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The annual fall bird migration has been happening for about two months. Would you believe it started with some shorebirds coming through in July?

The reason for migration is not hard to understand. Given mobile animals and a seasonally fluctuating food supply, the natural consequence is migration.

Of the close to 240 species of birds that nest in Minnesota, only about 20 don't migrate. Some include several grouse and owl species; four of our common woodpeckers (downy, hairy, red-bellied and pileated); the white-breasted nuthatch; black-capped chickadee; northern cardinal; house sparrow; and ring-necked pheasant.

Millions of American robins from Minnesota fly to Texas, Louisiana, Arkansas and Mississippi for the winter. Common loons, our official state bird, migrate through the Great Lakes to the Atlantic coast from North Carolina to Florida, or down the Mississippi Flyway to the Gulf of Mexico.

Listed below are just a few more wintering locations for some of our fall migrants. Nesting Minnesota birds do not raise families while in their winter territories.

• House wrens, eastern bluebirds, yellow-rumped warblers, red-winged blackbirds, and wood ducks head to southern states.

• Great blue herons fly to the southern United States and into Mexico.

• The ruby-throated hummingbird winters in a region from south Texas to Costa Rica.

• Baltimore orioles fly to Central America, and will return in early May.

• Barn swallows travel into Panama, and south to southern Argentina.

• Common nighthawks spend the winter in a stretch from northern South America to central Argentina.

Jim Gilbert's observations have been part of the Minnesota Weatherguide Environment Calendars since 1977, and he is the author of five books on nature in Minnesota. He taught and worked as a naturalist for 50 years.