Short photoperiods end autumn migration in a naïve diurnal migrant

Many migratory animals use daylength, or photoperiod, to signal when to migrate and transition between annual phenological states. Whether animals use photoperiod as a temporal or spatial cue while migrating, however, requires additional empirical support. We used hatch-year dunnocks, Prunella modularis (a songbird), caught during their first migration in southern Sweden to elucidate whether migratory animals incorporate photoperiod as a spatiotemporal cue into their endogenous migratory program during migration. We exposed the migratory-naïve to light environments that simulated either the local photic conditions or a shorter daylength and larger transitions between photoperiods. All birds experienced local geomagnetic conditions. We hypothesized that migratory dunnocks used photoperiod to inform their first migration and predicted that the experimental treatment represented either a spatial displacement to the north or a temporal advancement towards winter at the capture site compared to the local control conditions. We found, though, that the short photoperiods terminated the expression of the migratory phenotype compared to controls by reducing body mass gain and ending migratory activity, indicating that the endogenous migratory program integrates photoperiod during migration. The incorporation of photoperiod into the endogenous program may complement geomagnetic cues to ensure ending migration at the correct time and location. The incorporation of photoperiod can also provide a mechanism that facilitates poleward shifts of overwintering distribution under climate change by allowing migrants to overwinter in newly suitable habitat at higher latitude (i.e. short stopping).