The Evolution of Clutch Size and Hatching Asynchrony in Altricial Birds: The Effect of Environmental Variability, Egg Failure and Predation

A simple mathematical model is presented, in which hatching asynchrony, unpredictable offspring losses due to egg inviability, predation rate, costs of brood reduction and environmental variability are considered together, in order to evaluate their joint effect and relative significance in the evolution of the clutch size in a fluctuating environment. The model shows, when laying additional eggs to track environmental variability and/or insure against egg inviability is of selective advantage. Environmental variability seems to be a key factor governing the pay-off of brood size adjustment. However, in contrast to Lack's "brood reduction" hypothesis, asynchronous hatching ensures not only the establishment of competitive asymmetries among chicks, but also significantly reduces the impact of predation on fitness of birds. The model predicts that laying surplus eggs may evolve when the daily risk of predation is high, provided that the extra eggs hatch asynchronously. Laying additional eggs seems especially beneficial when the probability of hatching failure is high. This suggests, that the addition of surplus eggs to the clutch to track environmental fluctuations from one hand, and to insure against hatching failure from the other hand, seem likely to act jointly to favor larger clutches. The model also predicts that even if costs of brood reduction exist, asynchronous laying of at least one surplus egg should usually be beneficial.