Sibling resemblance in natal dispersal distance and direction in the Ortolan Bunting Emberiza hortulana

Natal dispersal distance and direction determine the likelihood that siblings will settle close together and hence the risk of inbreeding. Several studies have shown a sibling resemblance in dispersal distance, but few studies have analysed sibling resemblance in dispersal direction or the distance between siblings after dispersal at the landscape level. I studied the entire Norwegian population of Ortolan Buntings Emberiza hortulana, which is patchily distributed in an area covering c. 500 km2. Males and females did not differ in dispersal distance (overall median 3.7 km), but directions were different. Natal dispersal distances and directions were positively related within sibling pairs, but comparisons with control individuals suggested that any effects were due to spatial effects of configuration of habitat patches in the study area. Brother-sister pairs (n = 16) were at least as similar as brother-brother pairs (n = 18). Distance between siblings after natal dispersal increased with dispersal distance, but even so, five of 35 sibling pairs settled < 1 km apart, despite dispersal distances of 8.3-9.9 km for two of these pairs. Including movements later in life, eight sibling pairs were < 1 km apart at some time (four pairs of brothers and four brother-sister pairs), and in one case a brother mated with its sister. Another case of mating between close relatives (father and daughter) involved short female natal dispersal. These data indicate that female-biased natal dispersal and long-distance dispersal may reduce, but do not exclude, the possibility of inbreeding.