Fig. 1

a Proximate control mechanisms for migration: Two extremes along a continuous gradient. On one side facultative (environmentally induced) and on the other end, obligate migration (intrinsic control mechanisms). b Different migration types (ranging from irruptive to full migration) which are linked to specific proximate control mechanisms (see (a)). c Locations of study sites for Eurasian blackbirds (Turdus merula). While the Spanish population of blackbirds is fully resident, departures from partial (Germany and Finland) and fully (Poland and Russia) migratory populations are used in this study