Are yolk androgens adjusted to environmental conditions? A test in two seabirds that lay single-egg clutches

It is widely believed that female birds strategically allocate androgens to yolk in the manner that best equips offspring for feeding conditions during their development. Because most avian studies have focused on multi-egg clutch species, and interpreted results within the framework of sibling competition, we still know little about how yolk androgens might be allocated in direct response to environmental conditions. Most oceanic birds are long-lived and lay single-egg clutches, and their breeding success is tightly linked to highly variable marine production. That combination: a variable breeding environment, long lives, and single-egg clutches, makes oceanic birds good subjects to test hypotheses about yolk androgen allocation strategies. We measured concentrations of two yolk androgens, androstenedione (A4) and testosterone (T), in the single-egg clutches laid by early-laying Cassin's (Ptychoramphus aleuticus) and rhinoceros (Cerorhinca monocerata) auklets at Triangle Island, British Columbia, Canada, in 2002-2004. Environmental conditions including sea-surface temperatures and the timing and intensity of marine primary production varied over the 3 years, and in response, both the timing and success of seabird breeding varied. As in other avian species, concentrations of A4 and T varied markedly among individual eggs in both species (by factors of 3-8), yet contrary to expectation, little of the variation could be attributed to year effects. The high interindividual variation and the lack of interannual variation suggest a non-adaptive explanation for yolk androgen deposition relative to environmental conditions in these species.

Regulation of provisioning rate in the Thick-billed Murre (Uria lomvia)

Many studies of marine birds report that parents regulate provisioning, thus meeting their offsprings’ nutritional requirements at minimum cost. Others report that food availability limits provisioning, mediated by parental body condition. One hypothesis that might explain these discrepancies holds that seabirds are better able to regulate provisioning under more favourable feeding conditions. To test this, we studied provisioning behaviour in the Thick-billed Murre (Uria lomvia L., 1758) in three colony-years that, based on chick growth rates, spanned a wide range of feeding conditions. We considered that parents regulating provisioning would (i) deliver more food as their offspring aged, to meet their increasing requirements, (ii) space consecutive deliveries nonrandomly in time, and (iii) maintain additional mass to buffer against future deterioration in feeding conditions. As predicted, we found that (i) provisioning rates increased with chick age in all three colony-years, but more strongly when feeding conditions were better; (ii) consecutive deliveries were spaced nonrandomly in all three colony-years, again more strongly when feeding conditions were better; and (iii) adults were heavier at mid chick-rearing when feeding conditions were better. Future studies that investigate how feeding conditions influence seabird provisioning could improve our understanding of how long-lived species optimize reproductive effort in variable environments.