Differential responses to photoperiod in juveniles of two migratory songbird species.

Songbirds rely on an endogenous programme encoding spatio-temporal information necessary for naive individuals to perform their first migration. Timing programmes are generated by endogenous clocks and include responses to external cues, with photoperiod being the most important. Photoperiod can advance or delay the circannual cycle and, while it is well established as a trigger for initiating migration, it remains uncertain whether birds continue to integrate photoperiod into their programme during migration. This study investigated whether two passerine species incorporate photoperiod in their endogenous programmes during autumn migration. Photoperiod manipulation experiments were conducted on hatch-year individuals of European robin (Erithacus rubecula; short-to-medium distance migrant) and garden warbler (Sylvia borin; long-distance migrant), monitoring their migratory activity and fuelling. The two species responded differently to the treatment, possibly due to different migratory distances, though effects of stage-appropriate responses cannot be excluded. European robins were partially sensitive to photoperiodic cues regarding their migratory activity. Their fuelling was, however, unaffected by photic treatment. Garden warblers showed little sensitivity to photoperiod, suggesting perhaps a stronger reliance on other external cues during migration. Garden warblers, furthermore, exhibited an exceptionally high fuelling rate, compared with other species previously studied, probably an adaptation to their time-constrained, long trans-equatorial migration. The results suggest that different components of the migratory phenotype can be controlled by different cues, with species-specific differences in the response. Understanding how migratory timing programmes interact with environmental cues in various migration phases is crucial, especially as anthropogenic processes rapidly alter photic environments, risking threatening migratory birds.