Chap. 36 



BIRDS CONQUEST OF THE AIR 



745 



Syrinx 

 (voice box) 

 at base of 



trachea 



Interclavicular 

 sacs 



Intermediate sacs 



Fig. 36.16. The respiratory organs of a pigeori. The lungs fit closely to the ribs 

 and do not dilate. The air sacs are extensions of the lungs. Their thin transparent 

 walls are freely expansible and they communicate with one another directly or by 

 way of the lung cavities. Air sacs constitute a ventilating system which moderates 

 the high body temperature of the bird. The syrinx, the unique voice box, is located 

 at the junction of the bronchial tubes close to the lungs. In this figure it is hidden 

 by the air sacs. (Redrawn and modified from Muller: The Air Sacs of the Pigeon.) 



along the sides of the bills between which particles of food are strained from 

 the water; bristles about the mouths of the fly-catching phoebes are responsive 

 to contact with small insects. Next to sight, hearing is the most important sense. 

 Birds have no external ears, but near their peculiar bony eardrums the feathers 

 are especially open to currents of air. Barn owls have folds of skin near the 

 eardrums that they can lower or raise to make catch cups for sounds. 



