Many similar cases might be mentioned, such as the black-throated blue warblers still observed in the mountains of Haiti during the middle of May when others of this species are en route through North Carolina to New England breeding grounds. Redstarts and yellow warblers, evidently the more southern breeders, are seen returning southward on the northern coast of South America just about the time the earliest of those breeding in the North reach Florida on their way to winter quarters. Examples of the Alaska race of the yellow warbler have been collected in Mississippi, Florida, and the District of Columbia as late as October. [Illustration: _Figure 2. Isochronal migration lines of the black-and-white warbler, showing a very slow and uniform migration. The solid lines connect places at which these birds arrive at the same time. These birds apparently advance only about 20 miles per day in crossing the United States._] Students of migration know that birds generally travel in waves, the magnitude of which varies with populations, species, weather, and time of year. Characteristically, one will observe a few early individuals come into an area followed by a much larger volume of migrants. This peak will then gradually taper off to a few lingering stragglers. If we plot numbers observed against time, the rising and receding curve takes the form of a bell. In the northern part of the United States there are two general migration waves. The first one in early spring consists of "hardy" birds including many of our common seed eaters like the finches, sparrows, and others. The second wave occurs about a month later and consists primarily of insect-eating birds, such as flycatchers, vireos, warblers, and the like. Each of these species in turn has its own "curve" of migration in the major wave. Time of Day