The Wind Beneath Their Wings
If it weren’t for wind, most birds would never fly.
The 1989 number-one single Wind Beneath My Wings, sung by Bette Midler, soulfully describes the support gained by one individual from another during the journey through life. This poignant song, or at least the image, provides a perfect metaphor for the influence of wind on the flight of birds. Without the wind beneath their wings, birds would cease to be the aerially dynamic creatures that they are.
Wind is a physical force that influences birdlife in a host of ways. But to fully appreciate why wind is so critical, you have to understand something of avian flight mechanics. Basically, birds fly by taking advantage of lift, which is created as air passes under and over their wings, much as it surrounds the wings of an airplane. The changing air pressure above and below the wings of a bird as it moves through the air is pivotal to a bird’s ability to remain airborne while flying.
When the force of gravity is factored into the equation, particularly during windless flying conditions, thrust becomes equally critical. Without forward thrust to move a flying bird in a specific direction, no amount of lift can sustain flight. If there is enough wind to offset the effect of gravity, the amount of thrust needed to sustain flight is reduced. For many birds then, wind is the great equalizer in facilitating the ability of birds to fly.
For soaring birds including storks, vultures, and many raptors, the lift derived from thermals generated by rising warm air in conjunction with the wind makes it possible for these species to cover thousands of miles during their lengthy intercontinental migrations. North American species such as the broad-winged hawk and Swainson’s hawk virtually commute to their wintering grounds in Peru and Argentina by spiraling upward on warm air thermals, followed by lateral high-speed gliding on the wind. For familiar scavengers such as turkey vultures, wind makes it possible for them to scour dozens of square miles of countryside in their daily aerial search for carrion — a patchy food resource seldom concentrated in abundance.
Similarly, many seabirds rely heavily upon the wind in order to soar over ocean waves in their seemingly endless quest for food. Magnificent frigate birds, those huge swallow-tailed pirates of the sky that plunder flying fish from hardworking blue-footed boobies, spend hours gliding over tropical ocean waters on their 7.5-foot wings in their quest for booty. Maintained aloft by balmy ocean breezes hour upon hour during the daytime, frigate birds hang glide effortlessly on high until sunset, then return to shore to roost on remote tropical islands when the winds subside. In contrast, those penultimate soaring birds, the great albatrosses of the southern ocean, literally may remain on the wing for days or weeks at a time, all the while riding the “roaring forties” and “furious fifties” — the powerful cyclonic winds that are especially strong in the earth’s southern latitudes. Needless to say, to accomplish the type of gliding and dynamic soaring exhibited by species such as frigate birds, albatrosses and shearwaters, strong and steady winds are essential for optimal flight.
Not all avian locomotion involves soaring or powered flight, however. Some birds are effectively able to use the wind to hover, or hang suspended in the air without actually flying. Species such as the osprey and belted kingfisher routinely face into the wind and use it to hover in place by adjusting their wings just enough to maintain their aerial position over the water until they spy a fish below. Once a fish is located, they dive on their prey, only making final adjustments just before they strike. In a similar fashion, the rough-legged hawk and American kestrel routinely hover over open fields or meadows in their quest for voles or grasshoppers, usually with some irregular or steady flapping to help maintain a fixed position. Once prey is sighted, these aerial hunters drop to earth and snag their unsuspecting victims with their talons.
Geese, cormorants, and some other birds use the wind as they travel in fixed v formations rather than flying in irregularly bunched flocks. Birds at the apex of the v give added lift to the birds trailing behind them, much the way competitive cyclists and racecar drivers attain an advantage by drafting behind the bicycles or vehicles ahead of them. What actually happens is that eddies of disturbed air generated by the wings of birds in the lead of the v provide additional lift for those that are flying in their wake. In this way, all birds in the formation gain a slight flying advantage except the lead bird, which is probably why they frequently shift their positions so the same individual is not always at the apex.
During migration, wind is often responsible for facilitating or retarding the remarkable journeys that some bird species annually undertake to complete their migrations between breeding grounds in the north and wintering areas in the south. For many waterfowl and shorebird species that breed in the Arctic or sub-Arctic, the passage of cold fronts in late summer and fall is critical to initiating southward migration. Because changes in temperature, wind speed and wind direction are closely linked to weather fronts, the passage of an autumn cold front across Canada or the northern United States triggers a drop in temperature and a shift in the wind to the north or northwest. These are precisely the conditions and wind direction that will launch southbound migrants on their way — a “stimulus package” for migration, one might say. Many geese, ducks, and shorebirds actually remain in northern staging areas for days at a time prior to the passage of these fronts, waiting for just the right conditions to start the migration that will ultimately carry them toward their winter quarters.
Birds prefer to utilize tailwinds during migration, a fact that produces interesting migration variations in order to maximize the role of the wind. An example of this is demonstrated by several long-distance migrant shorebirds such as the American golden-plover, Hudsonian godwit, red knot, and white-rumped sandpiper. All of these species routinely depart from areas in eastern Canada including James Bay, the coast and islands in the Gulf of Saint Lawrence, the barrens of Labrador, and the Maritime Provinces. In autumn, they specifically utilize northwesterly tailwinds as they set a southeasterly course out over the Atlantic Ocean bound for South America.
In mid-flight, these powerful migrants begin to encounter the easterly trade winds that will gradually deflect them southwestward toward the northern coasts of Venezuela, Guyana, Suriname, and Brazil — destinations that for most birds will be their first touchdown. For some individuals, this course of migration represents a flight of several days, or upwards of sixty hours, of sustained flying without touching down for rest. Without northwesterly tailwinds beneath their wings to facilitate their Canadian departure, or easterly trade winds beneath their wings to deflect them back toward the South American coastline, many of the birds would probably never reach their tropical destinations.
Shorebirds and waterfowl are not the only birds that rely on the wind for transport assistance. Every spring, millions of songbirds cross the Gulf of Mexico bound for the Gulf Coast of North America en route from wintering areas in Central America and Mexico. Weather radar has revealed that the numbers of these migrants can be astounding, and annually thousands of birders from all over the country comb the Gulf Coast from Texas to northwestern Florida in an effort to witness this colorful tide of northbound migrants.
There is a bitter irony to this annual event, however, and curiously it is focused on the wind. The total trans-Gulf flying distance undertaken by many of the flycatchers, vireos, thrushes, warblers, buntings, and orioles making the spring pilgrimage each year to the Gulf Coast of the United States is anywhere between 500 and 600 miles. Much like the waterfowl and shorebirds headed south from Canada in the fall, these northbound songbird migrants rely heavily on tailwinds from the south or southwest to launch them on their perilous over water journey from the Yucatan Peninsula.
So what’s the irony? For birders headed to Texas in April to intercept spring migration, one of the conditions that many visitors hope for is the passage of a “norther” — a cold front with accompanying northerly winds that stalls somewhere over the Gulf of Mexico. Should one of these northers happen to catch thousands of migrating birds midstream over the Gulf of Mexico, the results can be both impressive and calamitous. When the favoring tailwinds from the south that existed when the migrants took off from the Yucatan suddenly switch to treacherous headwinds from the north, the tiny migrants are forced to expend excess amounts of energy in their battle against the adverse winds that stand between them and the Gulf shores that lie ahead. Under prolonged headwind conditions, the avian mortality that may result can be catastrophic. For the birders who eagerly await the arrival of waves of weary migrants, the results can be heart stopping. When conditions are extreme, it is sometimes possible to find windrows of tiny feathered bodies along Gulf Coast beaches, or to see dozens of weary migrants foraging for insects practically at one’s feet, exhausted from their struggle to reach the shoreline. In cases such as this, the sustaining wind has turned against them, with the consequence that countless weary migrants sometimes meet their demise as a result of exhaustion, dehydration, and starvation.
Possibly the most pernicious of all wind-related phenomena that can adversely affect birds are hurricanes. If these violent tropical cyclones happen to capture seabirds in their path or entrap them in their eye, the storms have the capability to displace and transport their victims miles inland, or miles north of their normal distribution. There are numerous New England records of displaced southern and tropical seabird species such as the brown pelican, white-tailed tropicbird, and bridled and sooty tern, to name but a few that have appeared in the aftermath of hurricanes from the south. Although many of these storm-buffeted waifs are found moribund on coastal beaches, others are transported relatively unscathed within the eye of such storms, with some birds seemingly able to return successfully southward.
Much like insects, bats, and aircraft, birds avail themselves of the wind in many ways. In fact, without the force of wind many would hardly be able to fly at all. Wind serves many functions in regulating the lives of birds. When we consider the significance of wind for humans, it is also increasingly apropos to recognize the importance of wind for other species.
Wayne Petersen is director of the Important Bird Areas program for Mass Audubon.
Reprinted with permission from Sanctuary: The Journal of the Massachusetts Audubon Society. Summer, 2009. Visit www.massaudubon.org or email sancmag@massaudubon.org. (781) 259-2167.
