Facts About Migration
- During a 20 year period between 1967-87, radar data showed almost a 50% decline
in the number of trans-Gulf migratory flights.
- Most migratory songbirds take off after the
sun sets--they are nocturnal migrants.
- Small landbirds migrate at speeds occasionally
over 50 mph, because tail winds of15-40 mph add to their own flying speed
of 18-25 mph.
- Most raptors (for example, hawks and falcons)
migrate during the day to take advantage of thermals created by air rising
off the warming earth.
- Using powerful thermals, some raptors can
reach heights of 5,000 feet above the earth during migration.
How does the weather affect migration?
Weather is one of the
chief external influences on migration. Cool air masses moving south
in the fall can trigger migratory flight. Cool air brings high pressure,
low or falling temperatures and winds moving in the direction of
flight and clear skies. If the cool air meets warmer air, clouds,
precipitation and fog may result. Fog, especially, causes birds
to descend to the ground and cease migration. Sudden changes in
the weather can be disastrous for birds. In the spring, a warm,
moist mass of air (low pressure with higher or rising temperatures)
moving north over the Gulf of Mexico can start a wave of migrating
birds to move northward from the American Tropics or southern United
States. A southward moving cold front meeting such a warm air mass
can result in heavy rains and high winds. This can stop migration
immediately or within 24 hours. These spring "fallouts"
or "groundings" of migrants may occur when the migrating
birds literally fall into sheltered areas seeking food and refuge.
This can be disastrous if the migrants are forced down into the
ocean drowning thousands of birds. Resumption of southerly winds
and rising temperatures starts migration northward again.
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