Why do juvenile Wood Thrushes make long-distance pre-migratory movements across a fragmented landscape?

The pre-migratory period for naïve juvenile migratory songbirds is a critical stage in development as they disperse from their natal territories and prepare for their inaugural fall migration. Little is known about this period because of the difficulty in tracking individuals once they make longer dispersal movements. We used the Motus Wildlife Tracking System combined with long life (~400 d) radio-tags to track 119 independent juvenile Wood Thrushes to fall migration departure and to detect their return the subsequent spring. For comparison, we also tracked 60 adults from breeding territories in the same years and study area. We found that 78% of juveniles made long-distance (> 5 km) pre-migratory movements (PMM) at night, mainly 2 h before sunrise, and in random orientations. We found no support for the habitat optimization hypothesis because adults rarely (8.3%) made PMM, and juveniles from small forest fragments did not disperse at an earlier age or make more extensive PMM than those from large fragments. The prospecting for future breeding territories hypothesis seems unlikely because PMM occurred primarily in the 2 wk prior to the onset of fall migration, when social cues of territory quality are less available, and juveniles that returned in spring did not breed closer to their previous fall locations than to their natal site. The extent of PMM also did not predict flight performance (departure date, pace, orientation) as birds crossed a large water barrier on their first migration movement. Our results best support the homing target hypothesis because, of the juveniles that returned the subsequent spring, a large proportion (81%) was detected in the study area rather than elsewhere by the extensive Motus network. Long-distance PMM by juveniles may be common in other forest songbirds and could have implications for landscape-scale habitat conservation.