We combine studies on wild migrants with controlled experiments on long-term captive migrants to understand A) the endocrine regulation of migratory behaviour, and B) the physiological adaptations to and consequences of migration. As model species we use the northern wheatear (Oenanthe oenanthe), which is an obligatory long-distance migrant, and the common blackbird (Turdus merula), a partial migrant that travels only short distances. In our studies we focus on stopovers, which are periods of rest and fuel replenishment in between flight bouts. Field studies are conducted on Helgoland, a small island in the North Sea some 50km from the German coast, where many migrants make stopovers. In the lab, at the Institute of Avian Research (IfV), stopovers can be simulated by fasting the captive migrants and subsequently refeeding them.
Some of the questions we address are:
a) Does corticosterone affect refueling rates?
Both in the field as well as in the laboratory we measure rates of refuelling and relate this to endocrine profiles. On Helgoland we measure refuelling rate of wild migrants by temporarily caging them. Captive birds at the IfV are used in controlled experimental work, e.g. through manipulation of birds’ corticosterone levels.
b) Is the decision to depart from a stopover site affected by corticosterone and/or ghrelin?
By fitting migrants with radio-transmitters we can effectively determine stopover duration and departure timing within the night on Helgoland and relate this to circulating hormone levels, while correcting for external and internal factors, such as weather conditions and body condition (fat score).
c) Are differences in migratory strategies reflected in birds’ physiology?
Helgoland supports a sedentary breeding population of common blackbirds and at the same time serves as a stopover site for migratory conspecifics. This means that we are able to compare the physiology of migrating and sedentary individuals at the same location during the same time period. Physiological parameters that we study are the birds’ immune function and oxidative state. Similarly, two subspecies of northern wheatear (O. o. oenanthe and O. o leucorhoa) make simultaneous stopovers on Helgoland, but differ in their migratory routes. In spring, O. o. leucorhoa are headed towards Iceland, Greenland, the Faroe Islands, and northeastern Canada. This means that, upon departure from Helgoland, these birds face a long overseas flight to reach their breeding grounds. O. o. oenanthe on the other hand do not face such a barrier when departing from Helgoland. Consequently, the two subspecies differ in their energetic demands when refuelling on Helgoland, providing another good setting for comparative field studies.