The function of green plants in nests of European starlings (Sturnus vulgaris)

European starlings add fresh green plants to their dry nest material. Male starlings of our 60-nest-box colony carried 68 different plant species into their nests. Some males were polygynous and had 3-6 clutches, others were monogynous and had 1-2 clutches per reproductive season. The 'nest protection hypothesis' proposes that insecticidal compounds in green plants reduce the parasite load of the nests. The 'courtship hypothesis' predicts that carrying nest greenery is a courtship activity to attract females. The aim of this study was to collect field data suitable for distinguishing these two hypotheses. 1. Some plant species occurred more often in the nest-boxes than expected from their frequency in the nest-box environment. A significant number of these preferred plants were rich in volatiles, some of which are said to be insecticidal. But volatiles could also attract females and/or influence their breeding activity and the chicks' development directly. 2. The males carried greenery into their nest boxes maximal around 5 days before the onset of laying, when pair formation took place, and ceased this behaviour with egg deposition. The total amount of greenery deposited in a nest-box was a function of the number of days of courtship a male needed to attract a female. 3. Polygynous males deposited the same amount of greenery in their first nest as monogynous males. In additional nests polygynous males deposited more greenery. However, this was due to the fact that these additional nests were advertised for a longer time. 4. Nest-boxes used again for replacement broods (after clutch loss) or second broods (after fledging of the Ist brood) in May contained fewer green plants than newly occupied nest-boxes at the same time, although according to the nest protection hypothesis re-used nests should contain more parasites. 5. Apart from green plants males also carried 'ornaments' such as flowers, pieces of bark, lichens, large feathers and human artifacts into their nests. There was a tendency of polygynous males to have a higher proportion of ornaments in their nests than monogynous males.