THE PHILOSOPHY OF BIRDS' NESTS. 227 



to the uniformity of the conditions under which each 

 species lives. Their range is often very limited, and 

 they very seldom permanently change their country, 

 so as to be placed in new conditions. When, how- 

 ever, new conditions do occur, they take advantage 

 of them just as freely and wisely as man could do. 

 The chimney and house-swallows are a standing proof 

 of a change of habit since chimneys and houses were 

 built, and in America this change has taken place 

 within about three hundred years. Thread and worsted 

 are now used in many nests instead of wool and 

 horsehair, and the jackdaw shows an affection for the 

 church steeple which can hardly be explained by in- 

 stinct. In the more thickly populated parts of the 

 United States, the Baltimore oriole uses all sorts of 

 pieces of string, skeins of silk, or the gardener's bass, 

 to weave into its fine pensile nest, instead of the single 

 hairs and vegetable fibres it has painfully to seek in 

 wilder regions ; and Wilson, a most careful observer, 

 believes that it improves in nest-building by practice 

 the older birds making the best nests. The purple 

 martin takes possession of empty gourds or small boxes, 

 stuck up for its reception in almost every village and 

 farm in America ; and several of the American wrens 

 will also build in cigar boxes, with a small hole cut in 

 them, if placed in a suitable situation. The orchard 

 oriole of the United States offers us an excellent ex- 

 ample of a bird which modifies its nest according to 

 circumstances. When it is built among firm and stiff 



O 



branches it is very shallow, but when, as is often the 



Q2 



