126 



ing birds are the last to start on their migrations lends additional 

 evidence in favour of this probability. We are driven to an 

 explanation of this kind in preference to admitting that birds 

 wintering a thousand or fifteen hundred miles away are conscious 

 that their breeding grounds are fit or unfit for occupation. 

 However, the foregoing theory will not sufi&ce in the case of the 

 autumn flight. It is undoubtedly a fact that this movement 

 commences with one or two species, a few weeks at most, after 

 the close of the breeding season. The early migration of the 

 young of the Starling is a well known case in point. In this 

 particular instance the explanation of this early migration on 

 the part of the young may perhaps be attributable to a diminish- 

 ing supply of food. The latter fact may arise from a simple 

 cause, i.e., the drying up of the countries inhabited, with the 

 consequent disappearance of worms and slugs on which the young 

 feed, owing to the summer heat, and the growing scarcity of larvae, 

 which at this period of the year will be undergoing their meta- 

 morphoses under ground, preparatory to their emergence as 

 imagoes in the autumn. It is interesting to note that in our more 

 pluvial climate, the young of this species do not migrate like 

 their relations on the continent. However, we know so little 

 of the former extension of the breeding ranges of any species, 

 so that such explanations must always remain more or less 

 conjectural. If we knew that the Starling at one time bred in 

 the high north, it would be within the bounds of possibility that 

 the early migration of the young was the survival of a former 

 necessary habit. It has already been pointed out that the 

 duration of vyinters in th«se latitudes has been subjectei, to 

 great variation. 



Herr Gatke further objects to the theory that migration 

 from the north is altogether due to a lack of food. He remarks : 

 "This explanation would have much to commend it if all the 

 individuals of a species left their breeding places in the highest 

 northern latitudes simultaneously, and if all followed a north 

 to south line of migration. I have, however, shown myself, 

 firstly that the young and old birds of a species rnigrate a 

 widely different times ; and secondly, that a number of species 

 perform their migrations on an east-to-west line of flight." The 



