Correlation between timing of juvenile moult and onset of migration in the blackcap, Sylvia atricapilla

In small bird species, energy-demanding life cycle stages such as moult and migration are generally separated in time. The extent of separation can vary considerably within and between species, but the causes of this variation are largely unknown. We studied the phase relation between postjuvenile moult and autumn migration by experimentally manipulating the timing of these events in the blackcap. In a split-brood experiment, we hand-reared 30 blackcaps and kept them under either natural daylengths or a time-shifted photoperiod that altered the timing and intensity of moult. We determined the onset and termination of moult and the onset of nocturnal migratory restlessness. In both groups, onset of migratory activity was correlated with termination of moult. The extent of moult-migration overlap was unaffected by the photoperiod manipulation, suggesting resilience of this correlation against environmental perturbation. Strong family effects explained a large proportion of phenotypic variation. The correlation between the timing of postjuvenile moult and migration is, therefore, likely to result from genetic covariation. We predict that selection for delayed termination of moult will result in more overlap between moult and migration. Because of this correlated selection response, adaptive changes in the timing of migration could be retarded, and independent adaptive evolution of moult and migration schedules could be constrained.