20 THE CONDOR Vol. XX 



air-sac was inflated, the distension varying from one-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 living 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-mflation 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) 



DURING 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 



