BIRD MIGRATION. 47 



It does not necessarily follow that any individual bird makes all 

 these changes in its speed of migration. The flight of the individual 

 can not be traced or timed under the present system of obtaining 

 records, and in the above statements it is meant that the general 

 advance of the van of the birds is marked by these great changes in 

 speed. It is quite likely that the first robins which reach central 

 Minnesota at an average speed of 13 miles a day stop there and nest, 

 and it is possible that those which continue the advance to southern 

 Mackenzie at an average speed of 40 miles a day are individuals that 

 have waited later in the winter home and have covered the whole 

 distance at the higher rate. 



That individual birds do increase their daily rate of progress as 

 they proceed northward seems probable from the records of the gray- 

 cheeked thrush (see fig. 17). The earliest migrants of this species 

 travel from southern Louisiana to northern Iowa (1,000 miles) in about 

 15 days, or over 60 miles a day. As at this time they are passing over 

 a country in which they do not breed, there is no reason to infer that 

 the same birds do not keep continually in the lead. Hence 60 miles 

 a day may be considered the actual average speed of individuals 

 forming the van of this species. Two weeks later the earliest gray- 

 cheeked thrushes appear in northwestern Alaska, 3,000 miles from 

 Iowa, and it seems unreasonable not to conclude that the same birds 

 that averaged about 60 miles a day as they moved north in the lower 

 Mississippi Valley have greatly increased this speed as they con- 

 tinued their journey northwestward and finally westward to Alaska. 



THE UNKNOWN. 



Interest in bird migration goes back to a remote period; marvelous 

 as were the tales of spring and fall movements of birds, as spun by 

 early observers, yet hardly less incredible are the ascertained facts. 

 Much has been learned about bird migration in these latter days, but 

 much yet remains to be learned, and the following is one of the most 

 curious and interesting of the unsolved problems. The chimney 

 swift is one of the most abundant and best-known birds of eastern 

 United States. With troops of fledglings catching their winged prey 

 as they go and lodging by night in tall chimneys, the flocks drift 

 slowly south joining with other bands, until on the northern coast of 

 the Gulf of Mexico they become an innumerable host. Then they 

 disappear. Did they drop into the water or hibernate in the mud, 

 as was believed of old, their obliteration could not be more complete 

 In the last week in March a joyful twittering far overhead announces 

 their return to the Gulf coast, but their hiding place during the 

 intervening five months is still the swift's secret. 



WASHINGTON I GOVERNMENT PRINTING OFFICE : 1915 



