10 INTRODUCTION. 



the brood being hatched. Sometimes, indeed, the 

 cock-bird takes her place, — sometimes he feeds 

 her while she is on the nest, — but female birds 

 always sit very close for a few days prior to the 

 exclusion of the young. The nests of the rook, 

 magpie, jay, wood-pigeon, &c. are pervious to the 

 air throughout, yet the eggs are duly hatched; 

 while the eggs of the golden-crested ^\Ten are en- 

 veloped in down and feathers. Instinct seems to 

 direct the bird to construct and suit the nest to 

 the size of the eggs. But it appears to us, that 

 the eggs of birds, even of the same size, requii'e a 

 different temperatiu-e. For instance : — the nest of 

 the sedge-warbler is a thin flimsy structm-e, and 

 placed in a damp exposed situation ; while that of 

 the common house-spaiTow is formed of moss, fea- 

 thers, thread, and other ^^^arm materials, thickly 

 lined mtli feathers, and placed in a warm spot. 

 We cannot account for this in any other way than 

 by supposing that the eggs of the house-sparrow 

 require a higher degree of temperatm-e to hatch 

 them than those of the sedge-warbler. Another 

 thing that has struck us as singular, with regard 

 to the materials birds use in forming their nests, 

 is, that the feathers or hair, which they make 

 choice of for lining them, are always white or 

 grey — never black. Whether the white colours 



