Photoperiodic response to early hatching in a migratory bird species.

A considerable number of bird species of the northern hemisphere have been breeding earlier over the last few decades, most probably in response to global warming. In migratory birds, there is also a trend towards later departure from the breeding grounds in autumn. Yet it is not known whether this trend in the timing of migration reflects an evolutionary process or is just an immediate phenotypic response to global environmental change. We conducted an experiment with migratory blackcaps (Sylvia atricapilla) to investigate how the photoperiodic conditions experienced by birds hatching earlier in the season affect the timing of post-juvenile moult and the onset of autumn migration. In a split-brood experiment, we reared 30 nestlings from six different families and kept them either under a photoperiodic regime simulating conditions 6 weeks before their actual hatching date or under simulated natural photoperiods. Time-shifted birds started moulting at an earlier age compared to controls and showed an early phase of nocturnal migratory activity. Under the influence of long day lengths moult was prolonged and migratory activity was interrupted. However, the termination of moult and the reinitiation of migratory activity were not delayed to the extent that birds compensated for the simulated early hatching date. Thus, we suggest that extant physiological responses to the photoperiod are maladaptive under the novel environmental conditions imposed by a global increase in temperature, leading to earlier autumn migration in juvenile birds with earlier breeding.