A common human activity in winter is to feed the birds, but doing this can have profound effects for the evolutionary future of the species. This is shown through a population of birds known as the blackcaps, which use to be a single speciess and has now been split into two reproductively isolated groups, despite the fact that they breed side by side in the same forest. The isolation between these two groups even though they live together for part of the year is stronger than that of two groups that live 800 kilometers or more apart. Birds that migrate northwest feed off the food humans give them unlike the birds that fly southwest which feed off of fruits as a consequence the northwest birds have rounder wings which provides better maneuverability but makes them less suitable for long distance flying. And they have longer, narrower bills that aren't are less equipped for eatting fruits and olives.
www.sciencedaily.com/release/2009/12/0912
B. Kohler
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