In the last 20 years, a westward expansion of the White-winged Tern (Chlidonias leucopterus) has been reported in Europe. Even though this species has become a regular breeder in some places, we lack basic information on the choice of breeding sites. The aim of the present work is to compare the habitat preferences of two closely-related species: the expansive White-winged Tern and the Black Tern (Chlidonias niger), which has been breeding in this part of Poland for a long time. The data were gathered in east-central Poland from late May to early July 2013. All potential tern nesting areas were searched over 40,000 km2, river valleys in particular. There were 41 colonies of White-winged Terns with a total number of 1,348 pairs and 55 colonies of Black Terns with 666 pairs. A Generalized Linear Model (GLZ) showed differences between both tern species in habitat parameters. However, only two predictors—area of open habitat (meadows, arable fields) and habitat type (ox-bow lakes vs. waterlogged sedge fields)—were significant. White-winged Terns located their colonies in waterlogged sedge fields three times more often than Black Terns did, while the latter nested three times more frequently on ox-bow lakes. There was also a statistically significant greater proportion of open habitats around the colonies of White-winged Terns than around those of Black Terns. The colonies of the two species were roughly the same size in all habitats. This study showed that habitat type was the basic factor governing the choice of nesting site by these two closely-related tern species.
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