A broad-winged hawk flies above the Hawk Mountain Sanctuary in Kempton, Pennsylvania. In just one hour on a September day, 126,516 broad-winged hawks were counted above Veracruz, Mexico, on way to winter homes in Central and South America. AP Photo/Carolyn Kaster
Some impressive diurnal migrations were observed recently in Maine. On Sept. 4, Reed Robinson counted 612 common nighthawks passing over Portland. On Sept. 5, Alison and Jeff Wells reported a flock of 190 in Harpswell. On the same day, migrating flocks were noted in Cape Neddick and Brunswick.
On Aug. 28, Aletha Boyle saw thousands of swallows flocking at Biddeford Pool.
Birds aren’t the only fall migrants. Keep an eye out for migrating insects as well. The remarkable migration of the monarch butterfly is well known but painted ladies and red admirals migrate through Maine in some years.
Dragonflies get in on the act as well. The large, colorful common green darner is the most conspicuous migrating dragonfly but is joined by wandering gliders, spot-winged gliders, black saddlebags and variegated meadowhawks. These dragonflies work their way south in large numbers to winter in Mexico and the West Indies.
To help identify dragonflies, download the free Dragonfly ID app to your iPhone or Android phone.
The pinnacle of diurnal bird migration has to be the migration of hawks through the Mexican state of Veracruz. Each fall, about 2 million broad-winged hawks, 1 million Swainson’s hawks and 200,000 Mississippi kites stream through this area from North America to their Central and South American wintering areas. Virtually all of the individuals of these three species pass through Veracruz.
On one September day, 126,516 broad-winged hawks were counted between 2 and 3 p.m. with 46,000 in one 10-minute period! Over a million diurnal raptors of many species have been counted on a single day.
However, most birds are nocturnal migrants. There are three advantages to migrating at night. First, the risk of predation by hawks, eagles and falcons is eliminated. Second, the atmospheric currents are typically smoother at night. During the day, the sun induces turbulence, greatly reducing flight efficiency. Lastly, the cooler night-time temperatures help birds with their heat balance.
A flying bird increases its basal metabolism by five-fold, even higher in some birds. The best that you can do with all-out exercise is to double your basal metabolic rate. Cranking the metabolism so high in birds necessitates the elimination of a lot of heat if a bird is going to maintain a constant body temperature.. Cooler air temperatures increase the rate at which the excess heat can be spirited away from the unfeathered legs, feet and beak.
Appreciating the magnitude of nocturnal migration is difficult since we can’t see the birds in the dead of night. But there are ways to use your eyes and ears to experience a nocturnal flight.
Grab your binoculars or, better yet, a spotting scope and train it on the surface of the moon. If migration is occurring, you will see the silhouettes of birds passing in front of the moon’s image. The technique works well for birds that are no more than several hundred feet above the surface of the earth.
You can also appreciate migrating birds at night from radar images. In the early days of modern radar in the early 1940s, mysterious blips, called angels, were detected on radar screens. Now we know that the angels are small flocks of birds.
The Doppler radar used now for weather forecasting is perfect for detecting bird migration. Here is the URL for a great tutorial on how to use the freely available NEXRAD radar images to monitor migration: woodcreeper.com/videos/NCAR_Tutorial-desktop.m4v.
Yet one more way to appreciate nocturnal migration is to use your ears. Nocturnal migrants are noisy, regularly emitting short flight notes. In some cases, the flight notes are similar to the calls the birds give while they are on the ground. In many cases, however, the distinctive flight notes are only given during a nocturnal flight.
Bill Evans has been a pioneer in the study of nocturnal flight calls. Visit his website at oldbird.org. He has sonograms for a number of warblers and sparrows.
Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions at whwilson@colby.edu
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