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 gl obal warming. In migratory birds, there is also a trend towards later depar ture 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 jus t an immediate phenotypic response to global environmental change. We condu cted an experiment with migratory blackcaps (Sylvia atricapilla) to investi gate how the photoperiodic conditions experienced by birds hatching earlier in the season affect the timing of post-juvenile moult and the onset of au tumn migration. In a split-brood experiment, we reared 30 nestlings from si x different families and kept them either under a photoperiodic regime simu lating conditions 6 weeks before their actual hatching date or under simula ted 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 m igratory activity was interrupted. However, the termination of moult and th e reinitiation of migratory activity were not delayed to the extent that bi rds compensated for the simulated early hatching date. Thus, we suggest tha t extant physiological responses to the photoperiod are maladaptive under t he novel environmental conditions imposed by a global increase in temperatu re, leading to earlier autumn migration in juvenile birds with earlier bree ding.