The effectiveness of mate guarding by male black-throated blue warblers

Abstract

In many socially monogamous birds, males maintain close proximity to their mates during the fertile period. This is often considered an effort on the male's part to prevent other males from copulating with his mate, but other functions have been suggested and the effectiveness of males in preventing extrapair fertilizations has come into question. Moreover, it is unclear whether mate guarding conflicts with other male activities, particularly the pursuit of extrapair fertilizations. We examined mate guarding by male black-throated blue warblers (Dendroica caerulescens). Behavioral observations showed that males that guarded their mates more closely were less likely to have extrapair young in their nests. Moreover, the experimental detention of a male for 1 h during the fertility risk period increased the probability that a brood would contain extrapair young. Thus, male mate guarding was effective in reducing the risk of extrapair fertilization. Males with many opportunities for extrapair copulations appeared to guard their mates less and consequently had more extrapair young in their broods than males with few such opportunities. This suggests that mate guarding may conflict with the pursuit of extrapair fertilizations.