WHITE-THROATED SWIFT. Aeronautes melanoleucus (Baird)

Field characters.—Resembling a swallow but wings much longer and more slender, and tail longer; outline in flight crossbow-like. Plumage black save for white on throat and mid-breast, and white patch on either side of rump. (See pl. 46g). Flight swift and erratic, with vary rapid beats of the wings which at times appear to operate alternately. Voice: A series of shrill twittering notes.

Occurrence.—Summer visitant locally in small numbers west of the Sierran crest, and below the Canadian Zone. Seen at Pleasant Valley and El Portal, and in Yosemite Valley. Extreme dates of observance, April 27 (1916) and September 29 (1915). Courses about in the open air, usually high over sheer cliffs.

Through our field observations we have come to associate the White throated Swift with open cañons or valleys flanked by bare rocky cliffs; and as the cañon of the Merced River affords locally just these conditions, it was no surprise to find numbers of these birds at Pleasant Valley and El Portal and in and about Yosemite Valley.

The White-throated Swift may be distinguished from any of the swallows by its crossbow-like outline of body and wings, the latter notably slender, its black plumage sharply relieved by white on throat and middle of breast (pl. 46g) and on sides of rump, and by its more reckless manner of flight. Its shrill twittering notes, of insistent quality, are also different from those of swallows.

The much frequented vantage places on the walls of Yosemite Valley, such as Columbia, Yosemite, Union, Sierra and Glacier points, afford good places from which to see White-throated Swifts on almost any day during the summer season. The birds often pass very close to an observer stationed on one of these points, sometimes dashing downward into the valley below at lightning speed, again pursuing a more level course with a half-mile of clear air between themselves and the Valley floor. They begin early in the morning and are active until late evening, even after the last rays of the evening sun have left the surrounding peaks and dusk is creeping into the gorge below. Thus on June 7 and again on June 23, in 1915, swifts were noted still abroad in the neighborhood of the Three Brothers at 7:30 P.M.

In all our watching of White-throated Swifts we have never seen one alight on the ground or upon any sort of perching place. But on one occasion, birds were seen to disappear in the face of the beetling cliff above the upper zigzag on the Yosemite Point trail. Their roosting and nesting places are located in narrow crevices of the rock walls, within which the birds cling, and such sites are, of course, practically inaccessible to any animal save the birds themselves.

From the Big Trees auto road above El Portal, on the morning of April 27, 1916, several White-throated Swifts and Violet-green Swallows were seen coursing about together over the Merced Cañon. Both species exhibited skill in their aerial evolutions, but the swifts were the more daring. They would dart downward, almost vertically, with such velocity that one would think they must be dashed to earth. Yet they checked their flight with apparent ease, and then circled lightly or sailed upward on set wings. Once three swifts swept past within a couple of yards of the observer, going at such high speed that their stiff-feathered wings made a distinct swishing sound as they cut through the air. On two occasions one swift was seen to pursue and grapple with another, as if to mate, and then the two went tumbling over and over, downwards through the air for a couple of hundred feet or more, to break apart and take opposite courses just before they reached the ground.