The natural breeding-places of these birds, before the settlement of the country, were caves, overhanging rocky cliffs, and similar localities. Swallow Cave, at Nahant, was once a favorite place of resort, and in the unsettled portions of the country they are only found in such situations. As the country is settled they forsake these places for the buildings of the farm, and their numbers rapidly increase. In the fur countries and in all the Pacific coast, they still breed in and inhabit caves, chiefly among limestone rocks.
Where the opportunity offers, they prefer to place their nests on the horizontal rafters of barns. Built in this situation, the nests have an average height and a breadth of about five inches. The cavity is two inches deep and three inches wide, at the rim. The nests are constructed of distinct layers of mud, from ten to twelve in number, and each separated by strata of fine dry grasses. These layers are each made up of small pellets of mud, that have been worked over by the birds and placed one by one in juxtaposition until each layer is complete. These mud walls are an inch in thickness. When they are completed, they are warmly stuffed with fine soft grasses and lined with downy feathers. When built against the side of a house, a strong foundation of mud is first constructed, upon which the nest is erected. In this case the nest is much more elongate in shape and more strongly made.
A striking peculiarity of these nests is frequently an extra platform, built against, but distinct from the nest itself, designed as a roosting-place for the parents, used by one during incubation at night or when not engaged in procuring food, and by both when the young are large enough to occupy the whole nest. One of these I found to be a separate structure from the nest, but of similar materials, three inches in length and one and a half in breadth. This nest had been for several years occupied by the same pair, though none of their offspring ever returned to the same roof to breed in their turn. Yet in some instances as many as fifty pairs have been known to occupy the rafters of the same barn.
In one instance Mr. Allen has known a pair of these Swallows to take possession of the nest of a pair of Cliff Swallows, placed under the eaves of a barn, driving off the rightful owners. The next year they built a nest in the same place, the old one having fallen down. But such instances are rare, and the attempt is often a failure.
The wonderful activity of this bird, its rapidity and powers of flight, are too striking a peculiarity of this species not to be mentioned. During their stay with us, from May to September, from morn to night they seem to be ever in motion, especially so before incubation, or after their young have flown. The rapidity of their tortuous evolutions, their intricate, involved, and repeated zigzag flights, are altogether indescribable, and must be witnessed to be appreciated. Wilson estimated that these birds fly at the rate of a mile a minute, but any one who has witnessed the ease and celerity with which they seem to delight in overtaking, passing, and repassing a train of cars moving at the rate of thirty miles an hour must realize that this estimate is far from doing full justice to their real speed.
The song of this Swallow, especially when on the wing, is very pleasing and sprightly. It is a succession of twittering notes uttered with great rapidity and animation. When alighted, their notes are delivered more slowly and with much less animation.
The attention of these birds to each other when sitting upon the nest, and to their young when hatched, is unremitting. The estimated numbers of small insects they collect for their own consumption and that of their nestlings is almost incredible. When the young are old enough to leave their nests the manœuvres of the parents to draw them out, and their assistance to them when practising their first short flights, are among the most curious and interesting scenes one can witness in his ornithological experiences; but space would fail me were I to attempt their details.
The number of the young is from four to six, and there are often two broods in a season. As soon as the second brood can fly, or early in September, they all prepare to leave. They usually collect in flocks of from one to several hundred, and depart within a few days of their first assembling. Large flocks pass along the coast of Massachusetts, from the north and east, early in September, often uniting as they meet, and passing rapidly on.
Their eggs have a ground-color of clear white, with a roseate tint when unblown. They are marked with spots of reddish and purplish-brown, varying in size and number, and chiefly at the larger end. They are smaller and more elongate than those of the lunifrons, and the markings are usually finer. Their greatest length is .94 of an inch, their least .75, and their mean .78. Their mean breadth is .56 of an inch, the greatest .62, and the least .50.