STUDIES ON THE SPECTRAL SENSITIVITY OF BIRDS. 



METHODS OF STUDYING SPECTRAL SENSITIVITY. 



In the literature bearing upon the homing sense (distant orientation) one 

 meets with the theory* that possibly those animals in which such a function 

 resides possess retinas sensitive to extremely long wave-lengths. It is further 

 supposed by the adherents of such a view that these long waves follow the 

 surface of the earth. f As a consequence, any animal with very keen vision, 

 having a retina sensitive to them, could directly respond to the visual stimulus 

 of a distant goal (cote, nest, etc.), the curvature of the earth by hypothesis 

 not interfering with the continuity of vision. Such a theory, while speculative 

 in the extreme and without good physical support, can not be dismissed with- 

 out some experimental tests, especially in view of the fact that no hypothesis 

 of return (logically) simpler than this has yet been advanced. 



During the spring of 1912 we decided to make a careful test of the limits of 

 spectral sensitivity in the noddy tern and in the sooty tern. It is clear that 

 the above theory has no standing if it can be shown that the animal under 

 consideration has a spectrum coextensive at the red end with that of man 

 (X = 7600). In order to make the work more complete we purchased some chicks 

 (of mixed breed) for testing in the same apparatus. We set up the apparatus, 

 consisting of a large spectrometer with suitable compartments for controlling 

 the responses of the Ijirds, in the Marine Biological Laboratory of the 

 Carnegie Institution of Washington, Tortugas, Florida. After capturing and 

 rearing 12 young noddies and 12 young sooties and constructing a rather 

 elaborate dark room, we found that the small electric-light plant which we 

 had installed could not be run with sufficient steadiness for use with the Nernst 

 lamp. We finally decided to abandon the experiment at Tortugas and to 

 remove the apparatus and the birds to the Psychological Laboratory of The 

 Johns Hopkins University. 



The l)irds arrived safel.y, standing the long journey by rail very well. They 

 were jjlaced in a large yard and fed twice ilaily with chopped fish. For some 

 reason they did not thrive, and died one by one. By the time the apparatus 

 could again be made ready all the terns had died. There remained only the 

 three chicks brought from Tortugas. We thought it distinctly worth while 

 to test them, since they are day birds and especially sensitive to red rays, as 

 has been shown by Hess.f 



*Facultfi de direction du pigeon voyageur. Ann. de Psych. Zool., i. 1901. p. 22; ii, 1902, p. 

 48. The above article was written by Haciiet-Souplet. Duchatel'a view appears in the article 

 in a series of letters. 



fDuchatel and Hacliet-Souplet, as has been pointed out by E. Branley, in the article referred 

 to above (ii, p. .57), are apparently not familiar with the properties of light. Duchatel at least 

 dra\vs an analogy between the Hertzian waves and the infra-red rays. Now the Hertzian waves do 

 apparently follow the surface of the earth (over water at any rate; as to their course over a moun- 

 tainous region there seems to be some question). But the Hertzian waves have a length varjang 

 from a few centimeters to many meters, and are not to be compared with the relatively extremely 

 short infra-red rays which may be obtained from prismatic spectra. Contrary to Duchatel, the 

 violet rays would come nearer fitting his theory, since they suffer greater refraction by the different 

 layers of the earth's atmosphere. Had his theory been based upon ultra-violet rays rather than 

 the infra-red, it would have had slightly better physical support. 



JHess, C, Ueber Dunkeladaptation bei Hiihnern und Tauben. Archiv f. .\ugenhcilkunde, 

 57, 298; also 57. 317. 



87 



