1921.] Bird-Migration />// the MarUng Metlwcl. 479 



subject, and isolated records must always be regarded with 

 suspicion and as at best suggesting the theoretical ex- 

 planations which they seem to indicate. The chances o£ a 

 faulty record are in themselves almost negligible if the 

 method is carefully and scrupulously followed : wrong ring 

 numbers have frequently been reported and have been 

 speedily detected by being inconsistent with the particulars 

 oi" marking of the bird to which the number really belonged. 

 There are, however, several records which suggest that the 

 individual birds concerned behaved in an abnormal manner 

 (c/. Mallard, Section VI.), and this makes it the more 

 necessary that all deductions should rest on a broad 

 foundation. 



It would obviously be desirable to collect a mass of data 

 sufficiently large to be treated statistically, but it cannot 

 be said, in view of the numerous unknown factors, that 

 this has yet been achieved. In the first place there is 

 to be considered the possibility that the material being 

 dealt with is not wholly homogeneous : even in the case 

 of birds of the same species bred in the same area there 

 may be migratory and resident individuals, and therefore 

 possibly migratory and resident races. In the case of 

 birds caught and marked in winter the material is' more 

 obviously of mixed origin and may even contain morpho- 

 logically distinguishable geographical races or subspecies. 

 Not only may some individuals of a species be migratory while 

 others in the same area are resident, but there is no ground 

 for assuming that all the migratory individuals perform 

 similar movements : the movements, indeed, certainly differ 

 in degree and may differ in kind, and it is not even fair to 

 assume that the same individual will act in an identical 

 manner in successive years. It follows, also, that great 

 caution is necessary in deducing routes of migration from 

 records relating to different birds. The obvious temptation 

 is to plot on a map all the localities of reappearance and to 

 consider them as points in a common path, but it is not 

 sound reasoning to say, for instance, that because many 



