HOUSE MARTINS. 



505 



Fig. 220.— House Martins. 



in a sycamore tree (familiarly known there as a button-wood) near 

 Louisvillein Kentucky, United States. These trees grow to an immense 

 size and are generally hollow. Some of this family prefer rocks or 

 caverns, and excavate in steep escarpments a gallery from two to 



