Long-term population monitoring reveals no condition-dependence but an agerelated

Abstract

The sexually selected eggshell colouration hypothesis proposes that conspicuous eggshell colours of many avian species (like the blue-green pigmentation caused by biliverdin) have evolved as post-mating sexual signals. A main prediction of this hypothesis is that the intensity of eggshell coloration reliably reflects female quality, therefore allowing males to modulate their parental care. In this study we used a large dataset resulting from a longterm monitoring of a spotless starling (Sturnus unicolor) population to study the patterns of condition-dependence in eggshell colour. We tested both the effect of pre-laying female condition and female condition as nestling, to explore the potential delayed effects of her early-life environment. We also carried out a longitudinal analysis to explore agedependent changes in egg colour. We found no evidence that eggshell colouration was affected by pre-laying female body condition. Similarly, we did not detect an effect of female natal condition on adult egg colouration. However, we found a moderate uniform decrease in eggshell colour as females aged, which is not the expected behaviour of a sexual signal. Thus, if eggshell colouration is an indicator of female quality as proposed, it should mirror other components of female quality more closely connected to biliverdin metabolism. The age-related pattern in egg colour found would be consistent with such mechanistic implication of oxidative stress.