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  How Birds Migrate

by Paul Kerlinger

This article first appeared in the Spring 1998 GCBO newsletter.

Understanding the rigors, difficulties, and perils of bird migration is almost impossible. We live in a world that is relatively stable: we find shelter in our homes and food in our kitchens; we are seldom considered food by larger animals; and we usually have maps, computers and sophisticated electronic gear to help usin our journeys. Birds do not have these luxuries, yet they undertake immense and wonderful journeys. By putting ourselves in the position of a bird undertaking a migration, we gain an appreciation for their incredible abilities.

Migration RoutesImagine that you are a Broad-winged Hawk or a Blackpoll Warbler, newly fledged from your nest in the forests of Minnesota or Quebec. In September, though you are only a few months old, you commence a journey that will take you out of your northern forests and over a wide array of habitats, many of which are inhospitable, all the way to northern South America. Though both species begin and end their journeys in about the same place, neither use the same migration pathways or strategies.

As a Blackpoll Warbler, you must take off after the sun sets. After takeoff you must decide how high to fly, how fast to fly, what direction to fly in, whether you should fly with other migrants, or whether you fly out over the large body of water ahead (the Atlantic Ocean). For a Blackpoll this means flying by yourself, or, perhaps, in small, loose aggregations which might be construed as flocks. We believe that most nocturnal migrants fly by themselves, but we still do not know for sure.

As a Blackpoll that flies out over the North Atlantic from Cape Cod you would climb to much higher altitudes than when migrating over land. You would find your way by a combination of magnetic cues from the earth, the stars at night, the plane of polarized light from the sun during daylight hours, and perhaps other, as yet, undiscovered types of information that can be used to orient and navigate. Radar studies have shown that small birds migrating over the North Atlantic from New England on the way to South America climb to greater than 5,000 feet, whereas overland they are mostly below 2,000 feet. When flying at these altitudes Blackpolls and other small birds maintain airspeeds of about 18-25 miles per hour, but their ground speed is often greater than 50 miles per hour. This difference is a result of flying with strong tailwinds of between 15 and 40 miles per hour, which results in a net energy savings of about 50% of what would be needed if they were flying without any wind.

Over the Atlantic you have nowhere to land, so you are faced with flying nonstop for up to three days, without food or water. Flight overland isn't so arduous. When morning comes, Blackpolls and other songbirds must find a forest or other habitat in which to rest, feed, avoid predators, and wait for the next night's migration. There are perils during both day and night. Hungry Sharp-shinned Hawks may chase you or you may fly into a guy wire at a 1,000 foot tall communications tower. You may also become lost in a storm or become drenched in rain and be forced down in the Atlantic.

The most insidious danger to migrants is loss of stopover habitat. Along the Atlantic and Gulf Coasts, much of the native habitat has been negatively impacted by coastal development and urban sprawl, reducing the amount of habitat suitable for making stopovers. This forces migrants like Blackpolls to fly farther to find good stopover habitat. Some do not have the energy reserves to make these flights and others fall prey to the many hawks and falcons that cruise the coastlines in search of tired and hungry songbirds. Mortality can be great for a migrant. You don't get many second chances.

If you were a Broad-winged Hawk you would face some of the same problems and some new ones. Instead of night flight, you would fly in the daytime to use the lift of thermals. Instead of flying alone, you would join with thousands of other Broad-wings forming large flocks. Instead of maintaining you're altitude during migratory flight, you would constantly change altitude as you climbed in thermals and descended between them. Taking off in the morning you must find the first usable thermals; columns of air that rise weakly to only a few hundred feet above the earth's surface. Toward midday these thermals become incredibly powerful and abundant. Some transport Broad-winged Hawks and other hawks like Ospreys, Swainson's Hawks, and even Peregrine Falcons to more than 5,000 feet above the earth.

Setting their winds at the top of their climb, these birds descend almost as rapidly s they climb in thermals. Realizing glide ratios of only 7 or 8 to 1 (gliding forward 7 or 8 yards for every yard they sink) is about average for these birds, much less than the 18 to 1 realized by albatrosses or 35 to 1 realized by sail planes. Over the ground they average only about 25 miles per hour or about 200-250 miles per day.

Just as Blackpoll Warblers and other songbirds need habitat in which to stopover, so do Broad-winged Hawks. It was once believed that they did not feed during migration and some authors have written that they gain up to 40% of their body weight in fat, similar to Blackpolls, to use as fuel for migration. There has been no evidence for migration fatlevels of greater than about 10% in raptors, so it is likely that they simply stop and feed here and there as they make their way southward. Without the large fat deposits they must rely on thermal currents to keep them aloft and save them energy.

Though the differences between Blackpoll Warblers and Broad-winged Hawks are great, they share one most important aspect of migration behavior. Both species must continually make decisions about the same aspects of flight: altitude, flocking, direction, speed and many more.

Understanding the difficulties and perils of migration is the best way to understand that birds must make decisions as part of a larger strategy for completing migration and that these decisions can result in success or failure (death or reproductive failure). These decisions and strategies are made on the ground during stopovers and they are made in the air as the birds move between destinations. The next time you see a migrant along the Gulf or elsewhere, try to imagine how it got there and how it will get where it is going. By doing this you will gain a new appreciation for how birds migrate.

Paul Kerlinger is the author of How Birds Migrate (1995, Stackpole Books 1-800-732-3669), an informative book about bird migrations. He is also an environmental and ecotourism consultant for industry and conservation organizations including the GCBO.

 

 
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