As raptors migrate over Pennsylvania this fall, the number of bald eagles among them is close to an all-time high, according to an expert in Penn State's College of Agricultural Sciences.
Starting at the end of August and continuing through the early part of December, thousands of raptors pass over the state, noted Margaret Brittingham, professor of wildlife resources and extension wildlife specialist.
"These raptors are migrating south for the winter. Some winter in the southern United States, while others travel as far away as Central and South America," she said. "The month of October shows the greatest diversity of raptors flying over the state. In November, you are likely to see high numbers of red-tailed hawks and golden eagles."
The raptors use the updrafts created by the mountains to their advantage as they are migrating, Brittingham explained. "The ridges run northeast to southwest, and the wind along the ridges provides lift to the birds as they travel south. On warm days, thermals form in the valleys between the ridges.
"The warmer air helps the raptors to fly higher and glide along with less effort for a much easier flight. The thermals and updrafts are the reason why we see the flying raptors so often in these mountain areas."
Groups such as the State College Bird Club have several locations where spectators can go to observe the raptors that are passing by, Brittingham noted. There are lookouts across the state at which "bird counters" watch and keep track of how many different types of raptors fly by.
For example, the State College Bird Club keeps a running total each migration season at Stone Mountain Hawkwatch, which is located on the Huntington/Mifflin County border. Jacks Mountain Hawkwatch in Mifflin County is another popular raptor-watching spot.
"The most common raptors that have been spotted so far this fall season at Jacks Mountain are broad-winged hawks, sharp-shinned hawks and red-tailed hawks," Brittingham said. "Hawk Mountain Sanctuary, located in east-central Pennsylvania, has counted a total of more than 19,000 birds this season."
Bald eagles are making a comeback in Pennsylvania, Brittingham said. "In addition, Pennsylvania is now known to have the highest population of migrating golden eagles in the East."
The migration continues on as the fall foliage changes its colors, the leaves fall and the weather becomes colder. Soon, all of the raptors making their fall migration will be at their new homes for the winter months, and the cycle will continue again in the spring.
"The raptors head north again in the spring -- the birds that migrated south latest are the first ones to head north," Brittingham said. "In spring, they are much less concentrated along the ridges, and so they are harder to see in large numbers. One exception is the golden eagle, which is frequently observed at the Tussey Mountain Spring Hawkwatch in Centre County."
Explore further: Hermit creepy crawlies: Two new taxa of wood-feeding cockroach from China