Showing papers by "Ruedi G. Nager published in 2001"

The cost of egg production: increased egg production reduces future fitness in gulls

Abstract: Measurements of costs of reproduction are essential for our understanding of the evolution of reproductive effort. While in birds the effects of increased chick-rearing effort on subsequent survival and fecundity have been relatively well studied experimentally, costs associated with increased egg-production effort have received relatively little attention. We experimentally increased the egg-production effort of individually marked Lesser Black-backed Gulls Larus fuscus and followed their breeding performance in the next year. In the season following increased egg production, females, but not males, were less likely to be resighted in the study plot and those that did return were less likely to produce a clutch compared to control birds. It is unclear whether the observed effect on local return rate represents differential survival, differences in breeding propensity or differences in dispersal between experimental and control females. In any event, all of these would adversely affect the fitness of experimental females. In addition, those experimental females that did breed invested less in egg production the following season, which again is likely to affect breeding performance. Thus, this study provides evidence that there is an inter-brood trade-off between current egg-production effort and future fitness in birds.

Increased reproductive effort results in male-biased offspring sex ratio: an experimental study in a species with reversed sexual size dimorphism

Abstract: Adaptive sex-ratio theory predicts that parents should overproduce the more beneficial offspring sex. Based on a recent experimental study of lesser black-backed gulls, we tested this hypothesis with the great skua, Catharacta skua, a bird species closely related to gulls but where females are the larger sex. When in poor body condition, the gulls overproduced daughters, the smaller and more viable sex under those circumstances. To discriminate between a mandatory physiological overproduction of female (i.e. non-male) eggs versus the overproduction of the smaller and presumably more viable sex, we conducted an egg-removal experiment with the great skua. Since the males are smaller, larger size and being male are separated. Through egg removal we induced females to increase egg production effort. Eggs were sexed using a DNA-based technique. Manipulated pairs produced a significant male bias at the end of the extended laying sequence, while the sex ratio in the control group did not differ from unity. Our results present an example of facultative sex-ratio manipulation and support the hypothesis that in sexually dimorphic birds parents overproduce the smaller sex under adverse conditions.