ENVIRONMENTAL AND GENETIC-DETERMINANTS OF REPRODUCTION IN THE HOUSE SPARROW - A TRANSPLANT EXPERIMENT

A transplant experiment was carried out to examine whether genetic dif ferentiation can explain geographical variation in the reproductive st rategies of house sparrows Passer domesticus. Individuals from an inla nd and a coastal population in Central Norway were released on a small island, near the coastal area. No directional selection was found on any of the morphological characters from the release to the onset of b reeding, but the proportion of the inland males that remained to start breeding on the island was smaller than that of the coastal males. Th e new environment influenced the time of egg laying which was, relativ e to the source populations, more delayed among the introduced inland females than among the coastal females. In 1992, chicks raised by inla nd females grew faster and were fed more frequently than chicks raised by coastal females. No difference was found between birds of inland a nd coastal origins in their breeding success and their relative number of surviving recruits. Transplanted parents from the inland fed their offspring more frequently than transplanted parents from the coastal area. This experiment shows that the plasticity of reproductive traits in combination with stochastic factors in the environment may lead to an establishment of introduced genes in small populations.