494 R. T. YOUNG 



which it had seized in order to pursue another frog which moved 

 nearby; and in Series III, Experiment 1 a martin passed dead 

 insects several times without noticing them, but seized a Uve 

 specimen which I had left in the cage by mistake. In many 

 other instances also, notably in Series VII, Experiments 9, 14, 

 23, etc., and Series IX, Experiments 8 and 9, the bird passed 

 close to one or other of the backgrounds without apparently 

 seeing the prey upon it. 



In order to definitely test this point in the case of the hawks, 

 (Buteo borealis krideri) I performed the following experiments: 



SERIES XI 



Experiment 1. A dead mouse (Mus niiisculus) placed at one end- 

 of the hawk's cage, while the bird was at the opposite end 4.5 m. dis- 

 tant, was not noticed by the bird for several minutes, until the latter 

 was driven across the cage to where the mouse lay, when it was taken. 

 A cage containing a living mouse, actively moving aliout, was now in- 

 troduced into the hawk's cage, and no sooner was the mouse liberated 

 from its cage than it was seized by the hawk. 



Experiment 2. A dead rat (Epimys norvcgicus) placed on the 

 ground at one end of the cage with the hawk at the opposite end was 

 seized in two minutes. A live rat released in the cage ran toward the 

 hawk and when about one half the distance across the cage it was 

 seized by the latter. 



Experiment 3. Two mice (Mus musculus) were placed at one end 

 of the cage with the hawk at the opposite end. One of these was dead 

 and the other alive, attached to a small wire by one leg. After seven 

 minutes the hawk seized the live mouse. 



Experiment If.. Same as Experiment 3, with same result in five 

 minutes. 



Discussion 



Birds are frequently given credit for an extremely acute 

 vision and to this faculty is ascribed, by the opponents of the 

 protective coloration theory, their power to discern their prey 

 at long distances regardless of whether the latter is or is not 

 protectively colored. 



Thus Entz (1906, p. 136) says: 



. . wo aber ist je eine Beobachtung dariiber angestellt worden, 

 ob sich die Schutzfarbe den Tieren des Waldes und der Luft gegeniiber 

 wirklich als solche bewahrt oder ob etwa Vogel mit scharferen Sinnen 



