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. Key words: black-throated blue warbler, breeding synchrony, extrapair paternity, mate guarding, microsatellites, time conflicts. [Behav Ecol 12:541–546 (2001)] n socially monogamous birds, females often copulate with males other than their social mates. Such extrapair copulations (EPC) can result in extrapair fertilizations (EPF), and thereby reduce the reproductive success of the cuckolded male. We therefore expect males to adopt behaviors that prevent their mates from engaging in EPC (Parker, 1970; Stockley, 1997). If, for instance, a male can remain near his mate and physically prevent other males from copulating with her, then such mate guarding could be strongly favored by selection. In a large number of species males maintain close proximity to their mates during the female’s fertile period (i.e., the period during which her eggs can be fertilized), and this behavior is often interpreted as mate guarding by males (Birkhead, 1998). The proposal that male birds guard their mates has generated some controversy, which centers on three points. First, alternative explanations have been proposed for the close association between males and females during the fertile period (Birkhead and Moller, 1992; Dickinson and Leonard, 1996; Gowaty and Pilssner, 1987). For example, males may be protecting their mates from predators (e.g., by acting as sentinels), and such protection may be most important during the egg laying period when female foraging demands are high. Of those studies that have examined patterns of male attendance to his mate (e.g., Dickinson and Leonard, 1996), most have concluded that males are indeed acting to prevent EPC (reviewed in Birkhead, 1998). Second, even if males do remain near their mates to prevent EPC, the effectiveness of this strategy has been called into question (Stutchbury and Neudorf, 1998). Time constraints