154 CALIFORNIA ACADEMY OF SCIENCES. 



are found in Sphyrapicus, Platypsaris and Hubia ludo- 

 vicia/aa. There is a curious resemblance in the col- 

 ors of this latter species with the becards (Platy- 

 psaris). In both cases the tendency is toward black on 

 the upper parts of the body, white on the lower parts, 

 and a rose colored breast. The red-winged blackbirds 

 (Agelaius), are unique in the red shoulder patch. It is 

 a noteworthy fact, and one which is too apt to be looked 

 upon, as a matter of course, that scarlet, or any other 

 brilliant color, when occurring only in limited areas, is 

 never in an inconspicuous place, as upon the abdomen, 

 for instance. 



The presence of red in connection with slaty gray in 

 a few instances, as in Pyrrhaloxia sinitata and Pinicola 

 cnudedior, may be worthy of note. The most significant 

 interrelation of colors, however, and one which I 

 believe to be of wide application in the explanation of 

 bird colors, is that between red and yellow. Whether 

 these two colors are produced by the same or a different 

 pigment I am unable to say; but, however this may be, 

 there is a high degree of probability that the red is 

 simply an intensification of the yellow. There is much 

 evidence to show that yellow is a more primitive stage 

 than red, and that the latter has always, or nearly 

 always, been developed from the former. What the 

 factors in this intensification may be I will not attempt 

 to state, merely suggesting that heat, moisture and food 

 may play an important part. The term Coeeelative 

 Colors will be an appropriate one by which to indicate 

 this interrelation. 



Now, as to the evidence of this dependence of red 

 upon yellow. Plate V indicates a number of instances 

 of correlative colors. What I have attempted to show is 

 that when I'ed occurs in a group of birds, yellow will be 

 found in the same group, and that the yellow represents 



