In concluding this discussion of orientation it is pertinent to point out that the migratory instinct appears to be more or less transitory, that it is not persistent over an extended period. Migratory birds may be arrested en route, either by natural conditions, such as unusual food supplies, or forcibly by the act of man, and detained until the end or nearly the end of the migratory season, and then may not attempt to finish the journey, apparently having lost the migratory impulse. In the fall and early winter of 1929, abundant food and an open season caused an unusual number of mallard ducks to arrest their migration and remain in western Montana and northern Idaho. Later, however, a heavy snowfall with subzero temperatures suddenly cut off the food supply, with the result that great numbers of the birds starved to death, when a flight of a few hours would have carried them to a region of open water and abundant food. SEGREGATION DURING MIGRATION During the height of the northward movement in spring the woods and thickets may be suddenly filled with several species of wood warblers, thrushes, sparrows, flycatchers, and others, which it is natural to conclude have traveled together and arrived simultaneously. Probably they did, but such combined migration is by no means the rule for all species. As a group the wood warblers (Compsothlypidae) probably travel more in mixed companies than do any other single family of North American birds. The flocks are likely to be made up of several species, spring and fall, with both adults and young. Sometimes swallows, sparrows, blackbirds, and some of the shore birds also migrate in mixed flocks. In fall, great flocks of blackbirds frequently sweep south across the Plains States, and occasionally one flock will contain bronzed grackles (_Quiscalus quiscula_), red-winged blackbirds, yellow-headed blackbirds (_Xanthocephalus xanthocephalus_), and Brewer's blackbirds (_Euphagus cyanocephalus_). On the other hand, many species keep strictly to themselves. It would be difficult for any other kind of bird to keep in company with a bird of such rapid movements as the chimney swift, which is rarely found associated with any other species at any season. Nighthawks, or bullbats, also fly in separate companies, as usually do crows, waxwings, crossbills, bobolinks, and kingbirds. Occasionally, a flock of ducks will be observed to contain several species, but generally when they are actually on migration the individuals of each species separate and travel with others of their own kind. The flocks of blue geese, previously mentioned in connection with speed of flight (p. 18), frequently have with them a few of the closely related snow geese (_Chen hyperborea_), particularly in the eastern part of their winter range. The proportion here is usually about 10 to 1, but farther west the numbers of snow geese increase until they outnumber their blue relatives.