228 Nichols, Warbler Problems. [April 



is a problem to the individual bird is evidenced by the adoption 

 of horse-hair by the Magnolia Warbler to supplant the very similar 

 "moss-stems" which doubtless were its original material. The 

 Chipping Sparrow must have substituted horse-hair for some pre- 

 civilization material, and its habits are such that horse-hair is 

 almost always obtainable by it and now almost the invariable 

 nest-lining for the species. It is clear that to be successful the 

 nest-building instinct of a given species must be pretty well fixed, 

 that a bird must know what material it will use, also were all the 

 Dendroicas dependent on, — let us say, feathers, horse-hair, or 

 rabbit fur, there would be less of it for each, and specific differentia- 

 tion is thus an advantage to the Dendroicine population as a whole. 



Secondly, what advantage to the species is there in their con- 

 trasted plumages — in the writer's opinion the colors of each act 

 as a uniform, facilitating the recognition by a bird of its own kind 

 just as they facilitate its recognition by a bird student. 1 



A varicolored group of animals such as Dcndroica, where many 

 related species occupy the same locality, — other such groups come 

 to the writer's mind, notably among tropical reef fishes, — should 

 be considered in formulating or accepting theories on species forma- 

 tion. In many cases isolation and reinvasion are doubtless the 

 succeeding steps in speciation, a process clearly indicated by work 

 recently done by Taylor on the mammals of California. 2 There 

 is no inherent impossibility of the many Dendroicas of eastern 

 North America having been similarly evolved, but with them it 

 would seem to have been a difficult and complicated process instead 

 of a simple and easy one, as with sedentary mammals in a broken 

 country, and may not the forms have arisen for biological advantage 

 without these steps? 



> Nichols, J. T., Auk. Jan 1912; pp. 44-48. 



> Taylor, Walter P. Univ. of Cal. pub. Zoology, Vol. 12, no. 15, March, 1916. 



