THE SPRING MIGRATION OF BIRDS. 203 



such temperate latitudes as the British Islands 

 (although instances are on record where the earliest 

 flights of Migrants have been delayed for days 

 together in countries immediately to the south of 

 them), where the transition from winter to spring 

 is very gradual. In the Arctic regions, however, 

 season change is much more rapid, winter merging 

 into spring after a south wind of twenty-four hours' 

 duration ; and then the interesting sight can be 

 repeatedly witnessed of birds arriving a day or so 

 too soon with the first signs of a thaw, and having 

 to retire south again for a few miles to the nearest 

 open water, or land free from snow. It is also 

 interesting to remark that many of the birds that 

 have the furthest to go are the last to start from 

 their winter quarters, and this appears to apply 

 equally to individuals as well as to species. Thus 

 the Swallows {Hirundo nistica) that breed in South 

 Europe begin to leave South Africa about the 

 middle of February, but those that breed in North 

 Russia delay their departure until the middle of 

 April, just as if they were perfectly well aware that 

 their summer quarters in the Arctic regions would 

 not be ready for them for several months longer ! 

 This great wave of spring migration lasts practically 

 for about four months ; beginning to set in towards 

 the north from the Antipodes about the middle of 

 February, and continuing until nearly the middle 

 of June, spending itself gradually at the latter 

 date in the highest Polar regions visited bv birds. 

 Gradually this great migration wave may be traced 



