Cliff Swallow Colonies of the Central Sacramento Valley in 1941
Cliff Swallow Colonies of the Central Sacramento Valley in 1941
Between April 20 and May 30, 1941, the writer had occasion to traverse approximately 850 miles of roads in the Sacramento Valley, California, between Sacramento and Marysville, an area of about 1600 square miles. Each flock of Cliff Swallows (Petrochelidon albijrons) encountered on these travels was investigated and recorded on a spot map together with pertinent data on numbers of individuals, nest sites and nesting activity.
A total of 68 colonies comprising approximately 8200 birds was recorded (fig. 70). Undoubtedly colonies were missed, but since every river and creek bridge in the area was visited, the survey is thought to approach completeness. Colonies ranged in size from a single pair (one instance) to 320 pairs, and averaged 60 pairs. Numbers were determined by direct counts of nests or birds, or, when this was impracticable, by rough estimate. Barn Swallows (Hirundo erythrogaster) nested in close association with the Cliff Swallows in 6 colonies, and English Sparrows (Passer domesticus) had appropriated swallow retorts in several instances. The average spacing of recorded colonies was one to every five miles, but distribution was far from uniform. Colonies were most numerous around irrigated farm lands near the river channels and least numerous in poorly watered grain and pasture lands. Foraging areas were small and colonies often were closely spaced. Two of the largest colonies (near Nicolaus) were scarcely half a mile apart, and three colonies of 75, 30 ahd 200 pairs, respectively, were located within a distance of two miles along the "river road" east of Woodland.
The main requisites for a nesting site in the ;area surveyed seem to be: (1) a protected vertical surface, preferably of stone, concrete or stucco, under an overhang, for the attachment of nests; (2) a reasonably open terrain in the immediate vicinity of the nesting site; and ( 3) a readily available supply of wet mud (and water?) close to the nesting site. All the colonies in the area were on man-made structures: 39 on concrete bridges, 13 on wood bridges, 7 under the eaves of concrete or stucco buildings, 8 under the eaves of wood buildings and 1 under a steel bridge. The 200 nests in this latter colony were situated on the draw of a draw bridge so that they were tilted through 90 degrees at each raising of the bridge, which occurred at least once. a day.
John T. Emlen, Jr.
University of California, Davis, June 24, 1941
