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. [Illustration: B3763M 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