Social Mating System Divergence Between North and South Temperate Wrens

Abstract

The Nearctic and the Neotropic differ in several abiotic and biotic features that have profound implications in avian ecology. Several of these variables are predicted to affect social mating systems. The Sedge Wren (Cistothorus platensis) and the House Wren (Troglodytes aedon) are distributed from Canada to Cape Horn and are ideal species to evaluate the differences in social mating systems between hemispheres. We compared the social mating system, male contribution to feeding nestlings, clutch sizes, and number of nestlings fledged in north temperate and south temperate wrens using original data gathered in three study sites and published data from four additional sites. In the north, polygyny rates were higher and clutch sizes were larger than in the south. Furthermore, in the north polygyny was achieved when a male attracted an additional female to his territory while in the south, polygyny occurred when a monogamous male replaced a neighboring male and bred with the resident female. We did not find a clear association between parental care patterns and social mating system in the north. Although northern wrens had similar rates of social polygyny, northern Sedge Wren males contributed less to feeding nestlings than northern House Wrens males. Our results suggest social divergence between hemispheres and social convergence within hemispheres, probably caused by variation in underlying common factors. We suggest that differences in polygyny rates between hemispheres can be caused by a combination of migratory behavior and life-history strategies that affect the control and manipulation that males can exert over female reproduction.