Factors Affecting How Male and Female Bobolinks Apportion Parental Investments

Abstract

Male and female Bobolinks (Dolichonyx oryzivorus) adjust the amount of time spent brooding and provisioning nestlings at primary nests in light of prevailing weather conditions and food availability. Males brood nestlings less and feed them more (mg/h) when food is scarce, regardless of weather conditions. Primary (i.e., first-mated) females brood nestlings more and maintain feeding rates when food is scarce and weather is stormy. When food is scarce and weather is good, primary females brood nestlings less and feed them more. Increased provisioning results from both faster delivery rates and larger numbers of food items delivered per trip for both sexes. In this 4-year study conducted in Oregon, male Bobolinks delivered about 60% of the food fed to older primary nestlings when weather was poor and food was scarce, about 50% when weather was good and food was scarce, and about 40% when weather was good and food was abundant. In the year when weather was good and food was scarce, males delivered about 70% of the food during the brooding period and about 50% thereafter. The results suggest that trade-offs exist among brooding nestlings, feeding nestlings, and self-maintenance. They imply that the relative value of investing in each of these activities differs between parents and changes according to weather conditions and food availability. Secondary (i.e., second-mated) females did not compensate for reduced male parental assistance by stepping up food delivery rates, except in one year when food was abundant and weather was good. Their cost of losing male parental help was substantial, especially in years when food was scarce and/or weather was poor.