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Early developmental experiences play an important role in development of the adult phenotype. We investigated the effects of neonatal handling on the hypothalamic-pituitary-adrenal axis in a free-living avian species, the American kestrel (Falco sparverius). In the handled group (H), kestrel chicks were handled for 15 min/day from hatching until 26 days of age, after which time blood samples were collected for analysis of adrenal responsiveness and corticosterone binding globulin (CBG) levels. The non-handled control group (NH) was left undisturbed until 26 days of age when blood samples were collected and analyzed as above. Handled and NH kestrels did not differ in body condition index. Both total corticosterone (CORT) and CBG capacity were dampened significantly in H kestrels. However, free CORT did not differ between the two groups. In addition, hormone challenges of corticotropin releasing factor and adrenocorticotropin hormone were compared to saline injections to determine if the pituitary or the adrenal glands, respectively, were rendered more or less sensitive by handling. There was no difference in the responsiveness of H and NH kestrels to either hormone challenge. It is clear from these data that handling had an affect on fledgling phenotypic development, although whether the effects are permanent or ephemeral is unknown., (Copyright © 2011 Elsevier Inc. All rights reserved.)

Although multiple condition dependence in dispersal is common, the proximate mechanisms that integrate information from multiple sources remain largely unknown. In the common lizard (Lacerta vivipara), earlier studies have shown that maternal plasma corticosterone level interacts with maternal phenotype to affect offspring phenotype and dispersal strategy, and may reflect the mother's external and/or internal environment. However, the mechanism by which these two types of environmental information are integrated has not been identified. We explored the possibility that the timing and duration of the corticosterone signal are the key factors by which the message is modulated. We found that the timing of corticosterone exposure affects the juvenile phenotype: an exposure to corticosterone early in development has negative effects on juvenile size, weight, and body condition, that can nevertheless be restored in the case of a prolonged exposure. The duration of corticosterone exposure affects the dispersal strategy, although the precise effect depends on the sex ratio of the clutch. That is, in female-biased clutches, a prolonged exposure during gestation promotes philopatry of juveniles, while a short exposure enhances their dispersal, a result that is consistent with kin competition theory. Therefore, our results demonstrate that while corticosterone titer may signal a mother's external and/or internal environment to her developing young, differences in the timing and duration of this endocrine signal produce offspring with specialized phenotypes that exhibit different dispersal strategies.

A model that explains natal dispersal in resident screech-owls is presented and examined. The model is based on interactions among hormonal changes, body condition and social stimuli. It predicts that corticosterone, an adrenal glucocorticoid known to stimulate locomotor and foraging activity, increases in blood plasma prior to dispersal through a combination of endogenous and exogenous events. This mediates the locomotor activity that underlies dispersal behaviour. Juveniles in good body condition (i.e. those with sufficient fat reserves) will disperse when corticosterone increases. Birds in poor body condition will not, but they will increase their foraging activity under the influence of corticosterone. Dispersal of siblings will reduce aggression and/or competition for food, enabling the remaining juveniles to improve their body condition and disperse. Initial studies on screech-owls, Otus asio and O. kennicottii, have produced results that are generally consistent with the model. For example, captive juvenile screech-owls showed increased locomotor activity in the weeks leading up to the time when free-living juveniles are dispersing, and activity levels declined thereafter. Peaks in corticosterone corresponded with periods of high locomotor activity (i.e. at the time of dispersal) in captive owls. Finally, field studies indicate that dominant juveniles, which are presumably in better physical condition, initiated dispersal before their more subordinate siblings. Copyright 1998 The Association for the Study of Animal Behaviour.

To assess whether alterations in the normal pattern of testosterone (T) secretion might be beneficial or detrimental, we studied a breeding population of dark-eyed juncos in which we elevated T experimentally and measured its effect on potential correlates of fitness. We treated both free-living and captive males with implants that were either empty (C-males, controls) or packed with T (T-males, experimentals). Timing of implant varied and was designed to mimic natural peak breeding levels except that peaks were either prolonged or premature. We bled the birds at recapture and analyzed their plasma, and that of their female mates, for T and corticosterone (B). We also measured body mass and fat score in free-living T- and C-males. In the field, T-implants elevated T and kept it elevated for at least a month. Experimental males also had higher B than controls. In captives, the effect of the implants on plasma T was detectable within 24 hr. B in captive T-males was again higher than in captive C-males. In females, neither T nor B differed between mates of T- and C-males. T-males implanted in early spring lost more mass between implant and recapture in late spring than did controls and also had lower fat scores when recaptured. When implants were inserted in summer, treatment did not influence mass. Elevated T in early spring apparently hastened the transition from the winter to the breeding mode of fat storage. We suggest that prolonged elevation of testosterone might be selected against because of the association between T and B. Premature elevation of T might be costly because of the resultant loss of mass and fat reserves, which could lead to mortality when spring snowstorms prevent access to food.

Free-living male brown-headed cowbirds were captured during the breeding season, implanted with testosterone-filled Silastic tubing, and released. Radioimmunoassay verified that the implants maintained circulating plasma testosterone values at maximal breeding season levels well beyond the normal time of decline. Survival to the following year of these implanted males was compared with survival of unimplanted birds captured in other years, and also with survival of males given empty implants. Androgen-implanted male cowbirds exhibited significantly reduced survival to the following year compared with either of the control groups, and also exhibited severe injuries not seen in other years. It is suggested that the increased risks associated with prolonged high testosterone levels act as a selective force to maintain reduced androgen levels except during the period of aggressive intrasexual interactions that characterizes the reproductive season. Further, it is proposed that the nature and importance of the different risks vary with a species' mating system.

Captive male brown-headed cowbirds exposed to long days exhibit gonadal growth and have elevated plasma testosterone (T) levels. This photoperiodic response is enhanced if males are housed with female cowbirds: Photostimulated males with females increase plasma testosterone levels sooner than do individually housed photostimulated males. Peak plasma T levels are similar in both groups, although peak levels are maintained longer in males housed with females. The gonadal cycle is similarly affected; males in the presence of females have earlier gonadal recrudescence and maintain mature gonads longer than do photostimulated males without females. Plasma corticosterone levels increase in the unpaired males, suggesting that removal of social cues is stressful for these birds. Free-living paired males have significantly higher plasma testosterone levels than do unpaired/unknown males early in the season, when social relationships are being established; the levels are similar thereafter. There is no difference between the two groups in testicular maturation rates; nor do they differ in plasma corticosterone levels at any time of the season. These results suggest that social stimuli are important in modulating the secretion of testosterone in males early in the season when pairing occurs, and possibly late in the season as well, probably to prevent termination of breeding prior to that of females.

In the laboratory and under semi-naturalistic conditions, male domestic fowl (Gallus gallus) produce more alarm calls in response to predatory birds flying overhead than females. This study tests the hypothesis that testosterone is a factor in the control of aerial alarm call production. Birds were castrated early in life and tested as adults in the laboratory. Aerial predators were simulated by cardboard silhouettes drawn overhead. Blood samples were taken at intervals and plasma testosterone levels were measured by radioimmunoassay. Each subject was tested in four conditions: with an empty Silastic implant, with a testosterone-filled implant, with an additional testosterone implant, and after removal of the implants. Results show that the presence of testosterone increases the production of aerial alarm calls, and that removal of the hormone supply results in a drop in calling rate. As a control a second vocal system, food calling, was considered. In contrast with alarm calling, there was no correlation between testosterone levels and variation in food-call production. We conclude that testosterone plays a specific role in activation of the production by cockerels of vocalizations that signal the presence of aerial predators.