Mass fluctuation in breeding females, males, and helpers of the Florida scrub-jay Aphelocoma coerulescens

Much evidence suggests that birds actively regulate their body mass reserves relative to their energy needs. Energy requirements during reproduction may differ in relation to sex-specific behavioural roles or, in the case of cooperative breeders, breeders relative to helpers. We measured body mass of free-living Florida scrub-jays throughout the nesting season by training them to land on an electronic balance. Jays exhibited a pattern of diurnal linear mass gain, from morning to afternoon. Day-to-day mass fluctuations, defined as the difference between mass on two consecutive days, were small (>80% were within 2 g, less than 3% of the mass of an adult bird) for all classes of jays: female breeders, male breeders and prebreeding helpers. The jays, which live in subtropical south-central Florida, did not exhibit changes in day-to-day mass fluctuation relative to weather or climate variables or calendar date. Day-to-day mass fluctuations influenced mass fluctuation between the following third and fourth days. These changes were usually compensatory, indicating that jays are able to regulate their body mass on a short-term basis, despite strong differences in their roles in reproduction. During reproduction, jays have a relatively predictable and abundant food supply, thus the appropriate strategy may be to maintain a stable body mass that balances some energy reserves against maintaining a low body mass for efficient flight, as required during reproduction.

Sex differences in helping effort reveal the effect of future reproduction on cooperative behaviour in birds

The evolution of helping behaviour in species that breed cooperatively in family groups is typically attributed to kin selection alone. However, in many species, helpers go on to inherit breeding positions in their natal groups, but the extent to which this contributes to selection for helping is unclear as the future reproductive success of helpers is often unknown. To quantify the role of future reproduction in the evolution of helping, we compared the helping effort of female and male retained offspring across cooperative birds. The kin selected benefits of helping are equivalent between female and male helpers-they are equally related to the younger siblings they help raise-but the future reproductive benefits of helping differ because of sex differences in the likelihood of breeding in the natal group. We found that the sex which is more likely to breed in its natal group invests more in helping, suggesting that in addition to kin selection, helping in family groups is shaped by future reproduction.

EVOLUTION OF AVIAN COOPERATIVE BREEDING: COMPARATIVE TESTS OF THE NEST PREDATION HYPOTHESIS

Comparative analyses carried out on two different phylogenies of cooperatively and noncooperatively breeding Australian passerine birds (parvorder Corvida) were unable to detect a significant difference in nest predation rates after controlling for body mass and risk of predation due to location of the nest (nest safety). Nest predation rates, however, decrease as nest safety and body mass increase. We suggest that cooperative breeding does not bring about a current net change in rates of nest predation among Australian passerines. Species breeding cooperatively may have developed antipredator strategies that produce results similar to those adopted by noncooperatively breeding species. The function of cooperative breeding may lie outside of antipredator strategies.

Cooperative breeding shapes post-fledging survival in an Afrotropical forest bird

For avian group living to be evolutionary stable, multiple fitness benefits are expected. Yet, the difficulty of tracking fledglings, and thus estimating their survival rates, limits our knowledge on how such benefits may manifest postfledging. We radio-tagged breeding females of the Afrotropical cooperatively breeding Placid greenbul (Phyllastrephus placidus) during nesting. Tracking these females after fledging permitted us to locate juvenile birds, their parents, and any helpers present and to build individual fledgling resighting datasets without incurring mortality costs or causing premature fledging due to handling or transmitter effects. A Bayesian framework was used to infer age-specific mortality rates in relation to group size, fledging date, maternal condition, and nestling condition. Postfledging survival was positively related to group size, with fledglings raised in groups with four helpers showing nearly 30% higher survival until independence compared with pair-only offspring, independent of fledging date, maternal condition or nestling condition. Our results demonstrate the importance of studying the early dependency period just after fledging when assessing presumed benefits of cooperative breeding. While studying small, mobile organisms after they leave the nest remains highly challenging, we argue that the telemetric approach proposed here may be a broadly applicable method to obtain unbiased estimates of postfledging survival.

Great spotted cuckoo fledglings often receive feedings from other magpie adults than their foster parents: which magpies accept to feed foreign cuckoo fledglings?

Natural selection penalizes individuals that provide costly parental care to non-relatives. However, feedings to brood-parasitic fledglings by individuals other than their foster parents, although anecdotic, have been commonly observed, also in the great spotted cuckoo (Clamator glandarius) – magpie (Pica pica) system, but this behaviour has never been studied in depth. In a first experiment, we here show that great spotted cuckoo fledglings that were translocated to a distant territory managed to survive. This implies that obtaining food from foreign magpies is a frequent and efficient strategy used by great spotted cuckoo fledglings. A second experiment, in which we presented a stuffed-cuckoo fledgling in magpie territories, showed that adult magpies caring for magpie fledglings responded aggressively in most of the trials and never tried to feed the stuffed cuckoo, whereas magpies that were caring for cuckoo fledglings reacted rarely with aggressive behavior and were sometimes disposed to feed the stuffed cuckoo. In a third experiment we observed feedings to post-fledgling cuckoos by marked adult magpies belonging to four different possibilities with respect to breeding status (i.e. composition of the brood: only cuckoos, only magpies, mixed, or failed breeding attempt). All non-parental feeding events to cuckoos were provided by magpies that were caring only for cuckoo fledglings. These results strongly support the conclusion that cuckoo fledglings that abandon their foster parents get fed by other adult magpies that are currently caring for other cuckoo fledglings. These findings are crucial to understand the co-evolutionary arms race between brood parasites and their hosts because they show that the presence of the host's own nestlings for comparison is likely a key clue to favour the evolution of fledgling discrimination and provide new insights on several relevant points such as learning mechanisms and multiparasitism.

Corticosterone, brood size, and hatch order in free-living Florida scrub-jay (Aphelocoma coerulescens) nestlings

It is well known that variation in developmental conditions can have profound effects upon lifetime fitness. In altricial avian species, nestlings undergo a substantial portion of development in the nest after hatching, often in the presence of nest-mates. This can result in the formation of brood hierarchies based on age, size, and competitive ability. Measurement of baseline corticosterone (CORT) levels in developing birds may provide a means to assess whether individuals within a brood experience stress due to sibling competition or nutritional state. However, few studies have attempted to correlate corticosterone concentrations with brood hierarchies in free-living, developing birds. We investigated the degree to which corticosterone levels in nestling Florida scrub-jays (Aphelocoma coerulescens) were predicted by brood size, hatching order, and body mass. Nestling corticosterone levels were inversely correlated with body mass but did not differ among brood sizes. Within broods of two, second-hatched nestlings had lower CORT than first-hatched nestlings, although there was no effect of hatch order in broods of three or four nestlings. The results of this study suggest that conditions within the nest, particularly those associated with within-brood hierarchies, are manifested through differential body condition and nestling corticosterone secretion. The consequences of this variation in nestling corticosterone are unknown, but development of the adult phenotype, as well as life-long survival, may be impacted.

Development of the adrenal stress response in the Florida scrub-jay (Aphelocoma coerulescens)

Nestlings of altricial species undergo a period of substantial growth and development in the nest after hatching. The hypothalamic-pituitary-adrenal axis regulates the release of stress hormones such as corticosterone, which in adults is critical in allowing an animal to respond to a stressor. However, activation of this axis in young birds may be detrimental to growth and possibly survival. The developmental hypothesis predicts that altricial nestlings should display a dampened corticosterone response to stress as a means of protection against the potentially harmful effects of elevated corticosterone. We examined this hypothesis in Florida scrub-jays, a cooperatively breeding species with altricial young. Blood samples were collected from nestlings, nutritionally independent young, and yearlings for measurement of corticosterone levels. Baseline corticosterone levels did not differ between age-classes; however, stress-induced corticosterone levels were highest in yearlings, intermediate in independent young, and lowest in nestlings. The nestling stress response was also of a shorter duration than the response in independent young and yearlings. This variation in stress responsiveness across ages may be an adaptive mechanism to protect the developing bird from the negative effects of corticosterone on growth and cognitive development.

Does initial suppression of allofeeding in small jays help to conceal their nests?

A neglected question in the study of communal breeding in birds concerns why alloparenting begins at variously late stages in the nesting cycle. We studied this phenomenon in the Gray Jay (Perisoreus canadensis), a species in which nonbreeders are excluded from the nest area by parental hostility and begin to feed young only during the fledgling period. We hypothesize that this pattern is favoured because of the risk of nest predation. By initially suppressing allofeeding, Gray Jay parents may reduce the frequency of predator-attracting visits to the nest when the young are most vulnerable. We evaluated this predator-avoidance hypothesis in a six-part meta-analysis using observations of 111 philopatric and immigrant nonbreeders associated with 647 pairs over 33 years and nest-visitation data for 307 other passerines. First, we found indirect evidence that Gray Jays and other corvids with non-allofeeding nonbreeders do reduce nest visitation by excluding nonbreeders. Second, we found that the hostility of adult Gray Jays towards nonbreeders peaks during the incubation and nestling periods, is stronger closer to the nest, and is directed towards both related and unrelated nonbreeders. We then found support for four predictions stemming from the predator-avoidance hypothesis. Compared with species having allofeeding nonbreeders, the Gray Jay and other corvids with non-allofeeding nonbreeders are characterized by (i) smaller clutches; (ii) lower parental nest-visitation rates (a consequence of larger food loads); (iii) a greater increase in parental feeding visits after fledging, paralleling the simultaneous relaxation of parental hostility towards potential allofeeders; and (iv) a smaller body size and smaller social groups, rendering them less able to deter nest predators. We discuss the potential value of the predator-avoidance perspective for understanding the occurrence or absence of allofeeding in other birds.

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.

Fledgling care and male parental effort in the Hooded Warbler (Wilsonia citrina)

We followed family groups of Hooded Warblers (Wilsonia citrina) from hatching through to fledgling independence to determine (i) the duration and extent of parental care of fledglings, (ii) the extent of brood division, and (iii) whether male parental effort in caring for nestlings predicts male effort in caring for fledglings. The 9-day nestling period of Hooded Warblers was followed by 4 – 6 weeks of further parental care of fledged young. Parental feeding rates increased from hatching to when the young fledged from the nest, and males fed nestlings significantly more than females did. At the fledgling stage feeding rates to fledglings were significantly higher than at the nestling stage, but there was no difference in feeding rates between the parents. Parents usually divided the brood of fledglings equally, so that each parent assumed full and exclusive care of a subset of the brood. However, many females (45%) initiated a second brood and the male assumed care of the entire first brood at the time when his mate began incubating. The proportion of feeding trips to nestlings made by the male was not predictive of his subsequent effort in the care of fledglings. Exclusion of the fledgling care period in studies of parental investment may give a biased picture of overall investment on the part of both male and female parents.