1 O Averill, Migration and Physical Proportions. [oct. 



of course in birds such as swallows and swifts and others especially 

 adapted for continuous flight the points I have called attention 

 to are not noticeable. It would be useless to multiply examples 

 as the other tables simply emphasize what I have shown in the 

 Warblers. 



SUMMARY. 



We have seen that the longest migrations in any group of simi- 

 lar birds are made by those with longer wings, smaller tails, and 

 smaller bills and feet, and from observation of birds of highly 

 developed powers of flight we conclude that flight is easier for birds 

 so proportioned. 



We know that migratory flights are a tax on the strength and 

 endurance of birds, that they cross considerable bodies of water 

 that in order to arrive in spring with the punctuality which many 

 of them attain, they fly under unfavorable conditions, against 

 adverse winds, in stormy weather, and are often found exhausted 

 by the struggle. Perhaps if we recall some of the cases of warb- 

 lers in distress we have witnessed or read of we remember that 

 such long winged species, as Blackpolls, Myrtles, Yellows, Oven 

 birds, Water-Thrushes, fared better than the shorter winged Yellow 

 throats, Parulas, Redstarts. It is logical to conclude that by 

 natural selection nature develops the characteristics of good 

 flight and the fittest survive. 



If birds extended their range by sudden expeditions to some 

 distant point then we might suppose the long winged birds had 

 simply beaten the short winged. Perhaps to some extent this has 

 happened. We may suppose that the Starling with its excellent 

 wing and tail for flight will extend its range more rapidly than some 

 bird of poor flight power. But when we look at the table of Yellow 

 Warblers or of Parula Warblers the differences in physical propor- 

 tions are so slight that it seems they could not be, as they are, 

 important factors in acquiring range. They seem rather to be 

 incipient developments that will increase with time. 



The forked tail accompanies the longer wing in our North 

 American migrants and is an evidence of good power of flight. 



The birds of the west, those of and beyond the Rocky Moun- 

 tains, while they may go far north to breed, many of them to 



