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ECOLOGICAL RELATIONS OF BIRD- 

 DISTRIBUTION. 



BY 



S. E. BROCK. 

 Introductory, 



That side of their subject Mhich is concerned AA'ith 

 distribution has throughout received a prominent degree 

 of attention from ornithologists. While the resulting 

 literature abounds in supplementary details, the methods 

 therein employed, with rare exceptions, have been 

 essentially faunistic. But the faunistic method is by 

 itself admittedly inadequate if the ultimate aim in the 

 study of local distribution is to get beyond the immediate 

 phenomena to their causal relations. Towards this goal 

 the local list is in point of fact but the preliminary step 

 in a survey which, in its ideal aspect, constitutes a 

 complete analysis of the organism in relation to its 

 environment. If such a conception is as old as field- 

 biology, it yet remains true that the realization of the 

 need for a systematic approach of the subject of animal 

 distribution from the environmental side is of but recent 

 origin — inspired in large part by the work of the plant- 

 ecologists. 



The environmental control in distribution becomes 

 increasingly complex, and its incidence indirect, in 

 accordance with the stage of evolution reached by the 

 group concerned ; and, in correspondingly increasing 

 degree, a restriction of attention to the relation between 

 environment and physical constitution, must lead to 

 barren results. The distribution of birds is insufficiently 

 accounted for by the localization entailed by structural 

 specialization. 



Factors in Distribution. 



It is sufficiently obvious that the avifauna of a given 

 country is distributed neither accidentally nor imiformly ; 

 its dispersal is on the contrary both definite and imeven. 

 Despite their exceptional mobility, birds are essentiall}^ 

 local ; the *■' universally distributed " species is at most 

 universally distributed onty \vithin the limits of a uniform 

 environment. In one sense, and from one aspect, the 

 environment may be said to determine the avifauna. 

 The localization of birds is du'ectly related to the 

 localization of environmental tj^Dcs, Since this is so, an 

 accordant classification of the phenomena of distribution 



