ORNITHOLOGIST 



— AND — 



OOLOGISTc 



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PUBLISHED BY THE FRANK BLAKE WEBSTER COMPANY. 

 Established, March, 1875. 



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Vol. XVII. 



HYDE PARK, MASS., MARCH, 1892. 



No. 3. 



Geographical Variation in Birds. 



In my first paper relating to surface geology 

 and tlie birds, I liave sliown liow different parts 

 of the country came to 1»e inhabited by pecu- 

 liar species. In this i)ai:)er 1 shall endeavor to 

 give an idea as to why such changes take 

 place. 



If we suppose an instance where a bird is 

 perfectly fitted in a special manner to lead its 

 life in a particular way: it may feed entirely 

 upon fruit, which tlie climate admits to mature 

 at all seasons and in sufficient quantities to 

 constitute a never-failing supply ; it may breed 

 in hollow trees, common in humid climates. 

 The food is healthy and sustaining, tlie nesting 

 places secure from many predatory animals, 

 and under the influences of this favorable 

 environment the species thrive and multiply. 



If they, on account of individual variation, 

 cliance to differ in any way from the parent 

 stock they are consequently in discord with 

 their surroundings, and their associates which 

 vary less have a better chance of survival, and 

 to leproduce their kind. In this way the 

 species is kept true to the parent stock. But 

 if tlie climate should undergo a slight change 

 the whole state of equilibrium is upset. A 

 drier atmosphere or a lower temperature may 

 strip the country of its fruit-bearing forests, 

 and new plants may take their places, and 

 thus materially alter the conditions of exist- 

 ence of the birds which were perfectly adapted 

 to the old regime. One of three things must 

 inevitably happen: the birds must die, they 

 must search for new quarters, or they must 

 conform to the new conditions of life. 



The last is accomplished by a natural, selec- 

 tion applied to the individual variation which 

 had pr 'viously been detrimental to the species. 

 jVow, however, any slight deviation from the 

 ortliodox habits, as, for instance, in the selec- 

 tion of the nesting-site, or an acquired appetite 

 f ■)r a new article of diet, would be a benefit to 



the race, and the advantage gained by such 

 individuals over their fellows would enable 

 them, by means of a competitive system which 

 has been termed natural selection, to live and 

 multiply where others would perish. Those 

 of their offspring which inherited the parental 

 traits would mature, only to leave more 

 descendants, and the variation once started by 

 chance would become more and more notice- 

 able and universal as time progresses. 



Nowhere is a knowledge of the laws of vari- 

 ation, and consequently of geographical dis- 

 tribution, more needed than in the study of 

 American ornithology, for it is in this country 

 natural selection has created varietal or sub- 

 specific forms and closely allied species, on 

 the grandest scale. As the process of their 

 restriction to comparatively small areas was 

 discussed in the first paper of this series, it 

 only remains to refer to the more general laws 

 of variation.* 



Tliere is an axiom, first established by Prof. 

 Baird, that birds having a wide range increase 

 in size from south to north, and from east to 

 west; and, as if for compensation, particular 

 organs, viz. : the beak and tail, decrease in 

 proportion to the increase in size, and, in some 

 cases, the measnrements were absolutely less. 



In color, the intensity is found to be great- 

 est in the more southern limits of the habitat, 

 and decreased toward the north. West of the 

 Rocky Mountains, and especially in the arid 

 regions of Arizona, New Mexico and Nevada, 

 a general bleaching or fading is the rule, but 

 on the middle and northern Pacific Coast the 

 colors are again intense, with a tendency 

 towards ferruginous tints in the former. 



This was one of the first steps to the proper 

 understanding of geographical variation, which 

 soon after resulted in the recognition of sub- 



* For the subject of fannal areas the reader is 

 referred to NortJi American Fouiia, No. 3, by Dr. C.- 

 Hart Merriani, and published by the U. S. Department 

 of Agriculture. 



Copyright, 18J2, by Frank Blake Webster Company. 



