Corticosterone: foraging and fattening puppet master in pre-breeding greylag geese.

Reproduction is one of the most energetically costly life history stages, which impose constraints, even outside the breeding period. Capital breeders typically accumulate energy in preparation for reproduction and the amount of body mass gain prior to reproduction partly determines reproductive outcome in such species. Understanding the physiological and behavioral interplay that governs energy storage is thus essential. Pleiotropic hormones such as glucocorticoids can modulate diel and seasonal energy allocation in vertebrates. Baseline corticosterone (CORT, the main glucocorticoid hormone in birds) fluctuation can induce changes in foraging behavior and/or energy storage. In this experiment, we slightly elevated CORT levels and monitored body mass and foraging behavior prior to reproduction in semi-captive greylag geese. Birds treated either with CORT or placebo pellets inserted subcutaneously were monitored during 21 days. Same individuals were sequentially submitted to both treatments. The increase of CORT levels measured in blood samples confirmed the slight CORT elevation in treated birds. Foraging behaviors increased (up to 9%) in the CORT treated group compared to controls only during morning observations. Birds treated with CORT increased their body mass gain by 6.3% compared to controls. This effect lasted during the first 11 days after pellet implementation. We thus confirm the central role of glucocorticoids on foraging behaviors and body mass gain in pre-nesting birds. This study opens new avenues to manipulate body condition in large-bird species.