184 Field and Forest Rambles. 



for by a study of the laws which regulate the- direction of the 

 winds. Thus between latitudes 32 and 50 in North 

 America, the regular direction of the prevailing winds is from 

 the west, the breadth of the atmospheric current being 

 about 20°, its greatest intensity culminating about latitude 

 45 c . But there are variations in the amount of intensity at 

 different seasons of the year in accordance with the rotation of 

 the earth and the sun's declination. Moreover, the great polar 

 and southern winds, at the times of the year when they pre- 

 dominate, are also influenced by the same cosmical forces, 

 which give them likewise a tendency towards the west. Now it 

 is a fact, both as regards the seasons of the year (spring and 

 autumn) and the birds transferred to Europe, that their trans- 

 portation is in strict accordance with the times of the migra- 

 tions of the species then on their routes to and from their 

 summer retreats. Thus, as elsewhere, the occasional transfers 

 of American birds to Europe are effected by the prevailing 

 winds ; moreover, these data explain why far more American 

 birds find their way to Europe than vice versa. 



As regards the European continent, it is the case that the 

 migratory birds lag longer on their way north in spring than 

 they do in autumn, whereas in Canada the very reverse would 

 seem to prevail. This, I think, might be explained by taking 

 into consideration the natures of the climates of the countries 

 where the majority of the birds of passage spend the summer. 

 Thus the Canadian winter extends over nearly half of the year. 

 There is no spring season ; summer advances with amazing 

 rapidity, and sharp frosts set in early in September, so that 

 the majority of migratory birds, we will say in latitude 

 50 , have barely time to rear one brood before the season 

 returns for their retrograde movements, thus imposing on 

 them the necessity of a rapid advance from their winter to 

 their summer quarters, in order to accomplish the duties of 

 incubation. 



Again, the young birds of the year not having attained 



