Green plants reduce fitness in the spotless starling

Although several hypothesis have been proposed to explain the function of fresh green plants in nests of many avian species, the consequences that this behavior have on fitness remains unknown. In accordance with the nest protection and drug hypotheses the volatile components of the plants should improve the health of nestlings; hence it is expected that green plants increase fitness return of breeders. However, if green plants were used by males to attract mates (the courtship hypothesis) no clear greenery-fitness relationship, or even a negative one, would be expected. We test these predictions by experimentally increasing the amount of green plants of nests in a wild population of spotless starling (Sturnus unicolor) during four years of study. Greenery reduced the probability of recruitment of both males and females as expected by the courtship hypothesis. We also found antagonistic sex-specific effects of greenery on fledgling condition: an increased tarsus-metatarsus length in females and a decreased body mass of males. Our results indicate that green plant addition behaviour induces maternal responses with complex short- and long-term consequences on both early development of nestlings and their future fitness.