From the 1st to the 10th of May a succession of bird waves comes from the south of such vast proportions as to the number of individuals and variety of species that all the previous migratory waves seem insignificant in comparison. It is the flood tide of the migration, bringing with it the host of warblers, vireos, orioles, tanagers, and thrushes that suddenly make our woods almost tropical in the variety of richly colored species and strange bird notes. It would take a volume to describe the wood warblers, sylvan nymphs of such bizarre color patterns and dainty forms that one is fain to imagine himself in the heart of some wondrous forest of a far-away land. Their curious dry notes, each different in its kind and expression, yet all of the same insectlike quality; their quick, active motions, now twisting head downward around the branches, prying into every nook and cranny in their eager search for food, or fluttering about the clusters of leaves, add to the strange effect. Their names, too, are richly stimulative to the color sense—the black-throated green, the black-throated blue, the chestnut-sided, the bay-breasted, the black and yellow, the cerulean, the Blackburnian, the blue-winged yellow, the golden-winged, the blue-yellow-backed or parula warbler, and the Maryland yellow-throat are each suggestive of a wealth of coloring. Others have names that carry us to southern realms, like the myrtle and the palm warblers; and others again tell of curious habits, as the worm-eating warbler, the hooded fly-catching warbler, and the black and white creeping warbler that scrambles about the tree trunks like a true creeper. There is nothing in all the year quite like the May woods. Then, if never again, you can step from your dooryard into an enchanted forest. The light yellowish effects of new green in the feathery masses of the oak catkins and the fresh, unfolding leafage of the forest trees are a rich feast to the eyes. Against this wealth of green the dogwood spreads its snow-white masses of bloom. In sunlit spaces of greenness the scarlet flash of a tanager, the rich blue coloring of the indigo bird, newly arrived from its winter quarters in South America, and the glimpse of a rose-breasted grosbeak among the high tree tops are strangely suggestive of a tropical forest. The ear, too, is charmed with a multitude of curious notes. The weird cries of the great-crested flycatcher among the topmost branches, and the loud chant of the ovenbird with its rising cadence coming from farther depths of the wood are two of the most characteristic bird voices of the May woodlands. If one would have the famous song of the mocking bird in this sylvan carnival he has only to loiter in the nearest grove to hear the wonderful performance of the catbird. The catbird is the real harbinger of summer. He is familiar throughout the countryside, liked or disliked according to the dispositions of folks, but when he appears amid the May-day throng every one knows that summer has come. As a countryman once said to me: "You can't place any dependence on the robin—it may snow the very day he comes; but a catbird never makes a mistake—it's summer with him for sure."

The passing on of the great warbler waves to the north and the ending of the migration likewise mean the passing of the spring. It is summer any time after the 15th of May, or, to be more accurate, after the last of the migratory warblers, thrushes, and tanagers have passed beyond our woods. To a New-Englander summer will come a little later, nearer the true almanac date of June 1st. To a dweller in Virginia the last of April is the passing of spring and the advent of summer.

Some ten or more years ago several enthusiastic ornithologists living in the neighborhood of Philadelphia began keeping records of the times of arrival of the different species of birds, and at the same time noted the conditions of temperature in relation to the abundance of individuals. After several years of these observations they were able to see clearly that these bird waves were directly related to the waves of rising temperature marking the advent of warm spells of weather. One of the most significant facts deduced from these observations was the remarkable regularity in the first appearance of certain species. For example, the Baltimore oriole in eight years of observation never arrived before the 1st of May, and only twice later than the 4th—viz., once on the fifth and once on the 7th. The list on the opposite page shows the date of first arrivals extending over a period of eight years, from 1885 to 1892.[10]

1885.1886.1887.1888.1889.1890.1891.1892.
Flicker April 10Mar. 24 Mar. 26 Mar. 30 Mar. 28 Mar. 26 Mar. 30 April 2
Chimney swift April 22 April 23 April 22 April 20 April 15 April 22 April 16 April 27
Hummingbird April 29 May 12 May 12 May 14 ....... May 7 May 11 .......
Kingbird May 6 May 11 May 7 May 6 May 6 May 14 May 1 May 4
Crested flycatcher May 2 May 12 May 3 May 1 May 8 May 1 April 30 May 3
Pewee April 3 Mar. 20 Mar. 21 Mar. 22 Mar. 27 Mar. 27 Mar. 31 April 3
Wood pewee May 6 May 15 April 30 May 13 May 12 May 14 May 6 May 17
Red-winged blackbird Mar. 4Feb. 19Feb. 19 Feb. 21Mar. 13 Mar. 12 Feb. 25 Mar. 9
Meadow lark ....... Feb. 10 Mar. 19 Mar. 21Mar. 14 Mar. 12 Feb. 23 Mar. 17
Baltimore oriole May 5 May 4 May 2 May 2 May 7 May 1 May 1 May 3
Purple grackle Mar. 16 Mar. 7 Feb. 19 Feb. 21 Mar. 2 Feb. 13 Feb. 18 Mar. 6
Chipping sparrow April 8 April 9 April 8 Mar. 31 Mar. 29 April 8 April 13 April 4
Field sparrow April 11 April 7 April 9 April 2 Mar. 29 Mar. 13 Mar. 15 Mar. 26
Chewink April 22 April 23 April 27 April 18 April 11 May 1 April 18 April 24
Indigo bird May 16 May 11 May 7 May 12 May 12 May 10 May 8 May 10
Scarlet tanager May 9 May 12 May 5 May 8 May 9 May 4 April 28 May 3
Barn swallow April 22 April 19 April 21 April 12 April 22 April 19 April 19 April 24
Red-eyed vireo May 7 May 11 May 4 April 29 May 5 April 30 May 2 May 3
Black-and-white warbler April 30 May 4 April 27 April 21 April 20 April 30 April 24 May 1
Yellow warbler May 6 May 4 May 2 May 5 May 11 May 1 May 8 May 4
Myrtle warbler May 2 April 10 May 2 April 25 April 20 April 27 April 18 April 7
Black-throated green warbler May 2 May 11 May 5 April 26 May 5 May 2 April 19 April 30
Ovenbird April 30 May 3 April 29 April 30 May 3 May 3 April 29 April 30
Maryland yellow-throat April 29 April 24 April 28 April 30 May 6 April 30 May 1 May 3
Chat May 2 May 12 May 5 May 5 May 11 May 5 May 1 May 3
Redstart May 2 May 4 May 3 May 1 May 4 May 3 April 29 April 30
Catbird May 2 May 4 May 3 May 5 May 5 May 5 May 4 April 30
Brown thrasher April 24 April 25 April 28 April 15 April 22 April 30 April 19 April 30
House wren May 3 April 27 April 24 April 28 April 14 April 30 April 19 May 5
Wood thrush May 2 May 1 May 1 May 1 May 3 April 30 April 23 May 2
Veery ....... May 11 April 25 May 3 May 6 May 2 April 28 May 4
Hermit thrush April 13 April 7 April 9 April 3 April 10 April 13 April 12 April 3
Robin Mar. 7 Mar. 10 Feb. 28 Feb. 19 Mar. 7 Feb. 26 Feb. 24 Mar. 9
Bluebird Mar. 18 ....... Feb. 17 Feb. 21 Mar. 8 Feb. 23 Feb. 17 Mar. 9

Another fact of great interest which bears on the south-to-north movement of migrating birds, and which these observations very clearly brought out, was the earlier appearance of individuals of various species at points nearer the river, the first arrival of the same species at points back from the river being, in many instances, several days later. The first report of the arrival of a given species usually came from a low, marshy tract of land immediately bordering the western shore of the Delaware. The second report came from a locality several miles back of the eastern shore of the river, but situated in the low plain of the river valley and within tide-water limits. The third report came from a place some miles back from the river on the uplands, but near the head of a stream emptying into the Delaware from the west. The last two places to report arrivals were situated farther up the river and some distance back from it. All this confirms the general idea that in migrating most, if not all, of the various land birds follow river valleys and invade the upland districts, lying back from either side, by way of the smaller tributaries.

The fact of greatest importance resulting from these observations was that relating to temperature. It was found that there was always a marked increase in the number of individuals of a given species following a warm wave of temperature as marked by a decided rise of the thermometer. The following graphic representation, based on the abundance from day to day of three common and easily observed species—the brown thrasher, chipping sparrow, and flicker—affords an interesting illustration of the relative movements of the two waves. It will be understood that the numbers in the extreme left-hand column refer to the relative abundance of individuals of the three species collectively. The inside column refers to temperature. The period of observation was twenty days, as shown by the line across the top of the figure.[11]

A, migration; B, temperature.

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.