Variation in marsh tit recruitment and survival creates gender imbalance

Our population of 18-31 Marsh Tit territories in eastern England had a gender imbalance in every spring. Up to 28% of potential breeding territories (mean 10%, minimum 3%) were occupied by lone males only, reducing potential productivity. Adult annual survival was similar for both sexes, but most females disappeared during spring. Juvenile settling and survival tended towards a male bias, but female-biased spring immigration was too little to balance the breeding population. This schematic shows how variation in mortality and recruitment at key stages of the annual cycle creates this gender imbalance.