Fighter-pilot parents: relationships of nest defense with species, nest stage, and brood quality

Abstract Predation is a major source of nestling mortality for many songbird species. Parental investment theory predicts that parents should defend nests based on offspring survival and recruitment potential and weigh current investment in nest defense against costs to their own survival and future reproductive opportunities, which may vary across species and be constrained by other life-history traits. We explored the relationships between parental investment in nest defense behavior and offspring age, number, and quality in eastern bluebirds (Sialia sialis) and tree swallows (Tachycineta bicolor), two species that breed concurrently in the same habitats and exhibit similar nest defense behavior but differ in their within-season re-nesting potential. We observed diving, alarm calling, and beak snapping by parents in response to repeated human visits to the nest box from laying through fledging over five consecutive breeding seasons and also measured nestling weight and size to assess quality over the later three breeding seasons. Tree swallows invested more in nest defense than bluebirds, and both species increased defense investment as nests aged. However, we found no association between defense and nestling quality or brood size. We discuss these findings in light of predictions stemming from parental investment theory as well as several previously proposed alternative hypotheses and suggest that the relationships between parental defense and fitness benefits for parents may be more subtle and complex than initially outlined by parental investment theory’s predictions.