isotherm of this temperature. Just as warm a spell of weather may occur in early April as in the first week of May, but it does not represent the permanent summer rise; and the majority of the warblers, the catbird, the tanager, the rose-breasted grosbeak, the two species of oriole, the vireos, and the kingbird, are rarely if ever seen in abundance in the Delaware Valley before the 1st of May. The migratory movement of such species is as regular as any other periodic phenomenon in Nature.
It is hard to realize the enormous multitude of birds that form a so-called "wave." During the whole period of migration there is a general northward movement of all the migratory species, but under the influence of warm spells of weather this more or less uniform movement rises into a vast wavelike sweep of birds. These bird waves, as already noted, follow the rise of temperature appearing at any given locality about a day or two after the first day of the warm spell. Many species of land birds migrate at night—such, for example, as the orioles, tanagers, warblers, vireos, wrens, the majority of the finches, the woodpeckers, and the thrushes, excepting the robin. During the passing of one of the May waves the darkness overhead is alive with flying birds. One may stand for hours at a time and hear the incessant chirping and twittering of hundreds of birds calling to one another through the night as though to keep from getting separated. The great mass of individuals are probably guided by these call notes.
The usually accepted notion that birds migrate from south to north in traveling to their breeding grounds is largely true of shore birds and waterfowl, but among many of the species of land birds conditions of topography tend to deflect a direct northward movement. The Atlantic coast plain, reaching up into southern New Jersey, and the Mississippi basin, each offers a broad south-to-north highway for birds leaving the Gulf shores of the United States on their northward journey in the spring. A great majority of species find in the wilderness of the Appalachian highland, from the Catskills to Georgia, breeding grounds quite as well adapted to their needs as the forests of Maine and Canada.-Large numbers of birds, according to their regional relations, will constantly turn from the Atlantic coast plain up the numerous rivers, which become great highways of migration, leading to the highlands. The northward movement has thus a large westerly deflection on the Atlantic slope of the middle United States. It is also quite certain that many birds winter in favorable localities on the Atlantic coast plain much farther north than is generally supposed. This is especially true of the holly thickets among the coastwise sand dunes of southern New Jersey and the cedar swamps and pine barrens in the vicinity of Cape May. Many