38 BRITISH BIRDS. [vol. viii. 



hand the advantage of clearness and simplicity, and it avoids 

 the admitted anomalies inevitable in the usual inclusive 

 method of grouping. But, accepting the customary view as 

 to the qualifications entitling a bird to a place in that palae- 

 arctic catalogue, the British bird-list, it is clear that the 

 breeding species in that list are in a category apart. Repro- 

 duction is the central fact in the life-history of the species, 

 and, in any classification of environmental relations, that 

 habitat in which reproduction occurs, takes premier position. 

 A classification of environmental types is necessary before 

 the phenomena of bird-distribution can be classified in turn. 

 The predominant importance of the vegetation as a factor 

 directly and indirectly governing bird -distribution suggests 

 the efficiency and convenience of an ecological classification 

 of the latter based preliminarily upon the plant-formations 

 and associations of the botanist : — in addition to the geological 

 factors which, in the form of cliff, sand-dunes, etc., exert 

 direct influence upon distribution. 



The Bird -Association. 



From the objective side bird-distribution is determined in 

 the main by such environmental agencies as vegetation, food- 

 supply, etc. But in view of the inter-relations of species 

 which proceed from the possession of a common habitat, the 

 term environment must be given a wider significance. A 

 bird's local position is influenced in varying degrees by its 

 associates. It may be entirely dependent upon their presence ; 

 or its numerical position may be affected by the presence of 

 others ; or the prior existence of another species may be a 

 barrier to colonization. The intricate inter-relations and 

 inter -dependencies of associated species indicate the need for 

 regarding such groups of species as vmits of classification. 

 Similar considerations have led to the conception of the plant- 

 association ; and, on similar lines, a bird-association may be 

 defined as that group of species supported by, and associated 

 in, an environment of specific type. 



The fundamental link between the species comprising an 

 association is the common attachment to a common environ- 

 ment. To this other inter-relations are necessarily subsequent 

 and secondary. The most simple and direct relations concern 

 competition for food, and for nesting-ground. The intensity 

 of competition in the latter is proportionate to tlie degree of 

 coincidence in nesting -habits. On the average, it will be 

 least in unrelated species, more active in congeneric, and will 

 reach its maxunum within the single species. Population- 

 density will be proportionate to the food-supply, or to the 



