Individual optimization of reproductive investment and the cost of incubation in a wild songbird

Despite keen interest in life-history trade-offs and the costs of reproduction, evidence that increased parental allocation reduces subsequent breeding productivity is mixed. Some of this uncertainty is attributable to environmental heterogeneity in space and time, necessitating multi-year field experiments. Across three breeding seasons, we cross-fostered clutches between nests to manipulate females' incubation duration in a wild population of Carolina wrens, a small songbird species in which only females incubate, to test for the cost of incubation on current and future reproduction. Prolonged incubation affected maternal productivity within seasons, impacting their current offspring post-hatching and their probability of breeding subsequently, both in a manner that depended upon the current environment and level of maternal investment (initial egg and brood size), suggesting incubation effort is optimized according to other components of investment and individual quality. Effects of incubation duration on subsequent fledging success and nestling condition varied between years, being costly in one year, beneficial in another, and neutral in the third. Thus, our results suggest that the costs of increased incubation effort were almost entirely dependent upon the maternal quality and environmental variation, thus demonstrating the importance of multi-year experiments for robust inference of the costs of reproduction and evolution of life histories.