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Where Have All the Songbirds Gone?
| Article
# : |
18611 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
4 / 1991 |
2,547 Words |
| Author
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Chandler S. Robbins
Chandler S. Robbins is a wildlife biologist for the U.S. Fish
and Wildlife Service at the Patuxent Wildlife Research Center
in Laurel, Maryland. |
For more than 60 years I have been fascinated by the ever-changing parade of brightly colored birds that distract me from my studies, my work, and even at times my sleep. Looking back over time, I recall many days when I saw truly impressive numbers of birds--specially during the spring and fall migration periods, when great hordes of feathered flyers are heading from their warm tropical winter quarters to their nesting habitats in the northern states and Canada. In my childhood, I knew only that these birds had spent the winter somewhere in the south and would nest either near my Massachusetts home or somewhere farther north.
As the years passed, I gradually gained a greater appreciation for the spectacle that had so fascinated me as a child. As I traveled around the continent, I became familiar with the places where each species spent the summer and raised its young. I learned that each kind of bird not only has a unique geographic range but that during the nesting season it is restricted to a particular type of habitat. We all recognize that a thrush needs woods in which to nest, while a meadowlark needs a meadow. But each species of thrush requires a particular type of woodland, and it must be large enough so the nest can be placed well away from the wood margin where predation rate is the highest. And the meadowlark, which hides its nest under a clump of grass, needs an extensive pasture or meadow, not just a half-acre field. Also, each pair of nesting birds must defend a feeding territory that is large enough to supply the needs of its family. This means that as the amount of available habitat declines, the birds cannot just nest closer together.
What happens at the end of the nesting season? After the young are able to care for themselves, the adults molt into fresh plumage and, depending on the species, the young birds molt some or all of their feathers. Then adults and young feed heavily and lay down a layer of fat that will serve as solid fuel during the trip to their wintering grounds. The tropical migrants are primarily insect eaters. They must fly far enough south to find a place where insects and fruits are available throughout the winter months.
Picture for a moment the shape of the North American continent. Millions of insectivorous songbirds from tens of thousands of square miles of forest in the United States and Canada must winter in an area perhaps a tenth that size in Central America or the West Indies. Other migrants stop in Mexico or continue to South America. Central America and the West Indies combined are less that half the size of Alaska. Not only are these migrants squeezed into a small geographic
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