Sandhill Crane

Bird migration, still not totally explained

woods a few days ago I encountered a flock of robins. There were robins in the trees and on the ground on both sides of the path. There were robins in the path ahead, each flying up into the trees when I got close. The robins, I knew, like the cranes were migrants. Like the cranes, they, too, would go south. But I wouldn’t see them flying south for they would go at night.

Bird migration, birds going south in the fall, returning north in the spring, is a well-known natural phenomena, as well known as the opening of leaves on the trees in spring, the coloring and falling of leaves in fall. But there is much more to bird migration than flying north in spring, south in fall, some during the day, some at night.

Sandhill cranes and robins, Canada geese and many other birds travel in flocks. Most hawks and eagles travel individually. Flycatchers, too, appear to travel individually. Swallows and nighthawks catch insects in the air and feed as they migrate, swirling about the sky and only generally working their way north or south.

While we think of migration as north-south, scoters, those northern nesting ducks, fly east or west in fall and spend the winter along the East or West Coast. Some Canada geese migrate and some do not making them, as a species, both migratory and non-migratory. Evening grosbeaks and pine siskins and crossbills go south some years and not others and we label them irruptive instead of migratory.

As for the season, some sandpipers and other shorebirds that nest in the tundra of the far north start going south in summer, the end of July. Some Canada geese and mallards and green-winged teal don’t go south until December. Many warblers don’t return to us in spring until May.

Once bird migration was called mysterious. How did birds find their way? Now we know they follow land marks, a mountain ridge, a river, a coast, they orient themselves by the sun and by stars, and they detect and orient themselves by the Earth’s magnetic fields. But that doesn’t explain how the robins that nested in a tree in my yard one year, or one of the pair that nested in my yard or the young of that pair, left in the late summer, then found their way back to my yard the next year.

Knowing that birds find their way by landmarks when they migrate and by the sun and stars and magnetic fields doesn’t explain why robins and red-winged blackbirds and many other birds go only as far south as the southern states while others fly to Mexico or to islands of the Caribbean or to South America. It doesn’t explain why blackpoll warblers fly to the East Coast, then across the ocean to South America, why tiny ruby-throated hummingbirds fly across the Gulf of Mexico.

Knowing how birds find their way doesn’t explain why sometimes a bird gets lost. The black-necked stilt I saw in northern Indiana last summer was lost. The western kingbird I saw in the Finger Lakes region of New York years ago also was lost. The long-tailed duck I saw off the coast near Corpus Christi, Texas one winter was lost. But with or without explanation, migration adds to the delight of bird watching.

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