The advent of spring is marked by the northward progression of the isotherm of 42.8° F., which is the initial temperature required to awaken the dormant reproductive and germinating activities in animals and plants. With the gradual invasion of the United States, from the south northward, by temperatures above this, there passes over the different regions the ever-old but ever-new panorama of the spring with its opening blossoms, its unfolding green, and its waves of migrating birds. The restlessness produced by the periodic development of the reproductive function under the stimulus of increased temperature causes the highly organized bird life to spread out from its winter quarters, wherever those may be, and follow the zone of new green that steadily widens northward with its increase of food supply in the form of myriads of insects. The comparative regularity in the recurrence of this phenomenon year after year is attested by the observations just noted. Each species has a certain, definite physiological relation to temperature, and its migratory movement toward the breeding ground is determined by the movement of the 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.