Evolution of monogamy in seaside sparrows, Ammodramus maritimus: tests of hypotheses

Seaside sparrows (Ammodramus maritimus) are monogamous throughout their range. We used male-removal experiments, measurements of territory quality and nest placement in relation to territory boundaries, and observations on foraging behaviour and on intraspecific aggression to assess the applicability of pertinent hypotheses that could explain the occurrence of monogamy in this species. Since experimentally widowed females can rear about two-thirds as many young to independence as pairs, made help is advantageous but not necessary for female reproductive success. Female-female aggression is probably not important in maintaining monogamy, but we need to learn more about it. We induced experimental bigamy in seaside sparrows in New York, thus showing that males will accept second mates. Territory quality (suitable cover, nest-sites and food) is very heterogeneous in the sparrow populations. However, ‘polygyny thresholds’ evidently are not exceeded either because resources on salt marshes are not limiting (cover, nest-sites) or because females can nullify the effects of resource (food) disparity between territories by feeding at distant sites outside their mates' territories.