BEHAVIOUR OF MK;HATORY MAI.KS 23 



with which the initial sta^e in the sexual process 

 was marked off, as a result of the incidence of 

 migration, would go far towards removing much 

 of the obscurity which appeared to surround the 

 earlier stages of the breeding problem in the 

 case of resident species. Recent observation 

 has shown that I exaggerated this difficulty, and 

 that it is generally possible to determine with 

 reasonable accuracy the approximate date at 

 which the internal changes begin to exert an 

 influence on the behaviour of resident species 

 also. Nevertheless, the specialised behaviour of 

 the migrants furnished a clue, and pointed out 

 the direction which further inquiry ought to 

 take. 



Those who are accustomed to notice the 

 arrival of the migrants are aware that the woods, 

 thickets, and marshes do not suddenly become 

 occupied by large numbers of individuals, but 

 that the process of " filling up " is a gradual 

 one. An individual appears here, another there; 

 then after a pause there is a further addition, 

 and so on with increasing volume until the tide 

 reaches its maximum, then activity wanes, and 

 the slowly decreasing number of fresh arrivals 

 passes unnoticed in the wealth of new life that 

 everywhere forces itself upon our attention. If 

 now, instead of surveying the migrants as a 

 whole, our attention be directed to one species 

 only, this gradual arrival of single individuals in 

 their accustomed haunts will become even more 

 apparent ; and if the investigation be pursued 

 still further and these single individuals observed 



C2 



