Differences in nest site characteristics and hatching success in White-winged Tern (Chlidonias leucopterus) and Black Tern (Chlidonias niger)

Nests should be built on sites that provide optimal conditions for reproduction, so nest site characteristics are assumed to have an adaptive value. In this paper, we compared the hatching success of two species of terns: Black Tern (Chlidonias niger) (a non-expanding species) and White-winged Tern (Chlidonias leucopterus) (an expanding species, new to the region since 1997) with respect to microhabitat nest characteristics on oxbow lakes in eastern Poland. The hatching success of White-winged Tern was lower than that of Black Tern (41.7 vs. 66.7%). Comparison of habitat parameters at the nesting sites between the two tern species showed a significantly greater depth of water and distance from helophytes in White-winged Tern. Successfully hatched White-winged Tern clutches were situated much closer to the helophytes and were initiated on average up to 8 days earlier than clutches that failed to hatch. In Black Tern, hatching success was not influenced by environmental factors describing nest location. In eastern Poland, oxbow lakes are probably a suboptimal habitat and constitute an “ecological trap” for White-winged Tern; because of these suboptimal habitats, the expansion of this species may be quite slow in predominantly dry years in eastern Poland.