136 DAVENPORT ACADEMY OF SCIENCES. 



the Mississippi is open at the time the van-guard reaches Iowa, 

 the water birds are apt to follow its channel in greater numbers, 

 barely touching the borders of Iowa, while if the Mississippi is 

 blocked by ice in early spring, the flight may be diverted into 

 the valleys of the Des Moines, Iowa, and Cedar rivers, and a 

 larger number reach the interior of the state. 



Throughout most of north central and northern Iowa, migra- 

 tion is more diffused. As has been pointed out by Mr. Wilmon 

 Newell,! in the region of the Wisconsin drift sheet, the uneven 

 dumping of the drift has resulted in lulls and moraines, and 

 numerous swales or marshy ponds. "The southern limit of 

 the region being near Des Moines, on the Des Moines River, the 

 spring migration of the Rallidar, GaUiniclidcs, Scolopacidtz , and, 

 to a certain extent, that of the Anscres, is diverted from the 

 river valleys to this territory, and passes through central Iowa to 

 the northern Iowa and Minnesota lakes. In the autumn, most 

 of the ponds being dried up, the .same birds follow almost exclu- 

 sively the river channels. However, the summer residents of the 

 pond region take their way southward over the prairie region, 

 following the line of their spring migration." 



This diffusion of the line of migration evidently induces a 

 more leisurely journey toward the north, the birds stopping to 

 rest and feed along the way. This appears to explain the condi- 

 tions noted by W. W. Cooke (Bird Migr. in Mi.ss. Val., 1884-5, pp. 

 61-5): 



"During the spring migration of 1884 it was noted by Mr. 

 Miller that ducks came to Heron Lake, Minn., not far from the 

 Iowa line, from the west, as if they were a part of the Missouri 

 River flight. In 1885 they must have come from the same direc- 

 tion, since in the region south of Heron Lake, in Iowa, in an 

 equally favorable locality (Emmetsburg), none were seen until 

 about two weeks later. There can be no doubt that in the 

 spring of 1885 the flight of ducks and geese along the Missouri 

 River was several days earlier than at corresponding latitudes 

 along the Mississippi River." 



While it is true that apparently delicate species of birds are 

 able to endure great extremes of temperature without apparent 

 di.scomfort, their occurrence and abundance is closely related to 

 the climatic conditions and food supply of the region. Graniv- 



I. "Topography as Influencing Migration." Wesltrn Oruilliologist, v. iii, 1900, 5S-9. 



