A method for studying post-fledging survival rates using data from ringing recoveries

Abstract

We present a method for studying post-fledging survival rates from data on national ringing recoveries. The approach extends the classical two-age-class models of Brownie et al. (1985) to include a third age-class of birds ringed as nestlings. The models can incorporate age-class-specific and year-specific variation in reporting rates and survival rates. Unlike intensive resighting studies at single sites, this approach does not confound death with emigration and provides estimates of survival which are representative of a larger geographical area. We implemented the models with SURVIV software, using likelihood-ratio tests to assess the significance of possible age and year effects on survival rates or reporting rates. We demonstrate this approach using a set of data on British-ringed Song Thrushes Turdus philomelos. In this case, we were unable to detect any age-class-specific or year-specific variation in reporting rates and, although the survival rates of first-year and adult Song Thrushes varied significantly between years, post-fledging survival did not show any detectable inter-annual variation. Song Thrushes ringed as nestlings had a 0.38 chance (95% CI = 0.30-0.45) of surviving the 63-day post-fledging period (here defined as the period separating the average dates of birds ringed as nestlings and birds ringed as independent first-year birds).