468 R. T. YOUNG 



Summary 



A summary of this series of experiments shows that of thirty- 

 seven insects eaten in those experiments (1, 3, 7, and 4, first 

 part,) with four combinations of insects and backgrounds, nine- 

 teen or 51 per cent were taken from that background with which 

 they made the greatest contrast, eighteen or 49 per cent from an 

 intermediate background and none from that of least contrast. 

 In those experiments in which there were but two combinations 

 of insect and background (5, 6, and 4, last part) the prey was 

 taken from the background of greater contrast in fourteen cases 

 or 73 per cent and from the background of less contrast in 

 five or 27 per cent.^^ Considering the first choice of the birds 

 rather than the total number of insects eaten, we find that in 

 the four combination experiments, the combination of most con- 

 trast was chosen in nine out of thirteen cases or 69 per cent, an 

 intermediate combination in four or 31 per cent and the com- 

 bination of least contrast in none. In those experiments with 

 but two combinations, we find the first choice of the birds to 

 have been the combination of greater contrast in five cases and 

 that of less contrast in none. 



SERIES IV 



In this series three young individuals of Krider's hawk (Buteo 

 borealis krideri) and dead mice (Microtus drummondi and 

 Mus musculus) and shrews (Sorex personatus haydeni and Blarina 

 brevicauda) were used. 



The hawks were taken from the nest just before learning to 

 fly and kept in a cage 1.5 x 3.9 x 3.7 m. in size. At one end 

 of this cage backgrounds of different substances were prepared 

 upon which the prey was placed.^^ During the latter operation, 

 the hawks were on a perch at the opposite end of the cage about 

 one meter from the ground, care being taken not to attract 



1^ See Experiment 5. The insects were taken from the mud (less contrast) 

 background in this experiment only after at least three (possibly all five) had been 

 eaten from the sand (greater contrast background). 



'^ Dorsum uppermost except as otherwise noted. 



