Page:Auk Volume 37-1920.djvu/690

 57 A Averill, Migration and Physical Proportions. [oct. Sitta pusilla. Brown-headed Nuthatch. In company with the preceding and with Red-cockaded Woodpeckers. Five. Baelophus bicolor. Tufted Titmouse. Fifteen. Penthestes carolinensis carolinensis. Carolina Chickadee. Twenty. Regulus calendula calendula. Ruby-crowned Kinglet. A sing- ing male at Orton, April 18. Polioptila caerulea caerulea. Blue-gray Gnatcatcher. Five. Hylocichla mustelina. Wood Thrush. Song heard at Wilmington, April 19. Silalia sialis sialis. Bluebird. Three. Eastern District School, Brooklyn, N. Y. MIGRATION AND PHYSICAL PROPORTIONS. A PRE- LIMINARY STUDY. BY C. K. AVERILL It is a matter of common observation that birds most capable of long sustained flights are long winged. Such are the swallows and swifts on land and the terns, plovers and sandpipers along the shore. A bird flying 35 miles per hour passes through the air at the rate of 51 feet per second and the form of the tail evidently has much to do with the resistance offered by the air. It is evident that the stream lines that pass under the body of the bird will converge at the rear of the body, striking against the tail and causing undue pressure. In birds of superior power of flight — terns, swallows, swifts, gulls, kites, the tail is either forked or it is short, in either case there is little tail beyond the end of the under tail coverts in the median line.* It is the mechanical function of the under tail coverts to fill in the angular space where the tail joins the body where without the coverts an area of reduced pressure would be formed increasing the resistance. The tail of the barn swallow, "In the soaring hawk or eagle the large broad tail forms one of the three planes which support the body.