Pair formation among experimentally introduced mallards Anas platyrhynchos reflects habitat quality

Using data from two independent field experiments, we address whether pair formation in introduced mallards Anas platyrhynchos is associated with habitat quality, specifically food limitation at the brood stage. Based on the concentration of total phosphorous in the water, the study lakes were divided into two groups, 'poor' and 'rich'. In one of the experiments we used mallard ducklings imprinted on humans to study mass change of ducklings in poor and rich lakes, respectively. It turned out that ducklings foraging on poor lakes gained less mass than ducklings foraging on rich lakes, the division of lakes thus reflecting habitat quality at the brood stage. Introduced mallards formed heterosexual pairs on lakes that were, in a relative sense, high-quality brood habitats, whereas they did not on lakes of low-quality brood habitat. Pair formation thus seemed to reflect the suitability of habitat for breeding.