EFFECTS OF FOOD ABUNDANCE AND MALE PARENTAL CARE ON REPRODUCTIVE SUCCESS AND MONOGAMY IN TREE SWALLOWS

Abstract

We examined the roles of food abundance and male parental care in the main- tenance of monogamy in Tree Swallows (Tachycineta bicolor). Mated, male Tree Swallows were removed from territories to simulate the lack of male parental care that would be incurred by secondary females. Removals took place in lakeshore and roadside habitats in central Alberta, Canada. Insect sampling over three breeding seasons indicated that the lakeshore had greater biomass of insects than the roadside during egg laying, but there was no difference during the nestling period. Both our male removal experiment and natural cases of polygyny suggested that loss of male parental care had little effect on reproductive success or survival of females or their offspring. Unaided females increased their per-capita nest-visit rate in two of three years, so the rate of visitation per nestling did not differ significantly between unaided females and both the male and female at control nests. Control females at the lakeshore produced more fledglings than control females at the roadside. This difference in productivity was due to earlier laying and larger clutches at the lakeshore than at the roadside and not to differences in brood reduction or insect abundance during the nestling period. In both habitats, male parental care was relatively unimportant to female reproductive success and, therefore, within a given habitat secondary females did not incur a reproductive cost. All natural cases of polygyny occurred at the lakeshore. We suggest that polygyny occurred at the lakeshore, and not at the roadside, because food abundance was greater during laying at the lakeshore, and this allowed secondary females to lay more eggs and produce more fledglings than females mated to monogamous males at the roadside. Received 11 March 1991, accepted 10 February 1992.