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

Despite avid interest in life-history trade-offs and the costs of reproduction, evidence that increased parental allocation reduces subsequent breeding productivity is mixed. This uncertainty may be attributable to environmental heterogeneity in space and time, necessitating experiments across a range of ecological contexts. Over three breeding seasons, we cross-fostered clutches between nests to manipulate incubation duration in a wild population of Carolina wrens, a species in which only females incubate, to test for a cost of incubation on current and future reproduction. Prolonged incubation affected maternal productivity in a manner dependent upon the current environment and initial investment in eggs, suggesting incubation is optimized according to other components of reproduction and individual quality. Effects of incubation duration on foster-nestling condition varied between years, being costly in one, beneficial in another, and neutral in the third. The proportion of young fledged, females’ probability of breeding again within seasons, and subsequent clutch sizes all declined with increasing incubation effort, effects which became more pronounced as seasons progressed. Therefore, costs of incubation were almost entirely dependent upon maternal quality and environmental variation, illustrating the importance of conducting experiments across a range of environmental settings for understanding the costs of reproduction and evolution of life-histories.

Methods
Data were collected from wild Carolina wrens (Thryothorus ludovicianus) breeding at the Edward J. Meeman Biological field station in southwest TN, USA.

Data were standardized using z-scores and analyzed using SAS v. 9.4 (more info can be found in the attached Statistical Code and README files).