Dominance, access to colonies, and queues for mating opportunities by male boat-tailed grackles

Abstract

Species characterized by female-defense polygyny have extreme variance in male mating success. Many studies have considered alternative male strategies for access to females, but few have considered age-specific strategies. Male boat-tailed grackles (Quiscalus major) compete for access to colonies of females and form linear dominance hierarchies. I observed two groups of males that competed for females at seven colonies. Dominance rank was significantly correlated with mass, but not after controlling for age. In contrast, the correlation between dominance rank and age remained significant after controlling for mass. Older males dominated younger males and dominance relationships were very stable. Thus, dominance hierarchies represent queues for mating opportunities. A male's rank in the hierarchy determined how closely he approached a colony. Furthermore, males of all ranks prevented lower-ranked individuals from approaching the colonies. Dominance rank thus determined access to nesting females. One top-ranking male's loss of mass over the course of the breeding season presumably reflected the energetic cost of defending females, but he maintained his position in the hierarchy despite the small loss of mass. One alpha male held a colony for at least 4 years, and the ages of males from two queues indicated that males wait 6 or more years before becoming an alpha male. Therefore, most males die before acquiring a colony of females. Spatial structure such as that documented here could obscure recognition of queues and explain why they have not been documented in more species.