20 TH]? CONDOR Vol. XX air-sac was inflated, the distension varying from o.ne-third full size to nearly two- thirds its complete capacity, according to circumstances. The air-sac lies between the trachea and the esophagus and could be felt with the fingers as a soft com- pressible bladder-like body that slipped about easily under the loose skin of the neck. In making the original dissections no sphincter muscle controlling the out- flow of air was found, but it was supposed that the muscle sterno-trachealis? which is expanded over the air-sac, with the anterior angle of the expansion at- tached to the esophagus, "may close the aperture of the air-sac by pressing the esophagus against it." Observation of these livin. g birds showed that this was true, and the contraction of this muscle was so strong that considerable manipu- lation was required to deflate the sac. I had supposed that the air-sac would always be deflated in diving, but this was not true in submersion to moderate depths at least. When birds were held under water it was possible at first to force out a small amount of air by compressing the air-sac between the fingers. This escaped by bubbles through the nostrils. Immediately, however, the muscle controlling the neck of the vesicle was contracted firmly, and a steady pressure of fifteen pounds or more on the sides of the air-sac failed to expel more of the air contained. It was possible however to deflate the sac completely by gentle manipulation with the fingers while the bird was not submerged. The process of re-?nflation was slow, and was accomplished in three or four stages during an interval varying from one to three or four minutes in duration. There was no visible effort made by the bird during this process. Examination of these living ducks substantiated the statement made that the tracheal air-sac was a secondary sexual character present in males but absent in females. Though the skin of the neck was full and loose in females there was no trace of a distended air-sac under it. Careful examination was made of several individuals in order to substantiate this point. That male birds habitually keep this air-sac partly inflated even while div- ing is a strange fact as it might be supposed that the increased buoyancy would render it more difficult for them to stay submerged especially as the air reservoir is near the anterior end of the body. The only apparent use that it might have under these conditions is the questionable one of furnishing a reserve supply of
air that might enable the bird to stay beneath the water for a longer period than
normal. Need to utilize this, however, would not arise save under unusual cir- cumstances. Washington? D.C., October 21? 1917. NOTES ON SOME BIRDS FROM CENTRAL ARIZONA By H. S. SWARTH (Contribution from the Museum of Vertebrate Zoology of the University of California) URING the summer of 1917 the writer spent several weeks in that portion of central Arizona traversed by the scenic highway known as "the Apache Trail", extending between Phoenix and Globe. The trip was undertaken at the instance of Mr. E. O. McCormick, vice-president of the Southern Pacific railroad, with the purpose of obtaining data for a popular account of the birds of the region. Specimens collected were, by agreement, placed in the collection
