About 40 percent of the world’s birds migrate in some fashion, whether it’s a short flight to a warmer locale or a long and arduous trek. Like other animals that undertake migrations, birds travel to find places with more resources or when breeding requires it. Plenty of variables play a role in how and when birds decide to migrate, including the climate and the availability of food and other resources. Above all, it’s a special balance for each species. As Cornell Lab of Ornithology explains, even hummingbirds can survive chilly temperatures provided there’s enough food to go around.