Scandinavian and central European subspecies of White-throated Dipper Cinclus cinclus interbreed in an isolated population in northern Poland

Abstract

Breeding ranges of Scandinavian and central European subspecies of the White-throated Dipper (Cinclus cinclus cinclus and C. c. aquaticus) are geographically separated by uninhabited lowlands. Between 1990 and 2006, 25 confirmed and 4 probable breeding cases of 23 individual dipper pairs were recorded in northern Poland. Dippers breeding in Northern Poland represented the Scandinavian cinclus phenotype in most cases (76% of males and 89% of females), while a minority phenotypically resembled the Carpathian aquaticus (24% and 12%, respectively). Offspring of a phenotypically mixed pair two males from the same brood displayed an intermediate phenotype. Ringing recoveries confirmed that immigrants from both ranges breed in N Poland. In 15 out of 23 breeding pairs, both mates represented the cinclus phenotype, one breeding attempt was by a pure aquaticus pair, and interbreeding pairs (cinclus × aquaticus) occurred in 7 out of 23 cases. Most breeding attempts were successful, and pair composition had only little if any effect on breeding success. A single individual hatched in Northern Poland was subsequently found breeding in Sweden. Out-of-range populations, such as the studied Northern Polish one supported by individuals from allopatric ranges, may promote the gene flow between allopatric subpopulations, contributing to the high morphological variability and the reported low genetic differentiation between the two studied subspecies.