[13] Tachornis. Generic Character.—Bill very short, depressed, gape very wide, the sides suddenly compressed at the tip, which is curved; the margins inflected: nostrils, large, longitudinal, placed in a membranous groove, the margins destitute of feathers. Wings very long and narrow; first quill tapered to a point: second longest. Tail slightly forked, a little emarginated. Tarsi rather longer than middle toe, feathered. Toes all directed forwards, compressed, short, thick, and strong, with compressed claws. Sternum immarginate, but with three foramina, one through the ridge, and one on each side.
Length 4²⁄₁₀ inches, expanse 9⁴⁄₁₀, flexure 4, reach of wings beyond the tail ⁹⁄₁₀, tail, outer feathers 1⁷⁄₁₀, uropygials 1³⁄₁₀, rictus ⁵⁄₁₀, beak ³⁄₂₀, tarsus ¼, middle toe rather less than ¼.
Irides dark hazel; beak black; feet purplish flesh-colour; claws horn-colour; inside of mouth, flesh-colour, tinged in parts with bluish. Head smoke brown, paling on the sides; back, wings, tail-coverts, and tail, sooty-black, unglossed, or with slight greenish reflections on the tail. Across the rump a broad band of pure white, the black descending into it from the back, in form of a point; sometimes dividing it. Chin and throat silky white, the feathers brown at the base; sides smoky-black, meeting in a narrow, ill-defined line across the breast; medial belly white. Thighs, under tail-coverts, and inner surface of wings smoky-black.
This delicately-formed little Swift, conspicuous even in flight, from the broad belt of white across the black body, is a very common species in Jamaica, where it resides all the year. Over the grass-pieces and savannas of the lowlands, the marshy flats at the seaward mouths of the valleys, as well as the pens of the mountain slopes, this swift-winged sylph daily urges its rushing course in parties of half-a-dozen to fifty or a hundred, often mingled with other Swallows, performing mazy evolutions, circling and turning, crossing and recrossing, now darting aloft, now sweeping over the grass, till the eye is wearied with attempting to follow them. The length of its wings, which is scarcely less than that of the whole bird,—renders it a fleet and powerful flier; an attentive observation will be able to identify it, when mingling in aerial career, by a more frequent recurrence of the rapid vibration of the wings, the momentary winnowing, by which a fresh impetus is gained. There is a very interesting structure in the sternum of this bird, which as far as I know is unprecedented. The sternum, though void of emarginations, possesses two oblong foramina of large size, one on each side of the middle of the ridge, and a round one perforating the ridge itself near the front margin. As all three are closed by the usual membrane, the object may be, the decrease of weight by the abstraction of bone, while the surface for the attachment of the muscles of flight remains undiminished. It would be interesting to know whether this structure is found in the Collocaliæ of the Indian Archipelago, to which the present bird bears a strong outward resemblance.
The stomach of one, a female, which I dissected, shot while hawking among many others over Bluefields’ grass-piece, in April, was distended almost to bursting with minute insects, which on being dispersed in water, and examined carefully with a lens, proved, I believe exclusively, the winged females of a small species of ant, exceedingly numerous, all more or less comminuted.
On the 20th of March last, visiting in company with Mr. Hill the estate called Dawkins’ Saltpond, the residence of the Spanish Admiral, at the time of the conquest,—I observed several small Swallows flying above some cocoa-nut palms; they uttered, as they flew, a continued twittering warble, shrill but sweet, which attracted my attention. I commenced a careful search, with my eye, of the under surface of the fronds and spadices of one, and at length discerned some masses of cotton projecting from some of the spathes, which I concluded to be their nests. This conjecture proved correct; for presently I discovered a bird clinging to one of these masses, which I shot, and found to be this White-rumped Swift. On my lad’s attempt to climb the tree, eight or ten birds flew in succession from various parts, where they had been concealed before. The tree, however, was too smooth to be climbed, and as we watched beneath for the birds to return, one and another came, but charily, and entered their respective nests. Although several other cocoa-nuts were close by, I could not discern that any one of them was tenanted but this, and this so numerously, whence I inferred the social disposition of the bird. At some distance we found another tree, at the foot of which lay the dried fronds, spadices, and spathes, which had been, in the course of growth, thrown off, and in these were many nests. They were formed chiefly in the hollow spathes, and were placed in a series of three or four in a spathe, one above another, and agglutinated together, but with a kind of gallery along the side, communicating with each. The materials seemed only feathers and silk-cotton (the down of the Bombax); the former very largely used, the most downy placed within, the cotton principally without; the whole felted closely, and cemented together by some slimy fluid, now dry, probably the saliva. With this they were glued to the spathe, and that so strongly, that in tearing one out, it brought away the integument of the spathe. The walls of the nests, though for the most part only about a quarter of an inch thick, were felted so strongly, as to be tenacious almost as cloth. Some were placed within those spathes that yet contained the spadices; and in this case the various footstalks of the fruit were enclosed in a large mass of the materials, the walls being greatly thickened. All the nests were evidently old ones, for the Bombax had not yet perfected its cotton, and hence I infer that these birds continue from year to year to occupy the same nests, until they are thrown off by the growth of the tree. The entrance to the nests, which were subglobular, was near the bottom.
Near the middle of May, my servant Sam, being engaged at Culloden, in Westmoreland parish, cutting the fronds of the palmetto (Chamœrops) for thatching, found these little birds nestling in abundance, and procured for me many nests of the present season. Their recent construction, and perhaps the diversity of their situation—for instead of the hollow of a spathe, these were attached to the plaited surface of the fronds,—gave them a different appearance from the former specimens. Many of these I have now in my possession. They have a singularly hairy appearance, being composed almost exclusively of the flax-like cotton of the Bombax, and when separated, are not unlike a doll’s wig. They are in the form of those watch-fobs, which are hung at beds’ heads, the backs being firmly glued by saliva to the under surface of the fronds, the impressions of the plaits of which are conspicuous on the nests when separated. The thickness is slight in the upper part, but in the lower it is much increased, the depth of the cup descending very little below the opening. The cotton is cemented firmly together as in the case of the others, but externally it is allowed to hang in filamentous locks, having a woolly, but not altogether a ragged appearance. A few feathers are intermixed, but only singly, and not in any part specially. One specimen is double, two nests having been constructed so close side by side, that there is but a partition wall between them. Many nests had eggs, but in throwing down the fronds all were broken but one, which I now have. It is pure white, unspotted, larger at one end, measuring ¹³⁄₂₀ inch by ⁹⁄₂₀. The average dimensions of the nests were about 5 inches high, and 3½ wide.
The genus Tachornis seems intermediate between Cypselus and Collocalia, with considerable general resemblance to the latter. This species is perhaps Hirundo 1, of Browne.