The Nature Writers of Texas

The best nature writing from the newspaper, magazine, blog and book authors of the Lone Star State . . .

Thursday, March 27, 2008

Bird Navigation is an Amazing Ability
by Ro Wauer

At this time of year, millions of birds of all sizes and kinds are passing through South Texas. The majority of these are neotropical migrants, those that spend their winters in the tropics but come north to nest in North America, in the United States and/or Canada.

Many of these northbound birds fly across the Gulf of Mexico, leaving Mexico’s Yucatan Peninsula during the early evening hours. They normally arrive along the Gulf Coast the following day between mid-morning and early afternoon. Others follow the Gulf Coast northward, passing directly through South Texas. Most are nighttime migrants, although a few, such as swallows and others that feed along the way, are daytime migrants.

In the big picture of migration, it is understandable that these spring migrants head north toward their ancestral nesting grounds. But what is less clear is how they are able to navigate from one tiny speck of habitat in the Tropics to another speck of habitat that may be as much as 4,000 miles to the north. How do they manage to find the exact place at Victoria’s Riverside Park, on the shore of Lake Michigan, or on the Alaska tundra where they were fledged?

The answer to that question is not fully understood, but some pieces of the avian navigation puzzle have been found. For instance, it is well known that birds are fully capable of following principle landforms, such as rivers, seashores, and mountain ranges. This suggests, of course, that once a bird has traveled the route once, even in the opposite direction, it can do so again. Domestic pigeons released miles from their roosts fly in circles until they are able to recognize their home surroundings. But what about nighttime migrants or those birds flying over fog or clouds?

Birds also are able to use celestial navigation; they can navigate by the stars. Biologists have placed spring migrants in a circular cage in a planetarium, and when stars come out the birds begin hitting the north walls. When the northern stars were reversed the birds begin hitting the south walls.

To take this experiment even further, spring migrants (equipped with tiny radio transmitters) were placed in a cardboard box that was suspended from a helium-filled balloon. When the box was opened from aloft on a clear night the birds flew erratically for only a minute or two and then set out on a straight northward course. If the birds were released on an overcast evening, when an afterglow was still visible in the west, they were able to orient themselves by that method. But when neither clue, stars or an afterglow, was available, they flew downwind, even if the wind was blowing in the direction opposite their destination. When that direction proved to be incorrect, they eventually had to turn back.

Migrants often remain in choice feeding areas, awaiting the right cue to head out. They make a go or no-go decision based on what the weather will be like over the next several hours. They apparently are able to detect the most reliable conditions, when they will be aided by a tailwind and not encounter severe storms along their route. They are correct the majority of times.

1 Comments:

At 12:44 AM, Anonymous Håkan Karlsson said...

Please feel free to visit:
http://orn-lab.ekol.lu.se/birdmigration/

Best wishes,
Håkan Karlsson
Lund University

 

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