63 



£be Hir Sacs of Biros. 



By Ivan D. Murray. 



In the issue of The Field, of March 2St.lt last, an article under the 

 above heading appeared. To my mind the worst part of the article was- 

 that the matter therein was inconclusive. Suffice it to say that after 

 advancing seven theories in all, the author leaves one in doubt as to which 

 is correct. Briefly the seven theories are : 



1st. That the reservoirs of air serve to erect the feathers. 



2nd. That they are resonatory organs to increase the strength and timbre of the bird's 



voice. 

 3rd. That they have a directly respiratory function. 



4th. That they serve to reduce the specific gravity of the body, as a whole. 

 5U1. That they act like the swim bladders of fishes. 

 6th. That they are sense organs comparable with aneroids. 

 7th. That when inflated Ihey serve to fix the wings, when extended, in a horizontal 



position. 



And lastly, the " theory mostly favoured of late years," that they 

 serve merely as a mechanical adjunct to the respiratory system. 



On the top of all these theories, each perfectly feasible to the lay 

 mind, Mr. I). M filler, of the Smithsonian Institution, at Washington, brings 

 forward a theory of his own. 



His theory is " that neither the air sacs nor air cavities in the bones 

 of birds have special or positive functions of their own," but he regards 

 them "as a series of empty spaces of which the value lies in their very 

 emptiness." He argues that "the ancestral bird adapted itself for flight by 

 divesting itself of all superflous material, ami occupied the body spaces 

 thus obtained with air sacs, and further mobility of the parts was secured 

 by surrounding them with air sacs." Hence we must believe that the 

 connection between the air sacs and the lungs is merely an accident of 

 development, and beyond to a certain extent renewing the air in the 

 windpipe have no physiological significance. 



Now without dreaming of posting up an argument with so learned a 

 man as Mr. Mfiller, may not these sacs besides being accidental in the first 

 place have at some later date been made to serve as reservoirs. I remember 

 some years ago an old Naturalist, who was my mentor in those young days, 

 telling me that the wind never blew continuously, really, but with slight 

 breaks which allowed the birds to fly. Where he got this theory from I can- 

 not say, but I have noticed over and over again, that no matter how high a 

 wind there is blowing, at any rate near the earth, there are constant pauses, 

 not at any defined periods, but the blasts are not one long continued blast 

 but with breaks between. 



Now may not these sacs serve as reservoirs in case of need. Before a 

 flight is started the sacs would be filled with air for the purpose of levita- 

 tion. That supply is held there, in reserve, in the same way as the 



