CLIFF SWALLOW. Petrochelidon lunifrons lunifrons (Say)

Field characters.—Body size about that of Linnet or Junco; wings long and narrow; tail short, practically square-ended. (See pl. 46d). Forehead creamy white; back, wings, and tail black-appearing; rump yellowish brown; cheeks and chin dark reddish brown, with a blackish patch on throat. Voice: A weak chuckle.

Occurrence.—Common summer visitant locally at the lower altitudes on both sides of Sierra Nevada. Recorded from Snelling and near Lagrange eastward to Bower Cave. Also, east of mountains, near Williams Butte and on Rush Creek. Local distribution controlled largely by availability of rough rock walls or of weathered buildings upon which to place nests. In colonies of few to many pairs. Forages over open fields or smooth water.

The Cliff Swallow is probably the best known of all our species of swallows because of its common occurrence about human habitations. Originally, as its name indicates, this bird placed its nests on the rocky walls of cañons and on river bluffs, in consequence of which it was correspondingly restricted as to local occurrence. With man's erection of barns and other rough-walled buildings, the Cliff Swallows took to nesting on these structures and so appeared in many new localities. In addition to extending its local range, it is certain that this swallow, in many parts of the west, has increased in aggregate numbers.

The Cliff Swallow is the most colonial of our six species of the swallow family. Wherever found it is represented in some numbers and its nests are placed in the closest sort of mutual proximity. At Pleasant Valley on May 25, 1915, ten or a dozen pairs had their nests on the weather-beaten station house, and near Merced Falls on May 28, the same year, a colony of about 20 pairs was nesting on the undercut walls of a small gully in the prairie. (See pl. 47a). Near Lagrange on May 6, 1919, an assemblage of fully 75 pairs was busily engaged in constructing nests on the face of a stratified cliff at the side of the Tuolumne River. At the McCarthy ranch east of Coulterville, and at Bower Cave, Cliff Swallow nests were seen on the inside of farm buildings, access to the interior in each case being provided by a large open doorway through which the birds could fly to and from their nests.

The swallows of the Yosemite section can be divided into three groups according to their manner of nesting. The Rough-winged Swallow nests in a hole in a bank, the Western Martin and the Tree and Violet-green swallows seek natural cavities in trees or, in the latter species, also in rocks, while the Barn and Cliff swallows being skilled masons build elaborate nests outside of any cavity, using mud for structural material.

The home of the Cliff Swallow is shaped like a gourd or retort, having a rather narrow entrance and expanding basally to accommodate the nest proper. (See pl. 47a). The structure is built entirely of mud (save for a slight lining of soft fibrous materials) which is gathered and applied wet in the form of small pellets. The building of such a nest is a labor which must extend over several days in order that the basal portion of the nest may dry and thus gain strength to hold the later additions. When a nest is well under construction the observer finds it composed of mud in several stages, from the entirely dry base to the wet, most recently applied, material at the rim. While gathering the small rounded pieces of mud the birds at most barely alight upon the ground, balancing with their wings upraised and quivering. As the source of supply for mud is often at some distance from the colony the total amount of energy expended in the construction of a nest by a single pair of birds is considerable. Moreover, work must be suspended at frequent intervals in order that the birds may hunt for food.

The Cliff Swallow arrives in the western part of the Yosemite region some time in March, but the birds there do not begin nesting until early May. This delay is probably due to the relative paucity of insect life in April as compared with the plenty in May and June. When the adults arrive they can find sufficient forage for themselves, but at that season there is not enough to enable them to feed a brood of young. Hence they delay until the food supply is adequate for the increased needs of nesting time. East of the mountains, near Williams Butte, in 1916, the birds returned for the season on April 27.