Natural selection is a central tenet of evolutionary theory, yet the estimation of the direction and intensity of selection remains problematic. Here, we assess the strength of selection on the early expression of a secondary sexual ornament, bill colour, in male European blackbirds (Turdus merula) using 5 years of capture-mark-recapture (CMR) data. The best-fitting model consisted of a quadratic relationship between survival rate and bill colour, indicating stabilizing natural selection on the early expression of a secondary sexual trait. There was no evidence for sexual selection acting on bill colour in the first year. We suggest that the consideration of early selection and the adoption of refined statistical methods may reveal patterns of selection in the wild that have, as yet, remained undetected.

Seasonal variations in microbial diversity and metabolite profiles of the gut of sea cucumber (Apostichopus japonicus),

The level of expression of secondary sexual characters has been suggested to signal male ability to resist parasitic infestations. To test this idea, several studies have examined the link between sexual signals and immunocompetence in birds. However, most of them have used only a single aspect of immune response to evaluate immunocompetence. We investigated the relation between bill colour and immunocompetence in captive male European blackbirds, Turdus merula, during the breeding season by assessing both cell-mediated and humoral components of the immune system. The blackbird is a sexually dimorphic species with bill colour varying from yellow to orange in males. Humoral immunity was assessed by measuring both primary and secondary responses to sheep red blood cell inoculation. Cell-mediated immunity was estimated with a delayed cutaneous hypersensitivity response to an injection of a mitogen (phytohaemagglutinin). No relation was found between male bill colour and the primary humoral response. However, males with orange bills showed a lower secondary humoral response but a higher cell-mediated immune response than males with yellow bills. Thus, the relation between immunocompetence and a secondary sexual trait may differ markedly depending on which component of the immune system is under consideration. We discuss our results in relation to mechanisms involved in sexual selection. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour

In many bird species, males may show brightly coloured traits and variance in male mating success may be explained by female preference and/or competition between males favouring the most coloured males. Male beak coloration has been suggested to play an important role in the pairing pattern of European blackbirds. Here, we investigate female preference and male-male interactions in relation to male beak coloration in this species. We used a field experiment to measure female and male responses toward stuffed decoys showing either of two beak coloration representing the extremes of the natural variation from yellow to orange. Decoys were situated on the centre of plots and behaviours of males and females approaching the decoy were recorded. The total number of males and females approaching the decoy did not differ between the two model treatments. Similarly, there was no difference in the mean time that males and females spent near the decoy. The number of males displaying aggressive postures did not differ between treatments and no female adopted a sexual posture. We discuss the relevance of our results and potential limitations associated with the experimental procedure.

Pairing patterns in monogamous birds are thought to be the consequence of mutual mate choice by males and females for characters linked to individual quality in the opposite sex. Although this may result in assortative pairing for a single character, there may be no simple match between male and female phenotype if the traits linked to individual quality are sex specific. For instance, in several passerine species, females tend to select males on the basis of territory quality or characters that may reflect health or vigour, whereas males benefit from pairing with females in good condition that breed early because early breeding increases reproductive success. We investigated pairing patterns in an urban population of the sexually dimorphic European blackbird in relation to bill colour in males and body condition in females, over a 3-year period. Females breeding early were in significantly better condition than those breeding late. The number of breeding attempts per season was significantly positively correlated with female condition, while the number of fledglings reared by a female in a breeding season was positively correlated with the number of breeding attempts, independently of female condition. Bill colour varied in males and was related to body weight, although not to condition. Female condition was correlated with male bill colour within pairs, with males showing more orange bills being paired to females in better condition. We discuss the observed pairing pattern in relation to individual or territory quality and to the constraints and costs associated with reproduction in each sex.

Over the recent years the role of females in maintaining or breaking the pair-bond in socially monogamous birds has received growing attention. Here, we review the overall evidence for a direct and predominant role of female behaviour in initiating or preventing divorce and its relevance for the understanding of both interspecific and intraspecific variation in divorce rate in monogamous bird species. The evidence is so far limited to a few species. We discuss the relevance of some alternative explanations and confounding factors. We conclude that the possible predominance of females in both initiating divorce or limiting its occurrence deserves further consideration. Future studies should favour experimental approaches, such as mate-removal experiments.