Examining the genetic influences of migratory status on island colonisation by naturally and recently established populations of a bird species

Seasonal migration influences long-distance dispersal and population establishment, which are the two important steps for successful island colonisation. Migratory behaviour facilitates the long-distance dispersal of individuals through frequent appearance outside a species9 normal range, while sedentary behaviour may lead to dispersal by post-fledgling flocks whose accidental immigration provide sufficient propagules for population establishment. We assessed the relative contributions of migratory and sedentary populations to island colonisation, by performing population genetics analyses on four recently and naturally established island populations of the Bull-headed Shrike (Lanius bucephlaus). Analyses using 15 microsatellite loci inferred colonisation by flocks of immigrants from possibly sedentary populations for all the islands studied. On one island, we found a correlation between wing length and the genetic probability of assignment to a migratory population, indicating a genetic contribution by migratory immigrants. Yet migratory immigrants to this island were inferred to be only two over eight years of sampling. We imply that sedentary populations were successful island colonists because more immigrants can be provided through their post-fledgling dispersal than through solitary migration. This is the first attempt to examine, using population genetics, influence of migratory behaviour on island colonisation thereby, bridging the gap between population ecology and island biogeography.