Chap. 36 



BIRDS — CONQUEST OF THE AIR 



747 



Fig. 36.18. An x-ray photograph of a living hen showing an egg about to be 

 laid, 25 1/2 hours after the last one was laid. Note that there are no bones below 

 the egg. The skeleton is strikingly open at the rear, a reminder that birds are the 

 only animals that produce such large hard-shelled eggs. Actually birds have a 

 pelvic saddle, not a pelvic girdle. (Courtesy, J. A. F. Fezzard: Series (1) of 

 Medical and Biological Illustration. Cambridge, England, Cambridge University, 

 1951.) 



The eyesight of birds is exceptionally keen. They can see to dart through 

 trees without striking a twig and to alight on one branch out of a thousand 

 others. This means constant shifts from far to near vision and reverse — great 

 power of accommodation. A sparrow hawk can drop down upon a beetle 

 after hovering 200 feet above it; by rapid peering this way and that chicka- 

 dees and warblers catch even the smallest insects on rough bark. The eyes of 



gins to develop; 13 and 14, lower regions of the oviduct; the latter is the part where 

 the shell is secreted; 15, the alimentary canal (cut off); 16, reflected body wall; 

 17, external opening of the cloaca. (Fertilization of the egg occurs before it is 

 coated with albumen.) (From Hamilton: Lillie's Development of the Chick. 

 Copyrighted 1952 by Henry Holt and Co. Reprinted with their permission.) 



