Nestbox availability affects extra-pair fertilizations and conspecific nest parasitism in eastern bluebirds, Sialia sialis

Abstract

Eastern bluebird pairs are usually monogamous, cooperating in caring for their offspring. However, alternative reproductive tactics including copulations with more than one partner and the laying of eggs in nests of conspecifics also occur. These alternative reproductive tactics could be adaptive when nest site availability varies, and different predictions have been made of who should employ these tactics, and when. Two experiments involving manipulation of nestboxes discriminated between these possibilities. The first experiment involved placement of nestboxes so that breeding pairs of bluebirds were relatively close together, a standard distance apart or relatively far apart. Nestlings non-descendant (measured using isozyme exclusion analysis) from one or both of their putative parents, the male and female care-givers, were least likely when nesting territories were relatively far apart and scarce, and increased significantly in experimental areas where nesting pairs were relatively close together and abundant, allowing the inference that neighbours were the source of non-directly descendant offspring. This is the first experimental evidence for birds demonstrating that individuals engage in genetically effective alternative reproductive tactics as a function of their opportunity to do so. The second experiment involved removal of nestboxes so that only two-thirds of known breeders had nesting sites. The rate of conspecific nest parasitism increased after removal providing the first experimental evidence for alternative reproduction by females when nest sites were not available. These two experiments were performed sequentially over two years. During these years extra-pair fertilization was significantly more frequent than conspecific nest parasitism.