THE COMING OF THE CATBIRD. 779 



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 repre- 

 sent 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 abun- 

 dance in the Delaware Valley before the 1st of May. The migra- 

 tory movement of such species is as regular as any other periodic phe- 

 nomenon 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 ex- 

 ample, 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 sepa- 

 rated. 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 move- 

 ment. 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, accord- 

 ing 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 thick- 

 ets among the coastwise sand dunes of southern New Jersey and the 

 cedar swamps and pine barrens in the vicinity of Cape May. Many 



