When the last Ice Age ended, the short cool summers favored animals like insects with short life cycles. Some bird species took advantage of the large food source and fewer competitors by expanding their ranges north. Migration may have started as birds began to make use of these areas made available by changing climate. However, they were forced to return south at the end of summer when insect populations dropped.
Over time as more northerly areas warmed, birds were able to move farther and farther north. It is believed that some bird species gradually developed migration strategies to take advantage of temperate climates. It is also thought that current migration routes follow the same path on which habitats expanded as the ice retreated.