Hybridization in birds as studied by cross-fostering in the wild

Hybridization between species generally reduces fitness. However, hybridization, and subsequent back-crossing, may lead to the transfer of genes between species and represent an important factor as a source of new variability in the subsequent evolution o f the species. The importance of hybridization in vertebrates has been recognized relatively recently and the mechanisms leading to such pairings are still poorly understood. A majority of heterospecific pairings are unidirectional, viz. males of one spec ies are paired with females of the other species, but not vice versa. This is also found in our study populations of great tits Parus major and blue tits P. caeruleus, because most heterospecific pairings are between male great tit and female blue tit. We suggest a new hypothesis to account for such mixed pairings based on constraints of realizing preferred mate choice caused by competition. We will test the hypothesis by analysing the settlement pattern of the birds in time and space, and by experiment a l removal of dominant birds. We will also analyse the fitness consequences for birds engaging in heterospecific pairings. The results will provide new insights important to sexual selection theory, sexual imprinting, hybridization and speciation, and to c onservation of endangered birds.