HIRUNDINUXE THE SWALLOWS. 218 



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the other, pairs of the two sometimes occupying adjacent burrows. 

 The Bough-winged Swallow, however, sometimes nests in holes in 

 the abutment of bridges or even in cavities among the timbers, a 

 habit which the Bank Swallow has not been observed to have ac- 

 quired. 



By the careful observer the Bough- winged Swallow may be im- 

 mediately distinguished from the Sand Martin (C. ripariaj by the 

 uniform "mouse-gray" or grayish brown color of the under parts, 

 gradually passing into white on the under tail-coverts, the lower 

 surface of the Bank Swallow being pure white, with a distinct band 

 of grayish brown across the breast. 



