J. A. ALLEN: BIRD MIGRATION. 17 



birds following, sometimes, in the case of the fall migra- 

 tion, much later. An instance that has attracted 

 attention is the European Cuckoo, in which the parents 

 depart from their summer home long before the young 

 leave. This condition, however, is by no means unusual, 

 since, in North America at least, it has been found to be 

 the. rule rather than the exception. This has led to the 

 question, How do the young birds of the year, left behind 

 by their parents, find their way in migration ? In 

 Mr. William Brewster's memorable paper, " Bird 

 Migration," published many years ago,* and based on long 

 field experience, he states (referring to North American 

 birds) that "the adults of many, or, as [he] believes, all 

 species migrate southward first, and often several weeks 

 in advance of the young. It is perfectly true, nevertheless, 

 that a few old birds are always to be found in the larger 

 flights, although the latest of these are certainly composed 

 mainly of young." Young birds, when left behind by 

 their parents, it is believed, join the general throng of 

 southward migrants, which always includes birds of many 

 species. 



All this goes to show that the present breeding stations 

 of birds are, generally speaking, of the highest antiquity, 

 and are an inseparable part of the evolution of the species ; 

 that the habit of migration in migratory species is likewise 

 inseparable from the history of their differentiation, and 

 has become as much a specific trait as any of their 

 physical characteristics. The origin of migration and 

 its doubtless gradual development is so remote that the 

 explanation must ever remain hypothetical, but an 

 hypothesis may be so well grounded that it may be 

 accepted as a reasonable certainty. Many birds not 

 much unlike existing types — belonging in many instances 

 to the same genera — existed in tertiary times. Since 

 this period the earth's surface has undergone great physical 

 and climatic changes, which have in turn immensely 

 modified not only the distribution but the physical 



* Memoirs of the Nuttall Ornithological Club, No. 1, 1886, pp. 1-22. 



