Effects of breeding density and plumage coloration on mate guarding and cuckoldry in blue grosbeaks (Passerina caerulea)

Abstract

For species in which females mate outside of the pair bond, selection should favor male mate-guarding behaviors that minimize investment in genetically unrelated offspring. Mate guarding may impose costs by diverting time and energy from activities such as foraging and seeking extra-pair copulations, so males should adjust their mate-guarding behavior according to the risk of cuckoldry. In this study, we investigated cuckoldry and mate guarding in the blue grosbeak (Passerina caerulea L., 1758), a socially monogamous, territorial songbird. Fifty-three percent of nestlings were extra-pair, and 70% of nests had at least one extra-pair fertilization. Males with dull plumage coloration were more likely to be cuckolded than males with more brightly colored plumage, but duller males did not guard their mates more. Instead, males appeared to adjust mate-guarding behavior according to the attractiveness of neighbors. Males with more brightly colored neighbors guarded their mates more intensely than males with fewer colorful neighbors. These observations suggest that in the blue grosbeak, mate guarding is a context-dependent strategy that is adjusted according to the density and ornamentation of neighboring males.