Hatching asynchrony in birds: Multiple nesting attempts and the nest failure hypothesis

Abstract

Hatching asynchrony in birds, which occurs when incubation begins before egg laying is complete, has been a topic of study for many decades. The "nest failure hypothesis" posits that the distribution of the risk of nest predation across the nesting cycle (from egg laying to the fledging of young) determines the optimal degree of hatching asynchrony. If such risk is higher earlier in the nesting cycle, then much asynchrony (e.g., incubation on the first egg laid) is generally favored. Alternatively, if the risk of nest predation is concentrated later in the cycle, then greater synchrony (a late start to incubation) is favored. Adult mortality during incubation favors greater hatching synchrony. Existing models suggest that optimal hatching asynchrony should depend on the ratio of daily mortality risks prehatching and posthatching, or early and late in the nesting cycle. We show that these classic theoretical expectations depend critically on the assumption of a single nesting attempt per breeding season. Allowing for multiple nesting attempts per season leads to a large range of possible outcomes, often with large deviations from the expectations of single-attempt models. We show further that daily nest survival probabilities across the entire nesting cycle combine to influence optimal hatching asynchrony, not just a subset of values, or a ratio of values, as suggested by earlier theory. In addition, assumptions about the transitions in daily survival values between nesting stages (abrupt vs. gradual changes) are also important determinants of hatching asynchrony. Overall, however, a consideration of multiple nesting attempts does not alter the general expectation that high risk early in the nesting cycle favors hatching asynchrony, and that greater synchrony is favored by high risk later in the cycle or adult mortality during incubation.