THE BIRD DEPARTMENT 



979 



at sea, but each year on exactly the same day they appear 

 on their breeding islands. 



The first birds to migrate, like the robins and black- 

 birds, are the least punctual because in the early spring 

 the weather is least settled. The later migrants become 

 more and more punctual as the weather becomes more 

 uniform until with those birds coming after the first of 

 May we can prophesy the day of their arrival at any 

 place with considerable accuracy. 



Now if birds had no enemies and always hatched their 

 first eggs successfully, it is probable that the fall migra- 

 tion would be equally regular, because after the breed- 

 ing season the reproductive organs begin to decrease in 

 size just as they grew in the spring. But many birds 

 have to make several attempts before they raise a brood 

 successfully, and while they are still feeding their young 

 in the nest, others of their species are ready to leave. 

 Some species, however, wait for a second or even a third 

 brood, but the majority are ready after the first brood to 

 enter once more upon a care-free existence. Some 

 species, like the swallows and blackbirds, assemble 'in 

 large, conspicuous flocks before migrating, but others 

 just slip away unnoticed, usually the old birds first, fol- 

 lowed later by young of the year. It is the periodic 

 changes in the reproductive organs, then, that tells birds 

 when to migrate and the concomitant instinct impels them 

 to go. 



But this does not explain how and why birds came to 

 migrate in the first place. It may explain how the in- 

 stinct is maintained, but not how it has been developed. 

 For this, of course, we have to resort to hypotheses. 

 Without going into detail, it may be interesting to re- 

 view the one which receives greatest credence today. It 

 is founded upon two beliefs which we now look upon 

 almost as facts: first, the origin of bird life upon this 

 continent, and secondly, the coming of a glacial period 

 or ice age when most of the birds were again driven out. 



In North America, it is undoubtedly true, that we have 

 received our birds from two sources : from South Amer- 

 ica and from Asia by way of Alaska. We know this 

 because some of our species are almost identical with 

 those of Europe and Asia, while others are very similar 

 to South American forms found nowhere else in the 

 world. If we can imagine the South American birds, in 

 ages past, gradually spreading northward because of an 

 overcrowded condition or because of the natural instinct, 

 inherent in most organisms, to cover as much territory 

 as possible, we would find them eventually coming into a 

 land which, while similar to that of their progenitors 

 during a part of the year, was entirely different during 

 winter and unsuited to their needs. They were, there- 

 fore, able to occupy it only during the summer months 

 and each winter had to retire southward. This would 

 have been sufficient to eventually form a migrating in- 

 stinct of considerable power and regularity, but it does 

 not explain all the variations in route and distance trav- 

 elled which we see today. 



During the numerous geological ages that have ensued 

 since birds first came into North America, the continent 

 and the climate have seen great changes. North America 



has changed from a mild, semi-tropical land to one cov- 

 ered with snow and ice and back again to a land of de- 

 cided seasons. If we think of birds as having become 

 established during the semi-tropical times, even without 

 any decided migrations, we can still think of them devel- 

 oping this instinct to migrate under the stress of the 

 slowly approaching ice age, the birds being driven south- 

 ward to seek quarters in an already overcrowded tropics, 

 and striving northward with each returning spring and 



THE TREE SPARROW 



This bird nests in northern Canada and in winter migrates to the 

 northern part of the United States. Just what the sense is which 

 directs migratory birds to return to the same place year after year 

 the scientists do not know. 



recession of the glaciers only to be forced back again 

 the following winter. 



One might follow this thought into great detail and 

 show how the various routes followed by birds between 

 North and South America may have been evolved, but 

 we cannot take space for it here. There remains one 

 other problem which seems even more mysterious than 

 the origin of migration, and that is how birds find their 

 way. 



Each year the bobolink after traveling 10,000 miles 

 comes back to the same meadow, and the oriole comes 

 back to the same tree. The robin and the phoebe come 

 back to the former nests and construct others close by 

 or even on top of the old structures. What is it that 

 guides them on their long journey and brings them back 

 sc precisely? It is instinct, we now say, a sense which. 



