Birds migrate for five main reasons

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What causes migration?

“What I cannot see, no matter how closely I look, is what drives this small creature, barely heavier than air, to make the journeys it must make,” writes naturalist Scott Weidensaul in his book, “Living on the Wind: Across the Hemisphere with Migratory Birds” (North Point Press, 1999). “Its secrets are locked in that tiny packet of brain and muscle and instinct ... It knows, and I do not.”

While there is some disagreement about the reasons birds migrate, most birders accept a few basic precepts on what keys migration:

■ Food — Or the increasing shortage thereof, is one big reason to leave. Insect eaters, particularly, find their larder getting a bit empty this time of year and head south to where insects are more plentiful.

■ Temperature — Cooler weather and the onset of fall rains cause many birds to migrate to more-temperate climates. Conversely, the summer heat of tropical regions causes birds to flee north for cooler temperatures.

■ Drought — This directly affects food supplies, and that normally means food shortage, but this year there is a bit of a twist. The draining of Miramonte Reservoir has left that lake with large expanses of mud flats, ideal feeding for a variety of shorebirds.

■ Offspring — The push to migrate is delayed until this year’s young are ready to fend for themselves or are strong enough to make the long flight to winter areas.

■ Location — Some birds, such as sandhill cranes and Western sandpipers, nest along the coast of Alaska and into Siberia, where winter comes earlier than for birds nesting closer to the equator.


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