In a population of doves in a forest a small group of long tailed doves migrated to another forest. over successive generations the population in the new habitat shows more of the long tailed doves than the short tailed ones. which factor could be responsible for this micro evolution

I believe the correct answer is the Founder effect.
This effect refers to the fact that genetic diversity is lost after a long period of migration. So, when the long-tailed doves migrated to another forest, there were more and more long-tailed doves, and fewer and fewer short-tailed ones, which means that there is not a lot of diversity anymore.