IV. 



Note on the Air-Sacs and Hollow-Bones of Birds. 



MR, F. W. HEADLEY, in his interesting paper in the November 

 number of Natural Science (voL iii., pp. 346-356), calls 

 attention to the fact that while the Gannet and the Cormorant 

 belong to the same order, the one has large air-sacs and a remarkably 

 pneumatic skeleton, while the other is devoid of air-sacs and has 

 practically non-pneumatic bones. I have said practically non- 

 pneumatic, because in Graculus cavho and related forms, the humerus 

 and sternum have small pneumatic foramina, while in the extinct 

 Phalacrocovax pcrspicillatus and other grooved-billed species these bones 

 are quite non-pneumatic. Mr. Headley suggests that light bones 

 would be out of place in, or unserviceable to, diving birds, and this is 

 undeniably the case, since the problem with them is how to keep under 

 water. 



While the solid body and dense bones of the Cormorant are 

 directly correlated with its mode of life, the air-sacs and pneumatic 

 skeleton of the Gannet appear to be equally good adaptive characters. 

 The Cormorant pursues its prey beneath the water, the Gannet plunges 

 upon its victims from above, dropping upon them headlong from 

 heights of one to two hundred feet. Now, as the cunning mechanic 

 has devised an air-chamber to break the accidental fall of an elevator 

 (American for lift), so nature has applied about the neck and breast 

 of the Gannet a series of air cushions to break the shock which occurs 

 when a bird weighing six or eight pounds strikes the water after a 

 drop of a hundred feet or so. 



In the Brown Pelican {Pelecanus fiirctis) there are many air-cells 

 present about the breast forming a thick mass of loose areolar tissue 

 located exactly where they would do the most good as a buffer when 

 this big bird plumps (dashes is hardly the word to use in connection 

 with a pelican) down upon a shoal of fish. I have never dissected 

 any of the white pelicans, and as these birds fish in quite a difierent 

 manner from their darker relatives, it would be interesting to know if 

 there is any corresponding difference in the arrangement or number 

 of the air-sacs. 



Another point possibly served by the air-sacs, as well as by the 

 pneumaticity of the bones, is that of equalising the pressure of the 

 air when a bird either dives from air into water, as does the Gannet, 



