Factors Affecting How Male and Female Bobolinks Apportion Parental Investments

Abstract

Male and female bobolinks (D. 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., 1st-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 faster delivery rates and larger numbers of food items delivered per trip for both sexes. In this 4-yr study conducted in Oregon [USA] male Bobolinks delivered .apprx. 60% of the food fed to older primary nestlings when weather was poor and food was scarce, .apprx. 50% when weather was good and food was scarce, and .apprx. 40% when weather was good and food was abundant. In the year when weather was good and food was scarce, males delivered .apprx. 70% of the food during the brooding period and .apprx. 50% thereafter. Trade-offs exist among brooding nestlings, feeding nestlings and self-maintenance. The relative value of investing in each of these activities probably differs between parents and changes according to weather conditions and food availability. Secondary (i.e., 2nd-mated) females did not compensate for reduced male parental assistance by stepping up food delivery rates, except in 1 yr 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.