ANDERSON — TIIK BIRDS OF IOWA. I35 



Among day migrants, according to Mr. Cooke, sight is prob- 

 ably the principal guide, and undoubtedly plays a part in guiding 

 the night journeys also, it being noticeable that thej' pass higher 

 overhead on clear, 1:)right nights, flying lower on dark, cloudy 

 nights. He believes that they also possess a power, whatever its 

 nature, that may be called a "sense of direction," which serves 

 to guide them unerringly over ocean wastes. "The truth seems 

 to be that birds pay little attention to natural physical highwaj's 

 except when large bodies of water force them to deviate from the 

 desired course. Probably there are man}- short zigzags from one 

 favored feeding ground to another, but the general course between 

 the summer and winter homes is as straight as the birds can find 

 without missing the usual stopping places." 



Commenting on the a1)ove. Dr. J. A. Allen' says: 



" It does not follow, however, that becau.se all the birds of 

 a district do not concentrate and move in masses along river 

 valleys and coast lines, that they are not guided in their courses 

 by the prominent features of the landscape, even in the case of 

 tho.se species which ])ass from the upper Mi.ssissippi Valley to the 

 coast of South Carolina and Georgia. Nor is it true that river 

 valleys, etc., do not form migration rou.tes for many species of 

 l)irds." 



The general north and .south direction of the rivers and streams 

 of the state and its boundaries renders this particularly true in 

 Iowa. The Mississippi River on the east and the Mis.sotiri on 

 the west form the main routes, from which in the eastern part 

 of the state the wave of migration diverges in general to the 

 northwest, following the trend of the larger tributaries of the 

 Mississippi, while in the western part of the state the streams 

 flowing into the Missouri lead it somewhat northeasterl)-. The 

 water birds naturally follow the streams, while the land birds, as 

 a rule, freqtient the natural woodland, which in a prairie state, 

 reaches its greatest luxuriance as a fringe along the water- 

 courses. 



The migration of most land birds in Iowa is fairh' constant in 

 spring and fall, while the migration of water birds is apt to fluc- 

 tuate, both in course and num1)ers, particxdarly in the .spring 

 flight. The great variation in the numbers of the .spring flight 

 from year to year has been explained by the hypothesis that when 



I. Aux, xxi, 4, 1904. 



