228 MIGRATION 



revived (Riippell, 1931), but again without definite evidence that they 

 can find their way alone. In one series of studies, the young from 

 46 nests left before the old, mostly 4-5 days earlier, while in 28 cases 

 they departed together or the parents first. It is not only quite pos- 

 sible that the mature birds overtake the juveniles, but Rodenbach 

 also cites observations in which the latter joined flocks passing over- 

 head, which might well contain older birds. The best evidence at 

 present of the lack of a sense of the direction of their ultimate goal in 

 young storks is advanced by Skovgaard (1929), who reports that of 

 five young banded from the same nest, two were found in Holland 

 and France, and another two in Hungary and Roumania. 



Though there can be no serious question as to the manner in which 

 adults find their way, there is still to be explained the migration be- 

 havior of juveniles. Since these too cannot possess a special sense, 

 they must likewise make use of perception and memory. The diffi- 

 culty naturally exists only when the young birds leave the breeding 

 grounds after the parents have departed. Departure takes place more 

 or less by groups and at successive intervals and in a definite direction. 

 Thus, it is probable that many flocks of young contain one or more 

 adults as actual if not intentional guides. Those that start south- 

 ward without such guidance must do so in consequence of a memory 

 of the direction taken by their parents or by virtue of physiologic 

 orientation in response to the incidence of the sun's rays. 



Here, again, it is assumed that an understanding of southerliness 

 in terms of warmth is as certain in connection with direction and path 

 of migration as it is in relation to local behavior. In evidence of the 

 latter, reference may again be made to the movement of ground birds 

 during hot days on the Great Plains, when they shift for hours to keep 

 fenceposts between them and the sun, leaving the shadow with re- 

 luctance and returning to it almost at once. Similarly, on the north- 

 ward journey, it seems that the need for cooler conditions favorable 

 to normal metabolism is realized by means of an orientation ap- 

 parently thermotropic but in reality directed by perception. Since 

 the three sets of evidence are all drawn from experiment, the prob- 

 ability is strong that the conclusions will stand the test of more ex- 

 tensive experimentation with some of the common migrants, but final 

 judgment must await such researches. 



