Song Thrush Turdus philomelos and Hawfinch Coccothraustes coccothraustes Exhibit Non-Random Nest Orientation in Dense Temperate Forest

Patterns of bird nest orientation in dense forest interiors are relatively unknown. Using data collected in primeval temperate forests at the Bialowieza National Park in Poland, we found that two arboreal nesting species, the Song Thrush Turdus philomelos and the Hawfinch Coccothraustes coccothraustes, located significantly more of their nests in the southern half of tree crowns than in the northern half. Nest orientation also depended on nest height: the higher Song Thrush and Hawfinch nests were in the crowns of trees, the stronger preference both species had for locating their nests toward the south. For the Song Thrush, tree species interacted with the date of clutch initiation to influence patterns of nest orientation: Song Thrushes located more of their nests on the ‘south-facing’ sides of live Norway Spruces Picea abies as the nesting season progressed. Spring temperature during the nest construction period affected Hawfinch nest orientation, with significantly more ‘south-facing’ nests during colder springs. The directional orientation patterns we observed may be viewed as a response of nesting birds to weather conditions during nest site selection and nest construction periods, when exposure to solar radiation could help regulate nest temperatures during cold mornings or days.