Food, weather, and population density, not number of helpers, drive overwinter survival in Florida Scrub-Jays.

Abstract

Survival is affected by many ecological factors including food, weather, population density, and predation pressure, but documenting survival rates together with all these associated variables requires long-term observational and ecological data from a large, marked population. We used 33 years of data on Florida Scrub-Jay (Aphelocoma coerulescens) demography, local weather, and food availability to assess determinants of jay survival during the non-breeding season (September to February). We used the known-fate model from program MARK to analyze the probability of survival for breeders and juveniles while independently testing multiple covariates. Breeder survival increased when territory size was larger, decreased when local population density was higher, and did not vary with mean daily minimum temperature in the winter. Juvenile survival slightly increased with mean daily minimum temperature in the winter but showed no association with territory size or local population density. For both breeders and juveniles, odds of survival increased with presence of both breeders in the territory and higher acorn availability, and decreased with higher total winter rainfall. Neither breeder nor juvenile survival was correlated with number of helpers or juveniles in the group. Our results emphasize the importance of environmental variables in driving adult and juvenile survival in this declining endangered bird.