Natal forest fragment size does not predict fledgling, premigration or apparent annual survival in Wood Thrushes

Determining the drivers and mechanisms for first year survival of migratory songbirds has been an understudied area in population dynamics due to the difficulty in tracking juveniles once they have dispersed from the natal site. With the advancement in miniaturization of radio-tags (battery life ∼400 days) and the development of the Motus Wildlife Tracking System, we tracked 189 Wood Thrush (Hylocichla mustelina) nestlings through independence and to fall migration departure, and their return the following spring. Natal forest fragment size and landscape forest cover (at different spatial scales) were not strong predictors of fledgling, pre-migration, or apparent annual survival; and onset of fall migration was best predicted by fledge date but not natal fragment size. Survival probability was lowest the first 16 days post-fledging (70%, or 0.86 weekly survival probability), very high for juveniles as they explored the landscape prior to fall migration (89%, or 0.99 weekly survival probability) and low during their first migration and wintering season (26%, or 0.95 weekly survival probability). To our knowledge, this is the first study to directly estimate annual apparent juvenile survival in a migratory songbird using year-round radio-tracking. Our study suggests that small forest fragments are important for the conservation for forest songbirds because they can support high survival of juveniles. Understanding the main drivers of survival during the full annual cycle of juvenile songbirds is urgently needed to better predict population growth. A key question for forest songbirds is whether nesting in small forest fragments negatively affects juvenile survival. The advancement in radio-tags (∼400-day battery life for small tags) alongside the Motus Wildlife Tracking System allowed us to estimate apparent juvenile survival at three life stages (fledgling, pre-migration, and migration/wintering) for a declining migratory forest songbird, the Wood Thrush. We show that natal fragment size does not predict survival probability for fledgling or juvenile Wood Thrushes. Weekly survival was lowest for fledglings on their natal territory, high for juveniles as they explore the landscape prior to fall migration, and lower during their first migration and wintering season. Our study suggests that even small forest fragments on the breeding grounds are important as they can support high juvenile survival for a forest songbird and that survival is driven primarily by f