Changes in plasma hormone levels correlate with fledging in nestling Leach's storm-petrels

Timing of Breeding Determines Growth and Development in a Long-Distance Migratory Bird

The timing of breeding can have significant consequences for adult and offspring fitness, yet our current hypotheses of seasonal timing focus on the parent perspective. When offspring survival is affected by timing of breeding, we expect to see offspring mechanisms to detect and respond to cues of seasonal timing. Avian embryos respond to photoperiod and seasonal cues during development and in this study we evaluate the influence of photoperiod and season on posthatching growth and development in Franklin's gull (Leucophaeus pipixcan). Early- and late-season chicks exposed to short and long photoperiods during development were reared under common garden conditions. Photoperiod had no effect on posthatching growth and development, but seasonal effects present in the egg (maternal egg effects or genetic) did influence posthatching growth and development. Late-season chicks grew body mass and primary flight feathers at faster rates than early-season chicks, which we hypothesize facilitates independence and readiness for migration. Growth rates are complex phenotypes and we propose a general growth model that incorporates delays in negative feedback systems regulating growth. We show that the timing of breeding programs intrinsic growth rates in offspring, which suggests that many of the metrics used to describe seasonal patterns of reproductive success may be biased.

Effects of Prenatal Environment on Phenotype Are Revealed by Postnatal Challenges: Embryonic Hormone Exposure, Adrenocortical Function, and Food in Seabird Chicks

The interaction between prenatal environments and postnatal environments is an important source of phenotypic variability. We examined the ability of prenatal steroid exposure and postnatal energy restriction to explain adrenocortical function and fledging age in captive seabird chicks. We proposed and tested two hypotheses: (1) the strength of prenatal effects is attenuated by challenging postnatal environments (postnatal override) and (2) the strength of prenatal effects increases with the severity of postnatal challenges (postnatal reveal). We reared common murre (Uria aalge) chicks and measured prenatal exposure to corticosterone (CORT) and testosterone (T) from allantoic waste. Adrenocortical function was assessed after 10 d of ad lib. feeding and then after 5 and 10 d on controlled diets. Postnatal override predicts that prenatal steroids will explain more phenotypic variation before implementation of energy restriction; postnatal reveal predicts that the contribution of prenatal steroids will increase with duration and severity of energy restriction. Energy restriction increased secretion of baseline CORT and the adrenocortical response to the standardized stressor of handling and restraint. The ability of prenatal steroids to explain baseline CORT increased with duration of energy restriction, and for day 20 free baseline CORT, there was a significant interaction between kilojoules per day and prenatal CORT levels; severity of restriction strengthened the relationship between prenatal hormone levels and postnatal hormone levels. Both maximum CORT at day 20 and fledging age were best explained by diet treatment and day 15 or day 20 baseline CORT, respectively. Overall, prenatal CORT increased fledging age and baseline secretion of CORT, while prenatal T decreased them. However, prenatal effects on adrenocortical function were apparent only under the energy restriction conditions. Thus, we found some support for the postnatal reveal hypothesis; our results suggest that some prenatal effects on phenotype may be more likely to manifest in challenging postnatal environments.

Feather corticosterone reveals developmental stress in seabirds

In nest-bound avian offspring, food shortages typically trigger a release of the stress hormone corticosterone (CORT). Recent studies indicate that CORT is passively deposited in the tissue of growing feathers and thus may provide an integrated measure of stress incurred during development in the nest. The current hypothesis predicts that, assuming a constant rate of feather growth, elevated CORT circulating in the blood corresponds to higher levels of CORT in feather tissue, but experimental evidence for nutritionally stressed chicks is lacking. Here, we examined how food limitation affects feather CORT content in the rhinoceros auklet (Cerorhinca moncerata). We (i) used captive chicks reared on control versus restricted diets, and (ii) applied this technique to free-living chicks with unknown nutritional histories that fledged at three separate colonies. We found that (i) feather growth was not affected by experimentally induced nutritional stress; (ii) captive chicks raised on a restricted diet had higher levels of CORT in their primary feathers; (iii) feather CORT deposition is a sensitive method of detecting nutritional stress; and (iv) free-living fledglings from the colony with poor reproductive performance had higher CORT in their primary feathers. We conclude that feather CORT is a sensitive integrated measure revealing the temporal dynamics of food limitations experienced by rhinoceros auklet nestlings. The use of feather CORT may be a powerful endocrine tool in ecological and evolutionary studies of bird species with similar preferential allocation of limited resources to feather development.

The effects of brood size on growth and steroid hormone concentrations in nestling eastern bluebirds (Sialia sialis)

Birds in multi-nestling broods often experience reduced growth and elevated nutritional and social stress as a result of competition for parental resources. While responses are often species-specific, experimentally increasing brood size has been shown to decrease growth and increase production of both testosterone and corticosterone in nestling passerines. To investigate the relationship between brood size, growth, and steroid hormone production in eastern bluebirds, we cross-fostered nestlings to small, medium, and large broods. Body mass, skeletal size, serum testosterone, and baseline serum corticosterone concentration were measured prior to fledging. Bluebird nestlings raised in large broods weighed less than nestlings in small and medium-sized broods and secreted elevated concentrations of testosterone. Nestling corticosterone concentrations did not vary in response to brood size or body condition, but concentrations were higher in males compared to females. Our results suggest that nestling bluebirds experiencing nutritional and social stress increase testosterone production. Elevated concentrations may enhance begging and competitive abilities in smaller young. Higher concentrations of corticosterone in males may be related to sex-biased provisioning. Further experimental work is needed to ascertain the effects of elevated testosterone concentrations in nestling bluebirds, as well as the causes and consequences of elevated corticosterone for male nestlings.