32 BIRD FRIENDS 



And when the cowbird has visited the nest again and 

 laid her egg in this second nest, the warbler has been 

 known to build a third nest on top of the other and 

 there rear her young, in the third story. This action 

 certainly strongly suggests some degree of intelli- 

 gence on the part of the warbler. 



Number of broods. The general rule among birds 

 is that one brood is reared each season, but a num- 

 ber of common birds, such as the robin, bluebird, 

 and wren, rear two, and perhaps occasionally three 

 broods; and the English sparrow has been known to 

 rear six broods in a season. 



Change in nesting-habits. Many birds have 

 changed their nesting-habits to adapt themselves 

 to the conditions brought about by man. Chimney 

 swifts, which formerly nested in hollow trees, now 

 nest in chimneys. Cliff swallows now nest under 

 eaves instead of on cliffs. The purple martin has 

 left its nesting-sites in hollow trees and now nests al- 

 most entirely in houses provided for it. The phoebe 

 has largely abandoned its nesting-place on the face 

 of cliffs and now nests around sheds and barns and 

 under bridges. Many birds which formerly nested in 

 cavities now nest in boxes provided for them. The 

 most common occupants of these houses are wrens, 

 bluebirds, and martins. 



