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Abstract: Optimal functioning of the hypothalamo–pituitary–adrenal (HPA) axis is paramount to maximizing fitness in vertebrates. Research in laboratory mammals has suggested that maternally-induced stress can cause significant variation in the responsiveness of an offspring''s HPA axis involving both pre- and post-natal developmental mechanisms. However, very little is known regarding effects of maternal stress on the variability of offspring adrenocortical functioning in free-living vertebrates. Here we use an experimental approach that independently lowers the quality of both the pre- and post-natal developmental environment to examine programming and plasticity in the responsiveness of the HPA axis in fledglings of a free-living passerine, the European starling (Sturnus vulgaris). We found that mimicking a hormonal signal of poor maternal condition via an experimental pre-natal increase in yolk corticosterone decreased the subsequent responsiveness of the HPA axis in fledglings. Conversely, decreasing the quality of the post-natal developmental environment (by decreasing maternal provisioning capability via a maternal feather-clipping manipulation) increased subsequent responsiveness of the HPA axis in fledglings, apparently through direct effects on nestling body condition. The plasticity of these responses was sex-specific with smaller female offspring showing the largest increase in HPA reactivity. We suggest that pre-natal, corticosterone-induced, plasticity in the HPA axis may be a ‘predictive adaptive response’ (PAR): a form of adaptive developmental plasticity where the advantage of the induced phenotype is manifested in a future life-history stage. Further, we introduce a new term to define the condition-driven post-natal plasticity of the HPA axis to an unpredictable post-natal environment, namely a ‘reactive adaptive response’ (RAR). This study confirms that the quality of both the pre- and post-natal developmental environment can be a significant source of variation in the responsiveness of the HPA axis, and provides a frame-work for examining ecologically-relevant sources of stress-induced programming and plasticity in this endocrine system in a free-living vertebrate, respectively. [Copyright &y& Elsevier]

Current research in birds suggests that a conflict should exist during reproduction for the role of the glucocorticoid corticosterone (CORT). While elevated levels have been correlated with the increased energetic demand of raising offspring, elevated CORT levels have traditionally been implicated in reproductive abandonment. We examined the relationship between CORT and nest desertion in breeding wild female European starlings (Sturnus vulgaris) incorporating analyses of both total circulating levels and ‘free’, unbound CORT through analysis of corticosteroid-binding globulin (CBG). Free baseline CORT levels of nest-abandoning birds were significantly higher than nonabandoning birds within each stage, with chick-rearing birds exhibiting the highest free baseline CORT levels, while concurrently remaining the most resistant stage to nest desertion. Elevated free baseline CORT levels in chick-rearing birds were not due to increased total CORT secretion, but rather to a decrease in CBG levels. Overall, our results suggest that CORT and CBG interact to play a role in mediating the increased energetic demand of offspring, while minimizing the chances of nest desertion, thereby alleviating any potential behavioral conflict for CORT during reproduction. Furthermore, these results demonstrate that the traditional view of the role of CORT during reproduction is much more complex than previously appreciated. Together with mounting evidence, we suggest that elevated corticosteroid levels are an inherent and necessary part of reproduction in nonmammalian tetrapods. [Copyright &y& Elsevier]

Although it is well documented that hatching asynchrony in birds can lead to competitive and developmental hierarchies, potentially greatly affecting growth and survival of nestlings, hatching asynchrony may also precipitate modulations in neuroendocrine development or function. Here we examine sibling variation in adrenocortical function in postnatally developing, asynchronously hatching American kestrels (Falco sparverius) by measurements of baseline and stress-induced levels of corticosterone at ages 10, 16, 22, and 28 days posthatching. There was a significant effect of hatching order on both baseline and stress-induced corticosterone levels during development and these effects grew stronger through development. First-hatched chicks exhibited higher baseline levels than later-hatched chicks throughout development and higher stress-induced levels during the latter half of development. Furthermore, there was significant hatching span (difference in days between first- and last-hatched chicks) × hatching order interaction on both baseline and stress-induced corticosterone levels during development. Hatching span was also positively correlated with both measures of corticosterone and body mass in first-hatched chicks, but was negatively correlated with these factors through most of the development in last-hatched chicks. It is known that hatching asynchrony creates mass and size hierarchies within kestrel broods and we suggest that hierarchies in adrenocortical function among siblings may be one physiological mechanism by which these competitive hierarchies are maintained. [Copyright &y& Elsevier]

Abstract: Sexual differentiation of the brain has traditionally been thought to be driven by gonadal hormones, particularly testosterone (T). Recent studies in songbirds and other species have indicated that non-gonadal sex steroids may also be important. For example, dehydroepiandrosterone (DHEA) – a sex steroid precursor that can be synthesized in the adrenal glands and/or brain – can be converted into active sex steroids, such as 17β-estradiol (E2), within the brain. Here, we examine plasma DHEA and E2 levels in wild developing European starlings (Sturnus vulgaris), from hatch (P0) to fledging (P20). Blood samples were collected from either the brachial vein (n =143) or the jugular vein (n =129). In songbirds, jugular plasma is enriched with neurally-synthesized steroids and, therefore, jugular plasma is an indirect measure of the neural steroidal milieu. Interestingly, brachial DHEA levels were higher in males than females at P4. In contrast, jugular DHEA levels were higher in females than males at P0 and P10. Brachial E2 levels were higher in males than females at P6. Surprisingly, jugular E2 levels were not high and showed no sex differences. Also, we calculated the difference between brachial and jugular steroid levels. At several ages, jugular steroid levels were lower than brachial levels, particularly in males, suggesting greater neural metabolism of circulating DHEA and E2 in males than females. At a few ages, jugular steroid levels were higher than brachial levels, suggesting neural secretion of DHEA or E2 into the general circulation. Taken together, these data suggest that DHEA may play a role in brain sexual differentiation in songbirds. [Copyright &y& Elsevier]