THE MIGRATIONS OF OTHER BIRDS 203 



and central Wisconsin and Illinois on the east, but 

 the full migration centres through a narrow region 

 comprising eastern Kansas and western Missouri. 

 Here this fine bird swarms in thickets and hedgerows 

 during October, and again in April, filling the air 

 with its rollicking whistled calls. At the height of 

 the migration thousands may be seen in a single 

 day, but outside this strip, which is barely 250 miles 

 wide, the bird is casual or rare. The cause for this 

 limited distribution is wholly obscure, for areas at 

 either hand seem equally suited for the needs of the 

 bird, which has the habits of its congeners. No 

 other bird has this distribution, which lies along the 

 lines where forms of the eastern half of the country 

 begin to disappear and those of the west to appear. 

 To observers in the eastern states the coming of 

 the hosts of wood warblers (Mniotiltidae) marks the 

 height of the spring migration. Trees and thickets 

 are filled with small birds, flitting among the twigs, 

 whose bright colors, revealed by field glasses or by 

 favorable light to the unaided eye, come as a never- 

 failing and refreshing surprise. The very fact that 

 more than thirty distinct species may be seen in a 

 single day adds to the excitement of their identifica- 

 tion and one never knows what rarity may appear 

 among their ranks. The tree-hunting forms (mainly 

 of the genus Dendroica) have the habit of moving 

 in mixed flocks which may contain a dozen or twenty 







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