Parker County Today November 2017 - Page 74

Rivers of Raptors BY THE WILD BIRD CENTER, WEATHERFORD, TX It’s autumn, and one of the natural wonders of the world is underway again: the birds are migrating. All across the United States and Canada, surging forward like water released from a dam, birds are flowing south along mountain ridges, coastlines and riverbanks. Of all the billions of birds that fly south each year, the ones that capture our attention the most intensely are the raptors. Perhaps it’s our desire to identify with their grace, speed, strength and apparent freedom that creates a sense of awe in us. Or perhaps it’s just that they are the most visible, and therefore catch our eye. But whatever the reason, man has revered raptors for all of recorded history, and migration creates a major part of our interaction with them. Because diurnal raptors use air currents to help them cover long distances with a relatively small invest- ment of energy, they tend to travel where differences in surface features create thermals, wind currents and updrafts. As a consequence, long mountain ridges, wide river bends and continental coastlines become “skyways” along which hawks, eagles, falcons and vultures travel on their way south. This causes most of the traffic to converge into five major “skyways”: the Pacific Coast, the Rockies, the Mississippi River, the Appalachian Mountains and the Atlantic Coast. Other pathways feed into these routes. Because they migrate during the day, unlike most other bird families, we can witness the spectacle of raptor migration. Hundreds, even thousands, of individual birds pass through the major flyways on any given day from September through November. Natural bottlenecks concentrate large numbers of birds in specific, reli- able spots (such as Cape May Point, NJ or the Marin Headlands, north of San Francisco in California) where people gather to watch. 72 The largest passage occurs each fall in the state of Veracruz, in Mexico, where literally millions of raptors travel down the narrow plain between the Gulf of Mexico and the coastal mountains. These “bottlenecks” are created by geography. The fact is that most birds are reluctant to fly over large expanses of water if they have a choice, and they will often spend a little time dithering about their options. In Cape May, where New Jersey runs into the Delaware Bay, great kettles of hawks and vultures bubble in the sky, testing the thermals, as the birds “debate” whether to go ahead or to go back up the coast where the narrower span of the Delaware River offers an easier crossing. The differences in migratory patterns, habitats and Barred Owl destinations are tributes to the birds’ wills to survive. For instance, Swainson’s Hawks travel from their breed- ing grounds in the Great Plains and Canada to the Argentine Pampas, in South America–a distance of up to 7,000 miles. The seasonal availability of their insect prey in both locations may account for their willingness to travel such a great distance. American Kestrels, on the other hand, do not attempt the same kind of journey, even though they also prey on large insects that are seasonal. Instead, some Kestrels make smaller adjust- ments to their territories (moving from Canada and the northern U.S into the U.S. mainland) and change their diets, relying more on mammals, small birds, lizards and crayfish during the winter months. In some species, such as Cooper’s Hawk and Sharp- shinned Hawks, the young birds tend to migrate before the adults. In other species, such as the Ospreys and Golden Eagles, the opposite is true. There may also be differences in migration schedule based on sex: Female accipiters (Cooper’s Hawks and Sharp-shinned Hawks),