1 92 1.] Bird-Migration hij the Marking Method. 525 



already been pointed out, the number of birds in a given 

 area at a particular time is only imperfectly represented by 

 the mortality rate, because the latter varies with time of 

 year and other circumstances. Moreover, the true mortality 

 rate is still less adequately represented by the " recorded 

 mortality " rate which forms the datum of the method. These 

 ditRculties cannot b(; altogether overcome, but they Avill be 

 minimised where the problem takes the form of comparisons 

 between sets of r(\sults collected under approximately similar 

 eircumstances. There is virtue, too, in the mere numbers of 

 records, and concentration of effort should lead to an increase 

 in these for the particular species selected. Where the 

 total numbers are small the quantitative errors due to mere 

 chance will obviously be great, and the extent of the possible 

 discrepancies from this source is strikingly shown b}' the 

 figures for the Lapwing [cf. Table II. p. 485), one of 

 the best subjects for study. 



The selection of suitable species does not exhaust the 

 question, for it is also important that the reappearance 

 records should fall into groups which are homogeneous as 

 regards the circumstances of marking. A concentration of 

 effort on particular areas would therefore have its advantages. 

 The question of season is probably still more important, and 

 a serious objection must be considered in the case of most 

 marking of birds at other times than the breeding season^ 

 namely, that the birds marked may consist of a mixture 

 of sedentary natives and visiting immigrants from other 

 summer-quarters. 



There arc various other points that need not be laboured. 

 The unreliability of isolated records which may well bo 

 abnormal will, for instance, be sufficiently obvious. The 

 danger of laying stress on negative evidence except in very 

 clear cases may also be mentioned : in considering the 

 Hedge-Sparrow and the Blue Titmouse, for instance, it has 

 been seen that almost neolioible numbers of marked birds of 

 these species are recovered where no special efforts are made, 

 whereas an exceedingly high figure may result if continuous 

 tra])ping be carried out. The absence of records from a 



