Green plants in nests reduce offspring recruitment rates in the spotless starling

摘要

Greenery added to birds' nests is a sexually selected trait of males that entails costs for females. Why some birds add green plants to their nests has long puzzled biologists. Some studies suggest that this behavior is promoted by natural selection because of its positive effects on nestling health. However, if greenery functions as a sexually selected signal of male attractiveness, it may promote costly female competition for preferred males. We experimentally increased the amount of plants in spotless starling's nests, causing a reduction in the number of recruits.Although several hypotheses have been proposed to explain the function of fresh green plants in nests of many avian species, the consequences of this behavior on fitness remain poorly understood. In accordance with the nest protection and drug hypotheses, adding greenery to nests should be naturally selected to facilitate positive fitness effects on offspring. Alternatively, the courtship hypothesis postulates that green plants function as a sexually selected signal of male attractiveness that might promote female competition for preferred males. We conducted a long-term study by experimentally increasing the amount of green plants in nests of a wild population of spotless starlings (italic toggle="yes">Sturnus unicolor). We experimentally decoupled the natural carrying behavior of males in half of the population and explored the consequences of this manipulation on offspring condition and their rate of local recruitment. Treatment did not affect the number of fledglings, but it did affect body mass and tarsus in a sexually antagonistic way: a positive effect on female tarsus length and negative on male body mass. The addition of greenery reduced the probability of local recruitment of both males and females. Our results suggest that the addition of green plants induces maternal response with complex short- and long-term consequences on the offspring body condition and recruitment success.