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),