276 THE BIRDS 



the midst of dense foliage, where the sun's rays cannot penetrate, 

 and when once made are used by the birds year after year ; that 

 they last for an exceedingly long time is evident, Gilbert having 

 found one with a tree a foot in diameter growing from the middle. 

 Although very conservative in the choice of site, building 

 material, and the manner of making the nest, birds may be said 

 " never to do anything invariably," and they will often depart 

 from the recognised custom of their family or species, in some 

 cases apparently from sheer eccentricity, but more usually to 

 adapt themselves to altered circumstances. Thus, though gulls 

 as a rule nest on the ground, they have been known to build in 

 quite high trees. Rooks, that habitually use dried sticks and twigs 

 in their building operations, have been observed, when these were 

 scarce, to substitute for them odd lengths of wire from refuse heaps 

 in the vicinity of the rookery ; and the robin and the titmouse are 

 both noted for the extraordinary places they often choose in which 

 to lay their eggs. 



