Flexibility and Control of Circadian Activity, Migratory Restlessness and Fueling in Two Songbird Migrants

Juvenile songbirds rely on an endogenous program, encoding direction, distance, fueling, and timing of migration. Migratory distance is species-specific, expressed as a period migratory restlessness, for which the length correlated with while fueling modified to meet anticipated flight distances controlled by geomagnetic cues amount day-light available foraging. How daylength affect onset level activity decisions in wild birds have so far received limited attention. Here we study how photoperiod controls onset, extent autumn juvenile diurnally migrating dunnocks, nocturnally European robins experimentally increasing daylength. For both species, kept control group indoors at location capture southern Sweden exposed natural photoperiod, experimental increased advanced 2 h morning. Dunnocks initiated sunrise (or artificial sunrise) groups, demonstrating highly responsive flexible component migration triggered light. Experimental end nocturnal predicting earlier immediately after time-shift this behavior already under darkness, supporting fast-resetting mechanism new diel period. Timing morning was not affected suggesting fixed that persisted throughout 13-day dunnocks higher overall lower fuel loads than controls, did change their load response shifted photoperiod. These results reveal important adaptations circadian timekeeping including open fast modifications more rigid end, differential effects treatment two species.