BIRD-MIGRATION IN PENNSYLVANIA
Some of our winter birds spend the year round in one region. Certain of them, like the Song Sparrow and Crow, migrate to an extent, the nesting individuals moving southward during winter, their place being taken by other individuals of the same species from farther north. Some winter birds, such as the Junco, Golden-crowned Kinglet, Brown Creeper, and Tree Sparrow, visit us from the north and return to their Canadian nesting-ground with the arrival of spring. Most of our familiar summer birds spend the winter to the southward, many of them in South America. They come to us for a few months each year for the sole purpose of bringing forth their young. Many species of birds pass through Pennsylvania en route from their home in the south to their nesting-grounds in the north, and back again in the fall.
By far the greater number of species migrate to an extent. The phenomenon of bird-migration has caused many a student to wonder. How did such a tremendous annual movement originate? How do the birds endure their great flights across bodies of water?
The probability is that the migration of birds developed in past centuries as the food-supply in the tropics became insufficient for all the nesting birds which tried to bring forth their young there. Urged by the need for solitude and a good food-supply, certain birds pushed out from the ancestral range and established a new summer home. After the young were reared, instinct drew them back to the region which was familiar to them, and so great migration routes have developed. Today the tiny Ruby-throated Hummingbird rears his young in our woodlands, then returns to South America with the young birds. Our Yellow Warbler, Red-eyed Vireo, Purple Martin, Rose-breasted Grosbeak, Scarlet Tanager, and many others, all go to South America.
Food-conditions, no doubt, have something to do with migration movements. Birds are well clothed with feathers, to be sure, but many of them depend on an insect diet, such as would be difficult to secure during cold winters. Some of our birds actually do not migrate if a food-supply is available.
Most of the smaller birds migrate at night, following streams or mountain ranges. Swallows and hawks usually migrate by day, ducks and geese by both day and night. The Ohio, Delaware, and Susquehanna river valleys are important routes of migration. The shore of Lake Erie is a resting-ground for birds which have flown over this large body of water. In fall, at Presque Isle, the trees may be alive with birds which have just made the flight. The Atlantic Coast is an important route of migration for many waterbirds. Since Pennsylvania has no salt-water shore-line, we do not find some species which are to be found along the coast of New Jersey and Delaware.
Many birds which occur in abundance at Erie, in fall, rest there until they are able to take another flight; then they start southward for a feeding or resting-ground south of Pennsylvania, and therefore skip over most of the Commonwealth.
The distribution of birds and the constancy of their migration routes is a source of much wonder to all of us. Why should the two Palm Warblers, for instance, so invariably be found each year, one to the eastward, one to the westward of the mountains? Why should some birds be here in fall and not in spring? Why should others be so variable in numbers? If you keep careful notes upon the migratory birds, you may eventually help to solve some of these problems.
HORNED GREBE
Colymbus auritus Linnæus
Other Names.—Dipper; Hell-Diver.
Description.—Neck long; no tail-feathers; toes flat and broad, feet at rear of body; sexes similar. Adult in spring: Large, puffy head, black, with stripe and silken plumes behind eye buffy; plumage of back blackish edged with gray; secondaries white; neck, breast, and sides chestnut; belly silvery white; eyes bright pink, the pupil encircled with a white ring. Immature birds and adults in winter: Grayish black above, silvery white beneath, grayish on the throat, with white cheek-patches which nearly meet on nape. Length: 13½ inches.
Range in Pennsylvania.—A migrant throughout the Commonwealth from March 20 to May 10 and from October 1 to November 30; occasional in winter when water is free of ice.