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Chapter 4—Striped Bass, Neotropical Migrants, Wild Turkey - page 2 / 10

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Apodidae) and hummingbirds (Trochilidae).

Though many of these species spend most of the year in the Tropics, they are very much a part of temperate ecosystems in the summer.  Their ecological and economic value is so great that it is difficult to calculate.  Literally tons of insects are consumed by neotropical migrants each summer and many plant species depend on these birds to disperse their seeds and pollinate their flowers.  Many of these birds are also sensitive indicators of environmental health. An example of birds as bioindicators is the decline of Peregrine Falcons, Bald Eagles, and Brown Pelicans due to DDT in the environment.   

WHY MIGRATE?

Especially among the migratory songbirds, annual mortality associated with migration is high, in some years probably exceeding 50%.  Increased nesting success achieved by migrating to the temperate zone must compensate for the high mortality associated with migration, or the whole system would collapse.  This pattern does tend to immerge as neotropical migrants are able to raise more offspring (4-6 eggs per clutch) than similar species that remain in the tropics (2-3 eggs per clutch).  A number of factors contribute to greater temperate productivity.  Species typically face less competition, less predation, and a seasonally abundant supply of food during the northern summer.  They are also able to spread out over 8 times the land area available to them in the tropics.  

4 Ovenbird eggs on a nest in north Georgia

This explanation of migration is a more positive one than an older notion of birds flying south to avoid cold.  As many species demonstrate, if a reliable food source is available year-round, they will not migrate.  Think of the tiny Golden-crowned Kinglet (Regulus satrapathat can survive nighttime temperatures of -30New England where they spend the winter.  

Trade

Winds

Southward migration of Blackpoll Warblers

HOW DO BIRDS MIGRATE?

Blackpoll Warblers (Dendroica striata) weighing less than an ounce leave the coast of New England on clear autumn nights when winds are favorable and fly straight out over the North Atlantic.  Their trajectory would take them straight to North Africa; however, the trade winds blow the flying birds back westward, guiding them to landfall in the tropical forests of Venezuela after a 3-4 day flight.  This astounding flight is carried out without stopping, sleeping, eating, or drinking.  This is the equivalent of a human athlete running 4-minute miles for 80 hours straight.  How are these tiny bundles of feathers, muscle and blood capable of such astounding feats of physical endurance?  Just as important, how do they know where they are going and find their way?

THE CONSTRAINTS OF FLIGHT:
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