128 Grinnell, Geographical Abrasion in Birds. I Apr. 



LApr. 



GEOGRAPHICAL VARIATION IN ABRASION. 



BY JOSEPH GRINNELL, 



A STUDY of pertinent material has led me to formulate the 

 following generalizations : (i) That fading of plumage colors pro- 

 ceeds more rapidly in direct sunlight than in the less intense re- 

 flected or interrupted light ; in other words, that color changes due 

 to fading are far greater in birds of a region of much average daily 

 sunshine, than in one with an extreme proportion of cloudy weather. 

 (2) That abrasion of feathers progresses more rapidly in a dry 

 atmosphere than in a humid atmosphere ; for extreme dryness 

 seems to make the finer structures of the feather more brittle. 

 Abrasion in general is from two causes: the attrition of feathers, 

 one against another ; and the wear produced by contact with 

 foreign objects. Proposition number two apparently holds good 

 in both cases. 



To illustrate, a series of Cyanocitta stellef-i from the cloudy, 

 humid Sitkan District taken in June and July show but slight traces 

 of wear ; while specimens of Cyanocitta stelleri frontalis from the 

 arid Sierra Madre Mountains of Southern California taken at the 

 same season are so ragged and faded as to almost completely 

 destroy the fresh fall coloration. Several parallel cases present the 

 same relative conditions ; examples at hand from the same two 

 regions are Jutico^ Empidonax, Reguhis, Certhia, Dryobates, 

 Melospiza, and Hylocichla. 



I am well aware that in the case of birds which live in dense 

 vegetation the nature of the foliage with which they come in con- 

 tact has much to do with the rate of abrasion, for I have at hand 

 two lots of Song Sparrows taken within three days of each other in 

 June, one from a tule swamp, and the other from a saw-grass swale. 

 The birds from the tules are but moderately worn, while the others 

 are so much abraded on the breast, sides, wings, and tail, as to 

 have lost much of their distinctive coloration. But differences in 

 vegetation, if any exist to such a degree, do not seem to me ac- 

 countable in the cases cited above ; surely not with the Thrushes, 

 Jays and in particular the Flycatchers. 



At any rate, however variation in wear is brought about, its 



