Glen E. Woolfenden

The Inheritance of Territory in Group-Breeding Birds

Recent advances in animal socioecology stress the importance of kinship and group living in the evolution of social behavior (see Wilson 1975). Although many animals exhibit a relatively simple social system in which individuals disperse and breed upon reaching maturity, among some higher vertebrate species certain individuals regularly delay breeding beyond normal physiological maturity and live for long periods in extended family groups. Group breeding, in which nonbreeding individuals help care for the young of a breeding pair, is now documented for a great variety of birds (Rowley 1976). Of special interest to evolutionary biologists is the question: Why do certain individuals seem to forfeit their own reproductive efforts to assist others with their breeding? Typical of the group breeders studied to date is close genetic relatedness between the helpers and the young they help raise (Brown 1974, Gaston 1976, Hardy 1976, MacRoberts and MacRoberts 1976, Maynard Smith and Ridpath 1972, Rowley 1965, Woolfenden 1975). Furthermore, the presence of helpers has been shown to increase the number of offspring produced by the resident breeders, who usually are parents of the helpers (Rowley 1965, Woolfenden 1975). Thus, by helping to raise kin, a nonbreeder may increase the representation of its own genes in the population without being a parent (Hamilton 1964, West Eberhardt 1975). This has led to the conclusion that kin selection underlies the evolution of helping behavior (Brown 1974, Wilson 1975). Our preliminary work with Florida scrub jays (Aphelocoma c. coerulescens), plus certain data available for other group-breeding birds, suggests the possibility of additional, more direct benefits to the helpers from delaying breeding and helping. Many group-breeding species studied to date occupy mature, often relict habitats, and show many characteristics of intense K-selection (sensu MacArthur and Wilson 1967; Brown 1974, 1975). Competition for space appears to be paramount among these characteristics. This observation, together with the data presented herein, leads us to hypothesize in this paper that, in certain species, remaining home and helping represents a strategy used by nonbreeders to inherit the space necessary for breeding.

A sentinel system in the Florida scrub jay

Abstract One postulated advantage of living in groups is increased protection from predators. Living in small, permanent groups of stable composition allows for the formation of a sentinel system, in which individuals coordinate their vigilance. Florida scrub jays, Aphelocoma coerulescens coerulescens , live in family groups, and coordinate their vigilance into a sentinel system. Observed vigilance behaviour was compared with models generating similar behaviour patterns at random. The occurrence of one jay terminating vigilance in the same minute that another initiated vigilance was greater than expected by chance, and the occurrence of more than one vigilant jay at one time was less frequent than expected by chance. The seasonal pattern of scrub jay sentinel behaviour suggests that it functions primarily for predator detection, especially of raptors. Most aerial predator alarm calls were given by sentinels, and a sentinel was most likely to give the alarm call first.

ACORN HARVESTING BY FLORIDA SCRUB JAYS

We documented numbers of acorns eaten, cached, and retrieved by Florida Scrub Jays (Aphelocoma coerulescens coerulescens), through one full annual cycle and part of another, in an oak scrub habitat characterized by abundant and reliable annual acorn crops. Jays consumed acorns during all months, with highest consumption from September through February and lowest in June and July. From August through December, most acorns consumed by jays were picked directly from shrubs; during the remaining 7 mo, acorns were recovered from ground stores. Acorn caching occurred throughout the fall, peaking in September and October. Data pooled across sex and age classes suggest that individual jays, on average, each cached between 6500 and 8000 acorns during fall of 1974. Only about one-third of those acorns were recovered later. Intact acorns recovered by Scrub Jays during fall usually were reburied, but by summer most recovered acorns were con- sumed. Acorn crops in the study area exhibit moderate annual variation but no crop failures. Acorns are available in substantial numbers every year, permitting Florida Scrub Jay territories to serve without fail as year-round warehouses of stored acorns. Those stores provide resources to carry group members through seasons of low arthropod availability, and perhaps facilitate delayed dispersal by juveniles. Use of a habitat characterized by relatively large and dependable annual acorn crops that are evenly dispersed, coupled with caching behavior, may contribute to the maintenance of permanent territoriality and co- operative breeding in this population.

Dominance in the Florida Scrub Jay

Studies of intraspecific dominance abound in the ornithological literature, but relatively few are based on the activities of free-living birds. The study of captive flocks, and even of wild groups attracted to feeding stations, may severely limit opportunities for discovering functions of dominance behavior in birds. Despite these limitations, several recent reports that in certain birds dominance is closely related to territoriality are convincing (Brown 1963, Dixon 1965, Wiley 1973, Smith 1976). More commonplace hypotheses relate dominance to differential allocation of limited food supplies (WynneEdwards 1962, Lack 1966: 284), though this relationship seems difficult to demonstrate in the field (despite Fretwell 1969). Florida Scrub Jays (Aphelocoma c. coerulescens) typically rear a single brood annually during a brief breeding season (Woolfenden 1973). Surviving offspring normally remain with their parents for at least one year during which they assist them in breeding (Woolfenden 1975). Eventually many of these helpers establish themselves as breeders either by replacing lost breeders of neighboring families or by founding new territories.

Multi-locus DNA fingerprinting supports genetic monogamy in Florida scrub-jays

Abstract Extensive behavioural and pedigree data on a colour-marked population of Florida scrub-jays (Aphelocoma coerulescens) suggested that this cooperatively breeding species is monogamous, with extremely rare exceptions in which males have two mates. We used multi-locus DNA fingerprinting to test these observations by determining genetic parentage. Despite restricted dispersal and high relatedness between behavioural parents and non-breeding members of the group, DNA fingerprints provided sufficient variability to determine parentage unambiguously in almost all cases. We found no evidence of extra-pair fertilisation of females or egg dumping, and confirmed a suspected case of polygyny in which a mother and daughter laid and incubated in the same nest. Our results confirm that detailed behavioural data allow accurate assignment of genetic parentage in this species. In Florida scrub-jays, large territory size may limit opportunities for cuckoldry, and persistent intense competition for limited breeding space may lead to low variance in the quality of established male breeders. These factors would reduce both the opportunity for, and benefits of engaging in extra-pair fertilisations. Delayed dispersal and cooperative breeding in this species have not evolved as avenues for direct reproduction by unpaired individuals.

Actuarial Senescence and Demographic Heterogeneity in the Florida Scrub Jay

Analysis of mortality from a 25-yr sample of 380 Florida Scrub Jays, Aphel- ocoma c. coerulescens, shows that actuarial senescence (increase in mortality with age) occurs. This refutes the notion that adult mortality is independent of age in birds, and has important implications for evolutionary dynamics. We point to two major factors that may act to mask underlying patterns of actuarial senescence: (1) Selection through time (de- mographic heterogeneity): because selection inevitably weeds out lower quality individuals first, overall mortality may appear to be constant or even decreasing, despite an increasing force of mortality acting on birds of higher quality. (2) Conflicting processes: one source of mortality may act to decrease mortality over time (e.g., group size effects), while another acts to increase it (e.g., degenerative senescence). Age-specific data, measures of intrinsic quality that are independent of mortality, and exposure of sources of heterogeneity and of conflicting processes allow us to demonstrate a clear pattern of actuarial senescence in an unmanipulated, natural population of long-lived birds. The slow rate of increase in mortality is consistent with the hypothesis that mortality increases more slowly with age in birds than in mammals.

Social learning of a novel foraging patch in families of free-living Florida scrub-jays.

Free-living juvenile Florida scrub-jays, Aphelocoma coerulescens, learned to forage in a novel patch (the centre of a ring) when in proximity to other family members that foraged successfully. We were able to distinguish the contributions of social learning and of individual learning, and to show that social learning occurred. The foraging task required individual jays to dig for peanut bits (chopped fragments) buried in sand in the centre of a 33-cm plastic ring. Jays were trained in their family groups to perform the task during a summer season, and were allowed to perform the task in the presence of juveniles (aged 40-85 days) in later years. Jays living in 18 control families received partial exposure to the training situation, but received no exposure to the ring before being presented with the task in the presence of their young. Juveniles in 16 families with trained jays were able to witness demonstrations and to scrounge peanut pieces from the models as they completed the task. These 41 juveniles learned more of the task than the 33 juveniles in control families. Seven juvenile jays and two older, nonbreeding jays in the trained families completed the task at least once, whereas no jays in control families completed the task. A modified task that prevented snatching also prevented transmission of the complete task, although the 22 juveniles that observed the modified task learned more of the task than the 33 control juveniles. Further analysis indicated that demonstrations had their greatest effect in increasing the probability that juveniles would enter the ring. Copyright 2000 The Association for the Study of Animal Behaviour.

Patterns of Dominance and Aggressive Behavior in Blue Jays at a Feeder

We studied interactions among Blue Jays (Cyanocitta cristata) visiting a feeder in south-central Florida over a 4-year period to examine the influence of sex, time of year, and body size on dominance and aggression, describe changes in dominance among individuals over time, and test for the presence of linear dominance hierarchies. Males dominated females throughout the annual cycle, and in all 24 of the male-female significant dyads. We cannot reject the hypothesis that male dominance over females results from the larger body size of males. We infer that males also were more aggressive than females because they were involved in more interactions than expected by chance. Females became more, and males became less, aggressive immediately prior to the breeding season, but fluctuations in aggression did not lead to shifts in intersexual dominance. Dominance relationships among a few high-ranking males were intransitive and changed over time. Dominance hierarchies, characterized by 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 suspect the variable constituency and instability of flocks precludes the emergence of strictly linear hierarchies in the genus Cyanocitta.

Factors affecting nutritional condition of fledgling florida Scrub-Jays : A ptilochronology approach

Although predation and starvation are considered the prime causes of post-fledging mortality, uncertainty exists about the importance and timing of these two factors. Reproductive success in Florida Scrub-Jays (Aphelocoma coerulescens) is positively correlated with amount of oak scrub in a territory. Using ptilochronology, we assessed whether scrub-jay fledglings in different territories differed in nutritional condition, because differential nutritional condition could be responsible directly (starvation) or indirectly (depredation) for territory-specific mortality. We measured three indices of feather growth in young jays: feather total length was significantly positively related to proportion of a territory covered by open oak scrub, whereas the relationship between amount of oak scrub and feather mass or daily feather growth rate approached significance. These results suggest that nutritional condition plays a role in survival of young Florida Scrub-Jays and that relatively subtle differences in territory composition may contribute to variation in overall reproductive success.

THE EVOLUTION OF CLUTCH SIZE. II. A TEST OF THE MURRAY-NOLAN EQUATION

We test the Murray‐Nolan equation for calculating clutch sizes of birds with demographic data from a long‐term study of the Florida Scrub Jay (Aphelocoma c. coerulescens). The predicted value, 3.43, is very close to the observed clutch size, 3.33. This result provides further support for the equations being an adequate description of the relationship among the factors affecting the clutch size of birds.