Breeding strategy and morphological characters in an urban population of blackbirds, Turdus merula

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.