Pre-migration behaviour and survival of juvenile Wood Thrushes (Hylocichla mustelina) in a fragmented forest landscape

Due to the difficulty in tracking juvenile songbirds once independent and dispersed from their natal areas, little is known about survival during the first year of life, despite being important for understanding population dynamics in migratory songbirds. Using technology advancements with the Motus Wildlife Tracking System and the miniaturization of radio-tags (~1-year battery), I tracked 189 Wood Thrush nestlings from the nest to provide survival estimates at 3 key life stages: fledgling survival, pre-migration survival, and migration/wintering survival. As small forest fragments have been identified as lower quality habitat, I tested if natal fragment size was a strong predictor of either survival or the extent of pre-migratory movement, and whether juveniles from small fragments suffered from carry-over effects that delayed fall migration departures or lowered annual survival. Lastly, I was able to test hypotheses that could explain the function of pre-migratory movements: habitat optimization, prospecting for future breeding territories, homing target; and for the first time, a flight performance hypothesis that proposes that the extent of pre-migratory movement improves first migration flight performance.

Natal forest fragment size was not a strong predictor of fledgling, pre-migration, or apparent annual survival and onset of fall migration was best predicted by fledge date. Weekly survival was lowest for fledglings on their natal territory, high for juveniles as they explored the landscape prior to fall migration, and lower during their first migration/wintering season. Most juveniles that were known to survive wintering/migration, returned to the study area (81%, 25/31) within ~12km of their natal site. Long-distance exploration of the landscape occurred for a high proportion (78%) of juveniles prior to onset of fall migration and these pre-migratory movements occurred at night, mainly 2 hours before sunrise. My results best supported the homing target hypotheses as pre-migratory movements were common in juveniles but not adults, were random in orientation, and a relatively large proportion of juveniles returned the next spring. Overall, my results suggest that even small forest fragments on the breeding grounds are important as they can support high juvenile survival for Wood Thrushes and that survival is driven primarily by factors outside of the breeding grounds.