Juvenile survival in common loons Gavia immer: effects of natal lake size and pH

Survival is a vexing parameter to measure in many young birds because of dispersal and delayed impacts of natal rearing conditions on fitness. Drawing upon marking and resighting records from an 18‐yr study of territorial behavior, we used Cormack–Jolly–Seber analysis with Program MARK to estimate juvenile survival and its predictors in a population of common loons Gavia immer. In addition, we investigated predictors of chick mass, survival and inter‐sibling size disparity in two‐chick broods. Both small size and low pH of natal lakes predicted poor survival among chicks and juveniles; thus, features of the natal environment have both immediate and lasting effects on fitness. The pH × stage interaction retained in our MARK models indicates that the detrimental impact of lake chemistry on fitness diminishes with time; the retention of pH × lake size as a predictor of chick mass and condition pinpoints small lakes as those where acidity impacts chicks most severely. Our adjusted estimate of 0.53 probability of for survival to age 3 suggests that loon populations are healthier than often supposed.