472 R. T. YOUNG 



Summary 



In eleven (Experiments 2, 4, 6, 8 to 10, 12 to 15) of the above 

 seventeen experiments or 65 per cent the combination showing 

 greater contrast was chosen by the birds, and in five (Experi- 

 ments 3, 7, 11, 16, and 17) or 29 per cent that of less contrast 

 was selected, while in one (Experiment 1) although the prey 

 was not seized by the hawk, the latter was evidently attracted 

 by the combination of greater contrast. Experiments 3, 11, and 

 15 are doubtful for reasons already stated. ^^ Omitting these and 

 Experiment 1, we have the following results: 



Combination of greater contrast chosen in ten cases or 77 per 

 cent, that of less contrast in three or 23 per cent. 



SERIES V 



In this series two^'' young hawks (Buteo borealis krideri) 

 and mice (Microtus drummondi, Peromyscus gambeli bairdi, 

 Mus musculus) and rats (Epimys norvegicus) were used. The 

 hawks were taken from the nest before learning to fly and 

 confined in a cage 5.0 x 2.4 x 1.5 m. in size, at one end of w^hich 

 were several perches placed from 0.3 to 1.0 m. from the ground. 

 At the opposite end of the cage backgrounds (described under 

 the different experiments) were prepared, upon which was placed 

 the prey.^^ Care was taken while placing the latter to avoid 

 attracting the attention of the hawks. This however was prob- 

 ably not a matter of great importance, as the latter seldom 

 noticed the prey until several minutes after I had left the cage. 



Experiment 1. Figure 9. Two backgrounds, one of moist earth, 

 and one of ashes, were employed in this experiment, on each of which 

 a Microtus was placed (venter uppermost). The mice were partly 

 imbedded in each background so as to expose only the under surface, 

 legs and tail. The white belly of the mouse on the earth presented 

 a much stronger contrast with its background than that of the mouse 

 on the ashes did with its background. Both hawks were on one of 

 the perches at the opposite end of the cage and remained there for 

 about a half hour evidentlj" not seeing the mice. Then I disturbed 



^^ See footnote '^ and italicized lines in Experiments 11 and 15. 

 1' In Experiments 15 to 19 three haAvks were used. 



