The blackcap's migration pattern is an example of natural selection and genetic behavior. Blackcaps are genetically predisposed to migrate during the winter. In the past, many of the birds travel towards Portugal with a few birds traveling elsewhere, such as Britian. Now, however, more and more Blackcaps are traveling to Britian. Experts say that this because of the length of day differences in Britain over the winter. This changes migration and breeding patterns, which causes the blackcaps in Britain to return home 10 days sooner. Better nests and mates can then be selected, allowing the British birds to produce more offspring. Because of the genetic disposition to head to Britain, the offspring will do as their parents did, passing on the British gene.