The Long-Term Pair Bond of Tropical House Wrens: Advantage or Constraint?

Permanent monogamy may be the most prevalent mating system in birds because most birds are tropical residents found as pairs in territories that are defended throughout the year. Long-term pair bonds may be advantageous to individuals with respect to current and future reproductive success. Conversely, each member may be constrained to remain with the current mate because of limited opportunities to acquire new mates. These alternative hypotheses were evaluated for permanently monogamous tropical house wrens in lowland central Panama. Mates gain no reproductive advantage by reuniting from one breeding season to the next or by reuniting within breeding seasons. In contrast, the scarcity of openings within the stable breeding population limits the opportunities for each sex to re-sort. Moreover, pairs must guard against encroachment by neighboring pairs and takeover by non-neighboring pairs. The same constraints that shape the long-term pair bond appear to influence the structure of the floater population, which includes transient pairs as well as individuals. Mates usually do not defend each other against single intruders, even when takeovers occur, suggesting that mate choice is a recurring phenomenon even in this permanently monogamous mating system. These results probably pertain to other tropical monogamous species with long-term pair bonds, but detailed studies of other species are required to establish generality.