Cross-fostering reveals sources of variation in innate immunity and hematocrit in nestling tree swallows Tachycineta bicolor

Innate immunity plays an important role in protecting birds early in development. The strength of innate immunity in nestlings appears to be affected by heritable and early maternal effects, as well as later environmental effects. To explore factors affecting innate immunity, we conducted a partial cross-fostering experiment in nestling tree swallows Tachycineta bicolor. The ability of nestling blood plasma to kill Escherichia coli in vitro was influenced by heritable or early maternal effects and was unrelated to body condition of nestlings or brood size of the foster nest. The relative importance of heritable and early maternal effects versus environmental effects on nestling hematocrit (packed volume of red blood cells) and body condition were also examined. Hematocrit was significantly influenced by the nest in which nestlings were reared, though it was not related to nestling body condition. Body condition was significantly influenced by the nest in which nestlings were reared, though it was also weakly influenced by heritable or early maternal effects. In summary, heritable and early maternal effects are stronger predictors than rearing conditions of innate immunity in nestling tree swallows. Furthermore, hematocrit is largely environmentally determined but is not related to body condition or innate immunity, as measured in this study.