Raptor Migration:
An Autumn Spectacle

By Bill Streeter

soaring eagle

Autumn in the northeast United States is a special season. The cooler temperatures, gusting winds, and spectacular foliage change from lush growing greens to shades of red, yellow, orange, and brown signal an end to summer and the earth's preparation for oncoming winter. It is also the signal for the birds of prey to begin their southward migration, a magnificent display that brings people to the ridges and mountains of our northeast flyway to watch and note the passage of some of our most impressive native birds.

For some of these raptors it is truly a rite of passage. Only months out of the nest, they are making their first long distance journey to wintering grounds that will not treat them as harshly as the northern territories where they were born and fledged.

The "fall" hawk migration begins in late August and ends in early December, although the greatest number of raptors pass through our area in September and October. As the days grow shorter there is a decrease in sunlight, and this triggers a biological clock in the bird signaling migration time. Birds of prey in northern North America will overwinter in Florida, the West Indies, the Gulf Coast states, Mexico. Central America, and northern South America. All seem to like great vacation spots. Certainly the weather is much warmer, but if you have observed in January the tiny songbirds at your birdfeeder when it is 10 F you might have surmised correctly that cold temperatures are not as much of a problem for birds as is a scarce winter food supply. Birds of prey like many passerine birds fly south to get to a better source of food, not to escape the cold.

Raptors fly south by following visual landmarks. The northeastern falcons which are the Kestrels, Merlins, Peregrines, and the occasional rare Gyrfalcon, prefer following the Atlantic Coast. Eastern river systems such as our Delaware River are used particularly by Bald Eagles and Ospreys. Finally the Appalachian Mountain chain is a landmark utilized by every raptor group.

Birds of prey use mountain ridge systems not only for visual guidance but more importantly for the air currents erected by solar heating of the valleys and slopes or by upward deflections of the northwest winds. A thermal is a mass of rising hot air produced by the intense heating which occurs on the south side of a mountain slope due to its position in relation to the autumn sun. Just like a hot air balloon will easily take us thousands of feet in the sky, a thermal will do the same for a hawk. That is the reason hawks are often seen flying in circles. They circle to stay in the thermal which lifts them to higher altitudes until very often they disappear to the human eye. As the thermal cools and dissipates the hawk glides out of it, losing altitude but gaining distance. The hawk glides lower to the next thermal and rings up again continuing the process. On a good thermal day a hawk can glide 250 miles without having to flap its wings once.

The other method of travel is to use deflective air currents on a windy day. The horizontal northwest wind strikes the north slope of a mountain and is forced upward creating strong up-drafts. Hawks flying south ride the updrafts and are bulleted along the slopes often reaching speeds surpassing 60 miles per hour. The stronger the wind, the closer the hawks will fly to the treetops where substantially less air turbulence occurs, and the more easily the hawks will be seen.

A textbook viewing day for hawk watching will occur in Pennsylvania, New York, and New Jersey under the following conditions: the passage of a low pressure system to the north over the New England states; an advancing cold front moving south from Canada; the northwest winds for a few consecutive days. Under these conditions migration would occur along northerly facing slopes along the ridges. When a high pressure system is positioned in the area, hawks disperse over large areas often resting and hunting while awaiting an increase in windspeed and thermals. As the high moves eastward, a southerly airflow moves in and hawks return using updrafts along south facing slopes. There is no movement during rain or fog. The best time of the day to hawk watch is generally between the hours of 9:00 A.M. and 2:00 P.M. although hawks will start moving earlier and will continue moving later depending on weather conditions.

During the fall migration it is not uncommon to see I5 species of raptors. In September you are likely to see Broad-winged Hawks, American Kestrels, Sharp-shinned Hawks, Bald Eagles and Ospreys. In October Sharp-shins, Cooper's Hawks, Northern Harriers. Red-tailed Hawks, Red-shouldered Hawks, Merlins, Peregrine Falcons, and Turkey Vultures move through in their greatest numbers. November is the time to watch if you are looking for Golden Eagles, Goshawks and Rough-legged Hawks. Birds of prey living north of us will often overwinter in the Pennsylvania, New York, and New Jersey area. January and February are excellent months to see Rough-legged Hawks, Bald Eagles, and an occasional Golden Eagle.

There are plenty of places in the tri-state area to hawk watch - most notably Hawk Mountain in Kempton, Pennsylvania which is one of the greatest hawk watching spots worldwide. However, a little research will generally turn up some mountain ridges in your local area. The Delaware Valley Raptor Center sponsors a fall hawk watch program and trip to Sunrise Mountain in Branchville, New Jersey - an excellent local viewing spot. The Pocono Environmental Education Center in Dingman's Ferry, Pennsylvania offers three "Hawk watch Weekends" every fall as well as a winter "In Search of Eagles" weekend.

Spotting and identifying swiftly moving raptors becomes easier with each day spent at a lookout observing the migration. Novice hawk watchers can usually he assured of at least one experienced observer sharing the lookout who will gladly help the beginner to learn the fine art of hawk watching. Varied hawk watching experiences bring a greater appreciation for the spectacle of the fall hawk migration. Many hawk watchers find themselves as compelled to take to the lookouts each fall as the raptors are to take to the air.

To find a hawk watching site near you got to hmana.org and click on Hawk Watching Sites.