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Numerous studies indicate interspecies variation in the ontogeny of the adrenocortical response in birds; however, little is known about the extent of interindividual variation in avian young. Toward this end, we examined the ontogeny and interindividual variation in the adrenocortical response in zebra finch (Taeniopygiaguttata) nestlings. We measured baseline and stress-induced total (bound and free) corticosterone, corticosteroid binding globulin capacity, and resulting estimated free corticosterone levels in nestlings of four different ages (days 5, 10, 16, and 21). In addition, we investigated the potential correlates of interindividual variation (brood size and mass). Nestlings at days 5 and 10 post-hatching showed no significant increase in total or free corticosterone levels in response to a standardized handling stress, whereas an adult-like stress response was seen by day 16 post-hatching. There was large interindividual (fivefold) variation in both baseline and stress-induced corticosterone among individual nestlings at any age. We estimate that half of this individual variation in the adrenocortical response could be explained by between-clutch variation (e.g., genetics), while the other half could be explained by other factors such as rearing environment (based on estimated intraclass correlation coefficients). Total baseline corticosterone, but not stress-induced corticosterone, was negatively correlated with fledging mass in this species. [ABSTRACT FROM AUTHOR]

Little is known about the energy costs of egg production in birds. We showed in previous papers that, during egg production, European starlings (Sturnus vulgaris) undergo a 22% increase in resting metabolic rate (RMR) and that the maintenance and activity costs of the oviduct are responsible for 18% of the variation in elevated laying RMR. Therefore, other energy-consuming physiological mechanisms must be responsible for the remaining unexplained variation in elevated laying RMR. Yolk precursor [vitellogenin (VTG) and very-low-density lipoprotein (VLDL)] production is likely to be costly because it signifies a marked increase in the biosynthetic activity of the liver. We documented the pattern of yolk precursor production in response to daily injections of 17β-estradiol (E[SUB2]) in zebra finches (Taeniopygia guttata). Based on this pattern we carried out an experiment in order to evaluate the metabolic costs of producing VTG and VLDL. Our E[SUB2] treatment resulted in a significant increase in plasma VTG and VLDL levels within the natural breeding range for the species. Although RMR was measured during the period of active hepatic yolk precursor production, it did not differ significantly within individuals in response to the treatment or when comparing E[SUB2]-treated birds with sham-injected birds. This could mean that yolk precursor production represents low energy investment. However, we discuss these results in light of possible adjustments between organs that could result in energy compensation. [ABSTRACT FROM AUTHOR]