Testing hypotheses for the evolution of long-term monogamy in Neotropical Buff-breasted Wrens (Cantorchilus leucotis)

Abstract

Over 90% of birds breed in socially monogamous partnerships. Among monogamous passerines, species vary from those with extra-pair copulations and short-term partnerships to others with exclusive and long-term partnerships. Study of long-term monogamy in passerines has lagged behind other forms of monogamy, and the factors favouring this extreme mating system are unclear. I investigated five hypotheses for long-term monogamy (LTM) in Neotropical Buff-breasted Wrens (Cantorchilus leucotis). Familiarity with breeding partners did not increase reproductive success relative to naive pairs, which is inconsistent with the mate familiarity hypothesis. Experienced females, and possibly males, had greater reproductive success than those breeding for the first time, supporting the hypothesis that LTM may be favoured by improved reproductive performance of individuals as they gain breeding experience. The low resource availability hypothesis was supported as territory availability was low, mates were rapidly replaced, and searching for alternative territories was costly, as birds did not always re-acquire territories following experimental removals. Evidence for territorial defence hypotheses was mixed, as pair members coordinated aggressive responses to intruders while simultaneously exhibiting sex-specific aggression. These results suggest fitness advantages, low resource availability and selection for resource monopolization by males and females may have favoured long-term monogamy in Buff-breasted Wrens.