Carry-over effects of day length during spring migration

The day lengths to which migratory birds are exposed depend on the timing and course of their journey. While winter day length is known to influence vernal events, it is not clear if birds also use day length during the spring migration as a temporal cue. We addressed this question by exposing captive stonechats (Saxicola torquata) to two different photoperiodic simulations of spring migration routes, following common winter conditions. One group experienced day lengths of the regular (fast) migration, and the other group, a slow, or more southerly originating, route. The resulting small, temporary differences in day length had lasting effects on the birds. The groups differed in migratory restlessness during and following exposure to different day lengths. Slow migrants continued nocturnal activity longer than fast migrants. Furthermore, all activities of the ensuing breeding season were delayed in the slow migrants, indicating a phase shift in their underlying annual rhythm. Slow migrants delayed terminating their reproductive stage by regressing testes and the cloacal protuberance later than the fast migrants. Molt started and ended later in slow migrants, but the duration of the molt was unaffected by spring day length. Finally, fast migrants resumed nightly restlessness earlier than slow migrants in late summer. These results demonstrate that Zugunruhe (migratory restlessness) and reproductive windows are not set exclusively during winter but can be modified by day length cues during the spring migration. Because migration modifies the day length exposure of birds, migration routes can have carry-over effects on the timing of breeding season events, including the completion of molt and initiation of autumnal nocturnal activity.