408 Dr. A. L. Thomson : Nesulfs of a Sfiidi/ of [Ibis, 



lieiglit in 1914, and that it would have continued at a high 

 level for a few years if circumstances had remained normal. 

 As it was, the work not only entered on a premature decline, 

 but a great deal of it was probal)ly rendered less fruitful by 

 unfavourable conditions for tlie reporting of reappearances. 



The Purposes of Bird-Marking. 



In its essentials the method consists of the marking of a 

 large number of birds, in some way or other, for the sake of 

 the data afforded by the subsequent reappearance of a small 

 proportion of ihem. The principle is identical with that of 

 the well-known method of marking fishes as a means of 

 studying their movements and life-histories, while an analo- 

 gous system has also been utilised in the study of ocean 

 currents. 



The method differs from other methods of studying migra- 

 tion in that it approaches the problems from the individual 

 aspect — it begins with individual birds and works from them 

 towards general movements. A marking record implies 

 that there are two or more times in the life of a particular 

 bird when one is able to state with certainty its whereabouts 

 and various other facts, these times being usually those of 

 infancy and of death. When a large number of these 

 records has been collected and correlated, there will exist 

 an array of facts which could not have been ascertained by 

 other means. 



To estimate the value of information of this kind, the nature 

 of the problems must first be considered. Perhaps the 

 greatest and most difficult problem of migration is that of its 

 origin — its ultimate cause. To an appreciable extent the 

 jmrpose served by migration, its ra'ison d'etre^ is known, 

 and the innnediate factors which periodicallv stimulate the 

 migrational habit into being niny be surmised : but the ques- 

 tion of the origin of the habit still lies completely Avithin the 

 realm of conflicting theory. A matter of theory and hypo- 

 thesis it must doubtless ever remain, but one may at least 

 })ut the theories to the test of facts and eliminate those that 

 are found wanting. One thing seems obvious, and that is 



