Abstract
The spectacular natural phenomenon of avian migration is evidently shaped by physical factors, but we know little about the underlying physiological regulation. This contrast is especially apparent for the process of departure on a migratory flight. The decision to resume migration is shaped by a suite of departure cues from innate rhythms, and intrinsic and extrinsic factors. It currently appears that these departure cues are translated into actual departure by the hormone corticosterone, but other hormones may play a role too and probably interact with corticosterone. We captured this concept here by investigating the role of the hormone ghrelin and its interaction with corticosterone for the departure decision in a migratory songbird. Ghrelin functions as an appetite-regulating hormone. It has also been suggested to be involved in the regulation of departure by upregulating corticosterone in migrants ready to depart, and by facilitating the breakdown of lipids to fuel migratory flight. We measured plasmatic ghrelin and corticosterone levels in migrating common blackbirds (Turdus merula) at an autumnal stopover site, and determined their departure timing with the use of a fully-automated radio-telemetry system. Against our expectations, ghrelin level was not correlated with the birds' lipid stores or with corticosterone level. Furthermore, departure likelihood and nocturnal departure time were not associated with ghrelin levels. Our study thus does not support the idea that ghrelin is involved in the regulation of departure from stopover, at least not in common blackbirds. We discuss possible reasons for the lack of confirmation of our expectations.