Ecological and Social Factors Affecting Hatchability of Eggs

Abstract

Hatchability, defined as the proportion of eggs surviving to the end of in- cubation that hatch, varies among populations of birds. Here, I examine the effects of a variety of variables on hatchability in a comparative analysis of 155 studies of 113 species. Of the ecological and geographical variables considered, latitude, diet, and nest type explain a significant amount of the variance in hatchability due to a significant increase in hatch- ability along a latitudinal gradient and a decrease in hatchability in carnivorous and hole- nesting species compared to herbivorous and open-nesting species. Of the variables related to sociality, all affect hatchability adversely as they increase along a sociality gradient (de- fined as the likelihood of increasing frequency of interactions with conspecifics), signifi- cantly so in the case of social organization and possibly incubation pattern. In addition, hatchability of eggs in nests of Acorn Woodpecker (Melanerpes formicivorus) groups con- taining more than one breeding male and/or female is significantly lower than that observed in groups with a single breeding individual of each sex. Although the causes of these trends in hatchability are unclear, these results provide comparative evidence to support the hy- pothesis that there is a direct, detectable reproductive cost to individuals pursuing strategies that are more social. Received 9 March 1981, accepted 11 February 1982. to exist, some of which can be correlated with both social and ecological factors. The corre- lations of the social variables with hatchability prompted a second analysis comparing hatch- ability among several subsets of a population of the cooperatively breeding Acorn Wood- pecker (Melanerpes formicivorus). Significant variability in hatchability is also shown to be present within this species.