60 HOMING AND RELATED ACTIVITIES OF BIRDS. 



so great that there could be no question of direct visual stimulation offered by 

 the goal (which in this case we may consider to be the lighthouse, 151 feet in 

 height) at distances greater than 100 miles (probably far less than this when 

 we consider the fact that there is a continuous haze over the water). Even 

 at 100 miles the bird must fly to a height of 0.9 mile in order to receive the 

 light rays reflected from the goal. 



(4) We have shown that the hazardous speculation of Duchatel, which was 

 designed to overcome the difficulties of the visual hypothesis discussed in 

 (3), is without foundation in fact. The investigation reported on page 87, 

 on the limits of spectral sensitivity in birds, shows clearly that neither the 

 chick nor the pigeon is sensitive to infra-luminous rays. Furthermore, it was 

 shown that the retina of the chick (and incidental confirmation has been found 

 in tests on the pigeon) is probably less efficient in responding to monochro- 

 matic light of low intensity than the retina of the human being. The single 

 exception to this statement is to be found in the extreme red, where the retina 

 of the chick is slightly more sensitive than man's. If these facts are considered 

 in connection with Johnson's work, which shows that visual acuity in the 

 chick is far less than in man, we may very well accept with caution the popular 

 view concerning the perfection of the organ of vision in birds. 



(5) Our work has shown further, in the terns at least, that there is no special 

 Spiirsinn special tactual or olfactory mechanism situated in the nasal cavity 

 which may function in homing. 



Our contributions are admitted to be negative in character. The difficulty 

 of explaining homing by current theories is seen to be great, but, while admit- 

 ting this, we do not suggest the assumption of some new and mysterious sense. 

 The task of explaining distant orientation is an experimental one, which must 

 yield positive results as soon as proper methods are at hand. Two lines of 

 investigation offer hopeful results : 



(a) The rearing of homing pigeons in a cote which is covered by a tent suf- 

 ficiently large to offer the birds wing practice and to make proximate adjust- 

 ments to their cote. Birds reared under such conditions may be sent out for 

 a test of the possibility of homing in an untrained animal. After shipping the 

 birds the tent which covers the cote could be removed. If they returned to 

 the neighborhood the visual stimulus offered by the cote would arouse the old 

 habit of entering it, etc. 



(6) The same possibility is offered on a smaller scale by a slightly similar 

 method. We could rear the birds in a wire-covered yard attached to a cote; 

 then, as was done by Hachet-Souplet with the birds living in the traveling 

 cotes (an experiment which certainly ought to be confirmed), we could tether 

 individual birds to the top of the cote by cords which would permit a view 

 only of the neighborhood immediately surrounding the cote. These birds would 

 next be sent into a region which could not have stimulated their vision previ- 

 ously. Again, if the birds effected a return to the neighborhood of the cote, 

 they would resume their old habits. 



With these experiments upon homing, work upon the sensory equipment of 

 the homing pigeon should be carried on. It is just possible that these animals 

 possess on certain parts of the body (eye-lids, ear covering, oral cavity, etc.) 

 sensitive tactual and thermal mechanisms which may assist them in reacting 

 to slight differences in pressure, temperature, and humidity of air columns. 



