Morphological shifts in the House Sparrows Passer domesticus of Helgoland: insular syndrome or founder effects?

Islands are natural laboratories for the study of evolution and help us to explain the patterns of diversification at large spatial scales. The colonisation of an island by a bird species inevitably involves divergence from the mainland mother population due to non-selective and selective evolution. On the island of Helgoland, House Sparrows Passer domesticus established a resident island population in 1958 which now numbers over 100 breeding pairs.We compared measurements of ringing data between Helgoland and two nearby mainland populations (length of 8th primary, bodymass) and found a significantly higher body mass and lower P8 length/ body mass ratio on the island. Further, we found that P8 length and bodymass significantly increased between the years 1989 and 2008 in Helgoland, indicating that the population changed in size towards larger individuals. This morphological shiftmight best be explained by the “insular syndrome” theory, which predicts increasing size of island birds as a result of reduced predation risk, reduced dispersal and increased intraspecific competition due to high population densities. On the other hand, founder effects could have added to the shift in the P8/ body mass ratio, assuming that the founder individuals yet showed this feature and the population had not much exchange with themainland since the colonisation in 1958. Future molecular analyses of the population structure may help to revealmore details on the isolation, colonisation process and geographical origin of the Helgoland House Sparrow population