Nest predation and the evolution of egg appearance in passerine birds in Europe and North America

Question: Does nest predation influence egg colour appearance among North American and European passerine birds? Analyses of species that lay their eggs directly on the ground provide support for such a contention, but it is less obvious in shrub and canopy nesters. We hypothesized that if predators locate nests by egg colour, nest predation could select for eggs with a less conspicuous background colour and more spots/markings so that they may achieve better camouflage. Thus, intra-clutch variation in egg appearance should be negatively related to rate of predation among passerines. Also, a reduction in intra-clutch variation would induce greater inter-clutch variation. Data studied: Data on egg appearance and nest predation rate, together with data on variables likely to affect the relationship between these two traits, such as suitability as hosts for brood parasites, level of brood parasitism, nesting habitat and nest location, were gathered for 23 European and 63 North American species of passerines, respectively. Search method: We controlled for similarity among species due to common descent by adopting the general method of comparative analysis for continuous variables that is based on multiple regression of statistically independent standardized linear contrasts. Conclusions: Egg appearance within clutches of open nesting passerines was explained by brood parasitic variables but was unrelated to nest predation in Europe. In contrast, neither nest predation nor avian brood parasitism explained variation in egg appearance of North American passerines. Globally, the lack of association between nest predation and egg appearance after accounting for the confounding roles of brood parasitism, nesting habitat and nest location suggested that egg coloration in open nesting passerine birds may be considered a neutral trait with regard to nest predation.