THE THKUSH GENUS 377 



Having fashioned the dry grass, moss, or other material into the 

 from of a cup, our Thrushes without exception normally plaster 

 the inside of it with a layer composed usually either of mud or of 

 the mixture of mud and vegetable matter which may be picked up 

 from any wet ditch, the margin of a pond or stream, or even from the 

 streets of our large towns. I have seen a thrush collecting a beak- 

 load in a spot no more rural than the gutter of Tufnell Park Road, 

 London, N.W. This plaster, when dry, is compact, hard, with a 

 smooth surface, and sometimes half an inch or so thick. It requires 

 some effort to break across and shows evidence of having been 

 subjected to considerable pressure by the bird. It is not, of 

 course, invariably present. Birds do not, any more than human 

 beings, adhere rigidly to the normal. I have found a blackbird's 

 nest composed externally of green moss and internally of fine 

 dry grass, there being nothing between the two but a little loose 

 soil. The bird's instinct, for it can be nothing else, had led it 

 to bring the soil to the nest, but at that point the instinct had 

 mysteriously ceased to function. This did not, however, prevent 

 the pair from successfully rearing their young, a fact which makes it 

 more difficult to understand the process by which the mud-lining 

 became general. 



As already noted, the plaster lining is not the final stage. All 

 except the song-thrush cover it normally with fine dry grass. Unless 

 enough of this is worked securely over the rim of the nest it tends 

 to be trampled down to the bottom of the cup by the feet of the 

 nestlings, so that the plaster beneath is exposed. 



The interior of the nest of the song-thrush is, I believe, 

 unique, 1 and like many common things, it has been largely taken for 

 granted, and has consequently not received the attention it merits. 

 I make no apology, therefore, for going into details ; they will show 

 that there is a greater variation in the materials used than might 



1 The bare-eyed thrush of S. America (T. gymnophthalmus) is said to build a nest with a 

 mud lining and no soft material on it, but whether it has the song-thrush's addition of 

 dry wood, etc., is not stated. See Seebohm and Sharpe's Monograph of the Turdidce, 1902. 



3c 



