VOL. vm] RELATIONS OF BIRD-DISTRIBUTION. 31 



seems to be necessary to a full illustration of the whole 

 subject. But the term environment stands for a complex 

 of varied and intricate factors, with differentiated reference 

 to the fauna, and the preliminary difficulties arise in the 

 search for the dominant or immediate agents in dispersal. 



The local existence of a bird as a breeding species 

 is dependent upon the local conjunction of certain 

 environmental units ; and distribution is uniform in 

 proportion as these essentials are imiformly and evenly 

 dispersed. A shortage in one direction is not balanced 

 by superfluity in another. The specific range is neces- 

 sarily contained within the region of efficient food supply ; 

 but it is of equal necessity contained within that of 

 suitable climatic conditions, and within that of nesting- 

 ground. A sufficiency of food and the necessary climatic 

 conditions do not permit existence if nesting -ground 

 is absent ; an abundance of food and nesting-groiuid is 

 useless if the meteorological conditions are inimicable. 

 Where the requisite conditions are unequally distributed 

 in space, or relatively unequal in supply, there the 

 dispersal of a species will be correspondingly uneven, 

 limited by the most local, or the least adequately 

 supplied, of the essentials. In the case of many birds, 

 an increased supply of nest-sites is alone sufficient to 

 allow of an increase in population, which impHes that 

 the possible maximum is normally not attained through 

 a relative shortage of nest-sites. In such instances, 

 distributional range is co-extensive, less with food-supply, 

 than with nest-sites. The latter is therefore so far the 

 limiting factor in distribution. If a common limiting 

 factor exists, and can be recognised, then on it a 

 classification of the facts of distribution might be con- 

 veniently based. But this can hardly be looked for ; a 

 varying supply of varying needs must be expressed by 

 a limiting agent differentiated according to species. 

 Nevertheless, it appears probable that this dift'erentiation 

 is confined within relatively narrow limits. 



The governing or limiting factors in bird-distribution, 

 apparent in part through direct observation and comparison, 

 are perhaps tlnown into clearest relief in the phenomena 

 of range-extension — ^which process may be graded 

 separately in degree as it represents predominantly 

 either (1) the colonization of new areas ecologically 

 homogeneous with those of origin ; or (2) extension of 

 range involving adjustment to new environmental 

 conditions. 



