Dispersal in Willow Ptarmigan Lagopus lagopus-who is Dispersing and Why?

The present study evaluates dispersal among breeding willow ptarmigan Lagopus lagopus in five closely located subareas in central Norway. During a six-year period, breeding territorial ptarmigan were live-trapped, aged, sexed and genotyped. Since it is believed that female willow ptarmigan are the dispersing sex, and foreign intruders are expected to be more genetically different than birds from the natal subarea, it is predicted that the genetic similarity between breeding birds of a given year (t) and the breeders in the same territories the next year (t+1) would be greater for males than for females. The extent of dispersal is believed to be positively correlated with the intruder pressure. Thus, the genetic similarity outlined above is predicted to be greatest in comparisons where chick production in year l was lowest. Analysis of genetic similarity among territorial birds in two consecutive breeding seasons revealed no significant difference between males and females. However, males showed greater genetic variability between years than females. For both yearling and adult males, the increased variation coincided with changes in the population density. Hence, a significant relationship between genetic similarity and chick production for both yearling and adult males was observed. This may suggest density-dependent dispersal for males, but not for females. The fact that there was a stronger relationship between genetic similarity and natal chick production, than between genetic similarity and mean chick production on all subareas, for both adult sexes, suggests that breeding adults are more often challenged by yearlings from their own subarea than by foreign intruders from neighbouring areas.