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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.
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As fall and winter bring colder temperatures, insects and other foods become scarce. Most insect-eating birds head south to find food.
Some species of birds spend the winter as close as the southern U.S. or as far as South America . Most insect-eating birds travel to Mexico, Central America and the Caribbean. These birds actually spend more time on their tropical wintering grounds than they do on U.S. or Canadian breeding grounds.
As spring arrives the birds return north. But why not stay in the warm tropics where food is available year round? The main reason is the massive burst of emerging insects in the northern spring and the longer days in which to forage for food. Migrant birds can produce more young under these conditions than similar species in the tropics. There are also fewer predators, parasites, and diseases in temperate regions. More young can be produced and survive to adulthood, making migration worth the cost.
In North America there are four distinct flyways; the Atlantic, the Mississippi, the Central and the Pacific. Northwest Indiana falls within the Mississippi flyway. In our particular area bird migration is affected by Lake Michigan. Many birds follow the shorelines as they navigate seasonally up and down the lake.
Many of them rest and feed in woods and fields at the south end of the lakeshore before proceeding north or south depending on the season. The lake itself attracts many species of waterfowl, which tend to concentrate at the southern most tip of the lake. Many shorebirds forage on the beach to fuel their long journey.