Green plant material in avian nests

Some bird species place in their nests fresh green plant materials, e. g. leaves, sprigs or branches of herbs, shrubs and trees, which are not a part of the basic nest structure. These additional materials are often characterised by a high content of volatile secondary metabolites and constitute a non-random, small fraction of plants available in the habitat. Several non-mutually exclusive hypotheses have been proposed to explain the function of green material in avian nests with three of them attracting the most attention. The courtship hypothesis proposes that green material is used by males in order to attract females to nesting sites. The nest protection hypothesis posits that birds place in the nest green material rich in volatile compounds to reduce the abundance of parasites and pathogens which, in turn, should mitigate their negative impact on the host. According to the drug hypothesis green material directly positively affects the health and development of nestlings, e. g. through stimulation of some components of the nestlings' immune system. Here, we present an overview of hypotheses explaining the phenomenon of green material in avian nests with a thorough description of three most frequently tested hypotheses and suggest the directions for future studies.