308 University of California Publications in Zoology. [^°^- ^ 



SEXUAL SELECTION AS AFFECTING WHITE PAT- 

 TERNS. 



It remains for us to touch on the question, does sexual selec- 

 tion enter into the problem of white patterns ? For open-ground 

 birds we can promptly answer that it does not. There are, how- 

 ever, many instances among arboreal birds where the white 

 marking is intensified in the male. This very fact militates 

 against the physiological contention that greater vigor in the 

 male sex accounts for all color differences. White blotches or 

 bars are caused by the absence of pigment. Were it not for 

 the more intense coloration of other parts we might conclude 

 on this basis that the white-marked male is deficient in vigor. 

 It also argues against the view that white patches afford the 

 best possible concealing pattern, for in that case the female 

 should not have them obscured. Adherents of the directive 

 theory may find support for their views in the fact that white 

 wing bars do actually persist in the female in many cases, as 

 in the pine grosbeak {Pinicola enucleator) and the white- 

 winged crossbill {Loxia leocoptera) and others, so that it cannot 

 be regarded as due solely to sexual selection. The analogy in 

 the case of open-ground birds where both sexes are alike, as a 

 rule, strengthens the directive interpretation. Further, if we 

 are right in assuming that the male is the more vigorous and 

 the leader of bird movements, then there is ground for believing 

 that white markings, even though intensified in the male, are 

 directive in some sense. That such a relation exists among 

 warblers was the belief of Dr. Coues when he wrote the following 

 paragraph descriptive of the warbler family 



"Some travel true to the meridian in hours of darkness, 

 stopping at daybreak from their lofty flights to rest and recruit 

 for the next stage of the journey. Others pass more leisurely 

 from tree to tree in a ceaseless tide of migration, gleaning as they 

 go. The hardier males in full song and plumage lead the way 

 for the weaker females and yearlings." 



