EVOLUTION OF THE COLORS OF BIRDS. 157 



tion of color changes from yellow to red is, the fact that 

 there are so few exceptions to it. The vermilion 

 flycatcher {Pyrocephulus rubineus mexicanus) is a partial 

 exception to it, although the female is sometimes tinged 

 with a salmon color which is not far from yellow. The 

 female cardinal grosbeak {Gurdinulis vivginianus) is dull 

 brownish or reddish, but the head and under parts are 

 more or less tinged with yellowish. Acanthis with 

 its red head is not far removed from the goldfinches 

 (Spinus). The becards (Platypsaris) and the red warb- 

 ler {Ergaticus ruber) form the only complete exceptions 

 to the rule with which I am familiar, but a knowledge 

 of allied South American forms might bring them also 

 under the law of correlation. 



There are no North American birds which are ex- 

 clusively yellow in color. In the following species it is 

 the predominant hue, or figures very conspicuously: 

 XanlhoceiJhalus xanthocepha- Piranga ludoviciana. 



lus. Dendroica cestiva. 



Sturne.Ua magna. Sylvania pusilla. 



Icterus. Geothlypis trichas. 



Goccothraustes vespertinus. Vireo. 



Spinus. 



It is conspicuously present among the warblers 

 (Mniotiltido}), and in such of the vireos as Vireo jiavi- 

 frons and V. flavoviridis. As a rule it is found upon 

 the under surface of the body, especially when in large 

 masses. In fact, when the tendency is for the whole 

 bird to be thus colored, it will be noticed that the yellow 

 upon the back is apt to be less pure, with more of an 

 olive or greenish tinge, as in Dendroica cestiva, the fe- 

 males of Piranga, etc. 



Two North American species are almost exclusively 

 blue in color — the indigo bunting (Passerina cyanea), 

 which has the lores black, and the arctic bluebird 



