Natal forest fragment size does not predict fledgling, pre-migration, 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 nestlings through independence and to fall migration departure, and their return the following spring. Natal forest fragment size was not a good predictor of survival at any of the main life stages and onset of fall migration was predicted by fledge date but not natal fragment size. The percent forest cover in the landscape (at 2-km scale) had only a weak effect on fledgling survival. Survival probability was lowest for fledglings on their natal territory (70%, or 0.86 weekly survival probability), very high for juveniles as they explored the local 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. 

Methods
Data was collected during the 2016–2018 field seasons at 29 forest fragments. Nestlings were tagged while in the nest and subsequently manually and remotely tracked from the nest to their return the following spring. Data were processed in R using RMark (for nest survival and fledgling survival) and GLMs (fall migration departures, premigration survival, migration survival and apparent annual survival).