NESTING 265 
scores of others will refuse to live with us. Birds that require 
some kind of crevice or hollow have still greater difficulty 
in finding accommodations. Hollow trees may be regarded 
as nature’s provision for them, and in their absence we must 
Birp Hovusrs 
To be fastened to a post, tree, or building, in a shady location. The open 
house at the right is for robins, the others for wrens and bluebirds. The floor 
of the robin house is about 6 by 8 inches. Robins want their nest 6 to 15 
feet {rom the ground; wrens and bluebirds, 6 to ro feet. The entrance hole for 
wrens should be an inch in diameter; for bluebirds, 1% inch. 
either supply proper boxes or do without Wrens, Bluebirds, 
and Martins. 
Materials ; Construction. — The nests vary greatly in many 
particulars. Generally speaking, the size is in proportion to 
that of the birds. The usual building materials are dead 
grasses, twigs, rootlets, plant-down and fiber, hair, and feath- 
ers. A number of birds, as Robins, Eave Swallows, and Barn 
Swallows, use mud, while Chimney Swifts get a gummy 
substance from their mouths to glue together little twigs and 
fasten them to the chimney wall. 
