Compensatory responses differ between parental tasks in a songbird species

In species with biparental care, the amount of care devoted to offspring is affected by the negotiation rules that the parents adopt. Theoretical models predict that biparental care can be evolutionarily stable if a decrease in parental investment by one parent is only partially compensated by its partner. However, empirical studies have found substantial variability in compensatory behaviour and have mainly used nesting provisioning as a single measure of parental effort. In this study, we investigated parental compensatory behaviour for two parental tasks, offspring provisioning and nest defence. These two tasks are likely characterized by different levels of risk as well as different cost and benefit functions for the parents, which may affect the expected level of compensatory responses. We experimentally widowed (by temporarily removing one parent) male or female Spanish sparrows, Passer hispaniolensis, and measured their compensatory responses to offspring provisioning and nest defence (after predator presentation of green whip snake, Hierophis viridiflavus, models). Parents differed in their compensatory responses in relation to parental task and sex: both sexes partially compensated for offspring provisioning, but females compensated by a larger degree than males. For predator defence, males instead decreased defence behaviour by increasing latency and reducing the number of attacks, while females did not change their behaviour when caring alone. This within-individual comparison indicates that parents adjust their compensatory behaviour according to parental task. We discuss how these differences could arise due to different costs and benefits of extra investments.