Biology Reference
In-Depth Information
Following Directions
Q Do birds migrate directly north and south? How do they compensate for cross-
winds?
A
Few birds fly directly north or south. Many species, such as Blackpoll Warblers, have
adaptations allowing them to capitalize on specific feeding or flying opportunities along
their route. Many of these birds start out heading east-southeast toward the Atlantic
Coast, and then take off over the ocean toward South America, aided by trade winds. To
aid their long flight, their primary wing feathers are longer than those of warblers, such
as Pine Warblers, that make shorter flights.
Ruby-throated Hummingbirds fly generally south until they reach the Gulf Coast, and
then take advantage of abundant food in the Mississippi Delta to build up their fat re-
serves. Then many of them take off over the Gulf, flying nonstop to the Yucatán. But
thereisevidence inbothcasesthatsomeindividuals hugthecoastline ratherthanmaking
such a long overwater flight.
Theflightdirectionofmigratorybirdsingeneraldependsonwheretheirfinaldestina-
tionis,whetherthestarsarevisible,andalotofindividualfactors.Forexample,thrushes
haveaverystrongsenseofdirection.Fromagivendeparturespot,though,eachbirdtakes
a different heading. They're all moving in general toward their species' winter range, but
a winter range can be large — it wouldn't be very good for all the birds to end up in ex-
actly the same spot!
Q Since they can fly away, why do some birds stay in extremely cold places for the
winter?
A
The average annual adult survival rate of year-round tropical residents is 80 to 90 per-
cent. The average annual adult survival rate of migrants is about 50 percent. And the av-
erage annual adult survival rate of temperate zone year-round residents is only 20 to 50
percent, because of the difficulties of surviving severe winters. Why don't these northern
residents head south to improve their odds?
The evolutionary game is wonnotbythe birds with the longest lives butbythose who
produce enough young to replace themselves. Year-round residents can select the best
territories long before migrants arrive and can work out territorial differences well in ad-
vance of the breeding season so they aren't depleted by the rigors of both a long journey
and territorial battles right when they start nesting. They can also get an earlier jump on
nesting, allowing them more time to re-nest if their first attempt fails.
Q Migrating seems so dangerous and the birds must use so much energy to fly such
great distances. Why do they do it?
