The quality of chicks and breeding output do not differ between first and replacement clutches in the Eagle Owl Bubo bubo

Abstract

Replacement clutches can be crucial to the breeding success of avian species, although se-veral factors may influence their outcome and the quality of chicks. Here we compare first and replacement clutches of Eagle Owls in terms of chick quality, timing of breeding, and number of eggs and fledglings. We hypothesize that the propensity to abandon nests and start a new clutch could have evolved because females are able to produce chicks that are as good as the ones of the first clutch. We found that nestlings in replacement clutches were not significantly different from nestlings in first clutches, in terms of immune system and body condition; nor were there significant differences in clutch and brood size. Our findings may be explained by the fact that clutches were laid early in the season, perhaps by high-quality parents, and that food resources are plentiful in the study area. In order to maximize fitness, individuals have to adjust optimally the investment of resources in dif-ferent life-history traits. Thus, the re-nesting behaviour of Eagle Owls may be seen as a way to solve the trade-off between one trait (survival) favoured over another (reproduc-tion), when one of them has a disproportionate effect on fitness.