There is no invariable law governing the distance of migration, although in general it is found that where a species has an extensive range, the subspecies that breed farthest north go farthest south to spend the winter
FALL FLIGHTS NOT FAR SOUTH OF BREEDING RANGES
Some other species that have extensive summer ranges, for instance, the pine warbler (Dendroica pinus), rock wren (Salpinctes obsoletus), field sparrow (Spizella pusilla), loggerhead shrike (Lanius ludovicianus), and black-headed grosbeak (Hedymeles melanocephalus), are found to concentrate during the winter season in the southern part of the breeding range, or to occupy additional territory that is only a short distance farther south. The entire species may thus be confined within a restricted area for the period of winter, and then, with the return of warmer weather, spreads out to reoccupy the full range.
There are many species, including the tree sparrow, slate-colored junco, and Lapland longspur (Calcarius lapponicus), that nest in Canada and winter in the United States; while others, including the vesper sparrow (Pooecetes gramineus), chipping sparrow (Spizella passerina), grackles, red-winged blackbird, bluebird, the woodcock (Philohela minor), and several species of ducks, nest in the northern United States and move south for the winter to areas along the Gulf of Mexico. This list includes the more hardy species, some individuals of which may linger in protected places well within the reach of severe cold, as, for example, Wilson's snipe, or jacksnipe (Capella delicata), which frequently is found during subzero weather in parts of the Rocky Mountain region where warm springs assure a food supply. More than 100 of our summer birds leave the United States entirely and spend the winter id the West Indies or in Central America or South America. For example, the Cape May warbler (Dendroica tigrina), which breeds from northern New England, northern Michigan, and northern Minnesota, north to New Brunswick, Nova Scotia, and nearly to Great Slave Lake, is concentrated in winter chiefly in the West Indies, its metropolis at this season being the island of Hispaniola.
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Figure 13.—Barn Swallow, a bird that has so long a migration route that some individuals breed north to Yukon and Alaska, while the winter range extends south to Argentina, 7,000 miles away.
LONG-DISTANCE MIGRATIONS
Some of the common summer residents are not content with a trip to northern South America, but push on across the Equator and finally come to rest for the winter in the pampas of Argentina, or even in Patagonia. Thus some species that are more or less associated with, each other in summer, as nighthawks, barn swallows, cliff swallows, and some of the thrushes may also occupy the same general winter quarters in Brazil. Some individual nighthawks and barn swallows ([fig. 13]) travel still farther, and of all North American land birds these species probably have the longest migration route, as they occur north to Yukon and Alaska, and south to Argentina, 7,000 miles away. Such seasonal flights are exceeded in length, however, by the journeys of several species of water birds, chiefly members of the suborder of shore birds. In this group are 19 species that breed north of the Arctic Circle and winter in South America, 6 of them going as far south as Patagonia, and thus having a migration route more than 8,000 miles in length.
