BIRD MIGRATION. 27 



to show for the 10 days' flight. By this time spring has really come 

 east of the Allegheoies, and the swallow travels GO miles a day to its 

 summer home in Nova Scotia. It is to he noted that the swallow 

 works up to high rates of speed only when it is traveling on the diag- 

 onal, and that except during the 10 days spent in crossing the moun- 

 tains each 10 days' travel covers approximately 5 degrees of latitude. 



One of the best examples of rapid migration is that of the gray- 

 cheeked thrush. This bird remains in its South American winter 

 home so long that it does not appear in southern United States until 

 late April — April 25 near the mouth of the Mississippi and April 30 

 in northern Florida (see fig. 17). The last week in May finds the 

 bird in extreme northwestern Alaska, the 4,000-mile trip from 

 Louisiana to Alaska having been performed in about 30 days, or 

 about 130 miles a day. 



Generally the later in the season a bird migrates the greater is its 

 average speed, but not necessarily the distance covered in a single 

 night. The early migrants encounter much bad weather and after 

 one night's migration usually delay several days before making the 

 next flight. The later migrant finds few nights too unfavorable for 

 advancing, so that short flights taken on successive nights greatly 

 raise the average migration speed. 



HOW BIRDS FIND THEIR WAY. 



How do migrating birds find their way? They do not journey 

 haphazard, for the familiar inhabitants of our dooryard martin 

 boxes will return next year to these same boxes, though meanwhile 

 they have visited Brazil. If the entire distance were made overland, 

 it might be supposed that sight and memory were the only faculties 

 exercised. But for those birds that cross the Gulf of Mexico, and 

 more especially for the golden plover and its ocean-crossing kindred, 

 something more than sight is necessary. Among day migrants 

 sight probably is the principal guide, but it is noticeable that these 

 seldom make the long single flights so common with night migrants. 



Sight undoubtedly does play a part in guiding the night journeys 

 also. On clear nights, especially when the moon shines brightly, 

 migrating birds fly high and the ear can scarcely distinguish their 

 faint twitterings; if clouds overspread the heavens, the flocks pass 

 nearer the earth and their notes are much more audible; and on very 

 dark nights the flutter of vibrant wings may be heard but a few 

 feet overhead. Nevertheless, something besides sight guides these 

 travelers in the upper air. In Alaska a few years ago members of the 

 Biological Survey on the Harriman expedition went by steamer from 

 the island of Unalaska to Bogoslof Island, a distance of about 60 

 miles. A dense fog shut out every object beyond a hundred yards. 

 When the steamer was halfway across, flocks of murres, returning to 



