The stomach of one dead were in dense tangles of mistletoe, and two were nestling contained a section of a torso, a adjoining bromeliads. 30-mm beetle larva, and numerous small Apparently, the normal clutch size is three. parts. The two nests found with eggs each had three, At least five groups were observed with fledg-

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lings of various ages, but never with more than dividual normally participated in defending the two fledglings. As in other Scrub Jay popula- territory at least as actively as the breeding fe- tions, very young fledglings were quite sed- male. entary, moving only to approach adults bring- (2) Predator defense: Although we have no ing food. Later, they were louder and more direct observations of extra individuals giving mobile, following the adults around the terri- alarm calls to warn the group of the approach tory begging for food. Although detailed ob- of a natural predator, alarm calls by all individ- servations of groups at this stage were not uals in the group could be elicited by our pres- possible, all members of at least one group (RR- ence in the general vicinity of the nest. In de- BBm, GA-BYf and one unbanded helper) fed fending nest and young against potential the fledglings. The stomachs of two full-grown predators such as ourselves, all members of the juveniles found dead contained mostly fruit and groups participated actively in mobbing. , and only a few insect parts. (3) Feeding nestlings: At the nest of A-BYBm Ectoparasites.-All jays handled were inspect- and B-AGAf, one or both extra individuals came ed for ectoparasites. In visual inspections of 30 to the nest tree an average of 0.189 times per adults (10 banded and 20 collected elsewhere hour (total of 10 times). On at least three (30%) in the same mountain range for genetic studies) of these occasions, the extra individual was car- and two full-grown juveniles, no ectoparasites rying food in its or throat, but on only one were encountered (as is more or less normal in occasion did an extra individual actually feed jays; Peterson unpubl. data), but mallophaga the nestlings. Although the extra individuals were collected by ethyl-acetate fumigation from took an active interest in the nest and nestlings, 2 of the 20 collected adults. However, all five on 5 out of 10 (50%) of their visits to the nest nestlings inspected carried at least four large tree, they were chased from the immediate vi- botfly larvae (mean = 6.8 larvae/nestling). These cinity of the nest by one or both of the breeding were encountered mainly on the abdomen, adults. Hence, while the potential helpers at- throat, and wings, and at times on the face, tempted to provide food, their help usually was where they appeared to impair vision. One dead not accepted. 73.5-g nestling had eight larvae with a com- (4) Feeding fledglings: Our general impres- bined weight of 1.07 g, or 1.46% of the bird's sion from incidental observations of five dif- body mass. That young jays can survive this ferent groups with both extra individuals and parasite is demonstrated by the presence of fledglings was that the fledglings begged food healed botfly sores on one of the two full-grown from whichever adult was closest, and that all juveniles inspected. adults actively provided food for the fledglings. Broodmortality. -Although the normal clutch On one occasion we witnessed a helper feeding size is three, observations suggest that the jays a fledgling. usually are not able to rear that many young. Therefore, given that the extra individuals The largest number of fledglings observed in present in this population actually help at least any group was two, and of full-grown juveniles in three of four phases of the breeding effort, in any group only one (five groups observed). we feel justified in calling them "helpers." The stage at which this apparent mortality oc-

curs and its causes are probably varied: in one DISCUSSION case, an egg disappeared overnight from an ac- tively incubated nest; in another, a large nest- Groupliving. -It is well established that Scrub ling of about 12 days age fell from its nest and Jay populations in the western United States died soon after. breed exclusively in pairs (Atwood 1980, Car- Helping behavior.-Given that "extra" individ- men 1988, Burt unpubl. data). The behavior of uals were present in 10 of 17 (59%) breeding Mexican populations, however, has long been units on the study area, the obvious question is a mystery (Hardy 1961, Woolfenden and Fitz- whether these individuals performed parent- patrick 1984). We have visited all major sections like behavior to young that were not their own of the Scrub Jay's range in Mexico, and have (Brown 1987). We examined this question at found them in groups only in Oaxaca and pos- four stages in the breeding effort. sibly in central Guerrero and eastern Puebla. It (1) Territorial defense: In response to both appears group living by Scrub Jays is limited territorial challenges from other jays and to our to Florida and the southern extreme of the range. recordings and vocal imitations, each extra in- Group sizes in the two populations are similar

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(compare Fig. 2 with tables 5.1 and 6.2 in Wool- helpers (RY-GYm, UNBDf) appeared during fenden and Fitzpatrick 1984) and, although be- territorial battles, but were absent or not de- haviors of both the Oaxaca and Florida popu- tected when all was quiet during four days of lations can be called helping, some intriguing regular observation. Possibly, given their in- differences exist. The Oaxaca Scrub Jays help in teractions (frequent rattle calls by the UNBDf territory defense, antipredator behavior and in response to RY-GYm), these two individuals feeding fledglings, but usually are chased away had an active nest elsewhere in the territory. when attempting to feed nestlings. Helpers are Second, in the RR-BBm/GA-BYf territory GA- not allowed access to the nest in at least one BYf and an UNDB helper spent most of their other cooperative breeding species, the Harris' time caring for a full-grown juvenile. RR-BBm, Hawk (Parabuteounicinctus). Dawson and Man- however, was strangely absent in three of five nan (1991) proposed social manipulation of sub- of our visits to the territory and, even when ordinates by breeders as one possible explana- present, did not join the main group. Again, he tion for limited nest access to helpers in this may have had another active nest with a dif- species. Helpers were active in territorial de- ferent female. These observations are at best fense, predator harassment and transporting suggestive of the occurrence of plural breeding captured prey to the nest, but typically were in the population. prevented from entering the nest. Occasional Habitat limitation.-Woolfenden and Fitzpat- access to the nest was allowed to encourage these rick (1984) have argued convincingly that se- beneficial behaviors, but frequent access might vere limitations in the availability of breeding induce full-breeding condition in helpers, caus- and marginal habitats are responsible for de- ing an increase in intragroup conflict. However, layed dispersal by juvenile individuals in Flor- data on hormonal regulation of breeding status ida Scrub Jay populations. The presence of Blue in the Florida Scrub Jay suggests this breeding- Jays may limit the Scrub Jays' use of taller wood- condition-induction explanation does not ap- lands and forests. Other surrounding habitats- ply to the Oaxaca population for the limiting open palmetto scrub, grasslands, and citrus of nest access to helpers (Schoech et al. 1991). groves-appear to be unsuitable for either Another possible explanation is that Oaxaca breeding or floating. Hence, Florida Scrub Jays helpers were driven away by the breeding pair appear to be hemmed in on all sides. because of disturbance-related anxiety caused We found no indication of strict habitat lim- by our observation blind. itation. We present three lines of evidence: Even in the Florida population, helpers are (1) The Oaxaca Scrub Jays use a wide range not included in all stages of the breeding cycle. of habitats. Although most frequently found in During nest building, incubation, and the first open oak woodlands, we also found them in few days after hatching, helpers are driven from closed humid and arid pine-oak forests, oak the nest by the breeders. Helpers are allowed scrub, cultivated fields, grazed lands, and pop- to participate in all subsequent breeding activ- ulated areas. Tall pine forest (height ca. 25 m) ities, including the provisioning of nestlings was used regularly during territorial interac- (Stallcup and Woolfenden 1978, Woolfenden tions and predator vigilance, and treeless, grazed and Fitzpatrick 1984). The proximate and evo- lands were used for foraging to a degree that lutionary reasons for the rarity of alloparental neither of us has seen in other Scrub Jay pop- feedings in the Oaxaca population require more ulations. study. (2) The habitats used by Scrub Jays in the Plural breeding.-The wide range of clutch ini- Sierra de los Mixes are arranged in a complex, tiation dates (April-July) suggests that Oaxaca ever-changing mosaic. Although the more re- Scrub Jays are able to renest frequently and, mote areas and steeper slopes are generally cov- thereby, replace lost young. However, two in- ered with closed pine-oak forest, the remainder complete observations suggest that more than of the region is a patchwork of habitats, each just renesting might be occurring in the pop- of which the jays use to varying degrees. The ulation. important point is that the activities of the local First, in the UNBDm/GA-GAf territory people do not destroy the jays' habitat, but rath- (UNBD = unbanded individual), the breeding er, open up the closed forest to make a contin- pair had a nest with three eggs that GA-GAf uum of habitats from completely unusable to was incubating almost continuously. Their two very suitable. Increased pressure on the land in

This content downloaded from 129.237.46.100 on Mon, 01 Jun 2015 14:07:13 UTC All use subject to JSTOR Terms and Conditions April 1993] CooperativeMexican ScrubJays 213 the last few decades for more agricultural, for- lifetime fitness by delaying dispersal and serv- est, and domestic- production has led to ing as helpers until their natal territory or an- vastly increased amounts of usable habitat in other of high quality is available. Another pos- the region, so spatial limitations that might have sibility is that cooperative breeding in this been present probably are no longer in place. population is not adaptive under current con- (3) Other jay species are not common in ad- ditions, instead reflecting selection in ancestral jacent habitats. Five jay species are found in the populations under very different ecological sit- Sierra de los Mixes (Binford 1989), but none is uations (Peterson and Burt 1992). More detailed common in habitats adjacent to those used by field studies of the southern Mexican Scrub Jay Scrub Jays. The White-throated -Jay populations are critical to resolve these possi- (Calocitta formosa) is present in the tropical de- bilities. ciduous forest and arid tropical scrub at the base

of the mountain range, but never ascends into ACKNOWLEDGMENTS the arid subtropical scrub (Binford 1989) that borders Scrub Jay habitat. The Unicolored and This study would not have been possible without Azure-hooded jays (Aphelocomaunicolor and Cy- hundreds of hours of help from our Mexican col- leagues: Ariel Ayala 0., Hesiquio Benitez D., Daniel anolyca cucullata, respectively) are restricted to Juarez A., Alonso Natividad S., Raphael Reyna H., cloud forest, while the ( Javier Salgado 0., Noe Vargas B., and Fernando Vil- nana) occurs in both cloud forest and humid lasenor. Scott Baker, Scott Burt, and Michael Stokes pine-oak forest. None of these three species was also provided many days of invaluable field assis- found even in the darkest barrancas high in the tance. The people of the pueblos of Santa Maria de hills above the study area. Steller's Jays (Cya- Albarradas, Santo Domingo de Albarradas, and San nocitta stelleri) were found near the study area, Miguel de Albarradas kindly allowed us to work on but only regularly in the darkest, most closed their land. Norman Slade provided help with statis- barrancas. During our stays in the area, we saw tical analyses. Members of the Nathaniel S. Goss So- them only three times, and they could not be ciety (University of Kansas) made suggestions based on early drafts of this manuscript. A.T.P. expresses a lured out into more open forest. Thus, other very special thank you to Frank Pitelka for his en- do not the barrier to jays present aggressive couragement and advice throughout this study, and Scrub Jay habitat use that they do in , to Amy Peterson for patience and understanding dur- Florida, or northern Mexico (Peterson and Var- ing his absences. He was supported in part by a grant gas 1992, Vargas in prep.). from the National Geographic Society. D.B.B. was For these reasons, we believe that ecological supported by grants from the Department of System- limitations, in the sense of the unavailability of atics and Ecology and the Panorama Society of the habitats for dispersal as modeled in habitat-sat- Museum of Natural History, University of Kansas. uration and marginal-habitat models, are not important factors maintaining cooperative LITERATURECITED breeding in the Oaxaca Scrub Jay populations. Of course, other ecological factors not consid- ATWOOD,J. L. 1980. Social interactions in the Santa Cruz . Condor 82:440-448. ered here (e.g. availability of food or nest sites) BINFORD,L. C. 1989. A distributional survey of the might exist that limit the jays' ability to disperse birds of the Mexican state of Oaxaca. Ornithol. successfully and breed as yearlings. We simply Monogr. No. 43. argue that the factors identified as critical in BROWN,J. L. 1974. Alternate routes to sociality in limiting habitat use of the cooperative-breeding jays-With a theory for the evolution of altruism Florida Scrub Jays do not appear important in and communal breeding. Am. Zool. 14:63-80. this case. BROWN,J. L. 1987. Ecology and evolution of helping More detailed studies of variation in habitat and communal breeding in birds. Princeton Univ. quality and fitness payoffs in the Oaxaca Scrub Press, Princeton, New Jersey. Jay may support alternative explanations for the BROWN,J. L., AND E. S. BROWN. 1981. Extended family system in a communal bird. Science existence of cooperative breeding in this pop- 211:959-960. BROWN, J. L., AND E. R. BROWN. 1990. The Mexican ulation. One possibility is that high variance in Jay: Uncooperative breeding. Pages 267-288 in habitat quality among territories may make Cooperative breeding in birds (P. B. Stacey and nondispersal the best choice for yearlings on W. D. Koenig, Eds.). Cambridge Univ. Press, New high-quality territories (Stacey and Ligon 1987). York. Yearlings on "good" territories may optimize CARMEN, W. J. 1988. Behavioral ecology of the Cal-

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