Vera Brust, Cas Eikenaar, Florian Packmor, Heiko Schmaljohann, Ommo Hüppop, Gábor Á. Czirják
Many species of songbirds breeding in northern Europe escape the cold times of the year, when food is scarce, and travel south. The small birds undertake journeys of several hundreds to thousands of kilometers twice each year. Travelling requires energy, but the amount of fat a bird can carry to fuel its flight is limited. Accordingly, migrants repeatedly land and take time to feed along the way. Leaving these so called stopovers, the birds are known to consider their current fat reserves and the prevailing weather to travel as economically and safely as possible. But is health also included in these decisions?
To check, we blood-sampled individuals of five species of songbirds wintering in central, western or southwestern Europe at coastal stopover sites and equipped them with light-weight radio-telemetry tags. Each tag emitted an individually coded signal every few seconds. After the birds’ release, receivers located all along the South-Eastern North Sea coastline automatically recorded these signals and thus the birds’ movements in our study area. From the blood plasma, we measured five components of immune function. Telemetry data revealed how long the birds continued stopover after release, how far they flew once they departed, and whether they preferred to follow the coastline or dared to cross the open sea.
Some, but not all of our plasma measures correlated with the birds’ behavior. In birds on spring migration, prolonged stopovers were linked to higher levels of natural antibodies and immunoglobulin Y. Both proteins may be abundant either due to a recent infection or as a precautious investment in immune function. Taking more time, birds may have waited for being in better condition to continue travelling and/or to start breeding once arrived at their migratory goal. During fall, we did not find such a link, but stopovers were generally longer. Route choices were not linked to immune measures, but longer flights were taken by birds with initially higher immunoglobulin Y levels in some species. Our findings indicate an interplay between health and migration even beyond stopover.