The Pursuit of Extra-pair Copulations by Female Birds: A New Hypothesis of Colony Formation

A previously unrecognized selective pressure is proposed here which may contribute to the formation of bird colonies: the pursuit of extra-pair copulations (EPCs) by females. The "EPC hypothesis" employs the mechanisms of models of lek evolution which explain how males in promiscuous species cluster as a result of females pursuing copulations. It is suggested here that the mechanisms which have produced leks also operate when monogamous female birds pursue extra-pair copulations. The consequent clustering of males and females may contribute to the formation of colonies. While other hypotheses of coloniality stress advantages which accrue mainly after egg-laying, such as enhanced food-finding and reduced predation, the EPC hypothesis explains how and why individuals in colonies cluster prior to egg-laying. Once clustering occurs, the stage is set for subsequent advantages of high density breeding to accrue.