Migration & Weather
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Right now is the time for many of the birds across the northern Hemisphere to head south. But how do they know it's time to go? Hi, I'm Dave thurlow for the Weather Notebook. I recently spoke to Scott Weidensaul, author of 'Living on the Wind' and an expert on bird migration.

A hypothetical weather system that could be ideal for mass migrations of waterfowl from their breeding grounds in the Canadian prairies wintering areas to the southern United States. Northern Prairie Wildlife Research Center

DT: "What triggers bird migration? What clues in the environment are they looking for?"

SW: "Of course, a lot of people assume that birds migrate because they're trying to get away from cold weather in the fall and that's not actually true. They migrate frequently long before the weather turns nasty. The real environmental trigger for migration is generally the photo period -- that changing ratio of daylight and darkness. In the fall, the days are getting shorter, the nights are getting longer. That sets off this cascade effect in the birds body of hormonal changes, which in turn triggers this instinctive, and it is almost an entirely instinctive phenomenon of bird migration."

DT: "But there are times, say there is a strong wind blowing in their face, there's still some reactions to things that are happening in the weather right?"

SW: "Sure and the photo period is kind of the big picture and the short term weather situation is the near term trigger for bird migration. Birds migrate when it's most advantageous for them. If they're heading south in the fall, very often they'll wait until after the passage of a cold front when you've got strong northerly winds blowing. It's tailwind for the bird. It gives them added lift. It gives them greater range as they're moving. They get more bang for their buck essentially for all the fat that they've stored up. In the spring, obviously warm southerly breezes tend to be the ones that are moving in advance of a warm front that tend to be what trigger migration north bound in the spring."

Tomorrow the effects of weather on hawk migration. You can find out more about "Living on the Wind" at mountwashington.org. Thanks to the National Science Foundation and Subaru.

 

Living on the Wind - "Across the Hemisphere with Migratory Birds" Scott Weidensaul is the author of Mountains of the Heart: A Natural History of the Appalachians and other books. A columnist for the Philadelphia Inquirer, he is also a federally licensed bird bander in the Pennsylvania Appalachians, where he lives. Read an excerpt.