Breeding performance of the White-winged Tern (Chlidonias leucopterus) in two habitat types at the edge of their distribution range

Abstrakti

Peripheral populations that are expanding their distribution often colonise habitats that are different from the centre of their distribution range. These less optimal areas could have an advers effect on their breeding success. Consequently, the production of individuals in these populations must be high if the expansion is to continue. The White-winged Tern (Chlidonias leucopterus) is a good model for studying breeding parameters in peripheral populations, because in the last 20 years, a westward expansion of this tern has been reported in east-central Poland. We evaluate the breeding success of White-winged Tern's in thwo habitats. Data from the centre of this species' distribution range indicates that waterlogged sedges are the typical nesting habitat, whereas ox-bow lakes ar less frequently occupied. However, clutch size and the mean volyme of eggs in a clutch were not explained by the habitat type. The hatching success was significantly higher and the number of hatchlings per pair of breeding terns was also higher (close to significance) in waterlogged sedges than on ox-bow lakes. the causes of nest losses differed by habitat type too. While there was no predation on the nests situated on ox-bow lakes, 30.8% of clutch losses in waterlogged sedges were due to predation. In contrast, 50.0% of clutches on ox-bow lakes failed because the nests sank into the water, whereas in waterlogged sedges this was the cause of just 1.9% of all brood losses. Waterlogged sedges are the preferred habitat, offering safer nesting sites than ox-bow lakes for terns nesting at the edge of their distribution range in east-central Poland.