THE NATURAL HISTORY OF NESTS 159 



a contrast in the cranes, which feed together, play together, 

 and rest together but nest separately. Sometimes the 

 gregariousness in nesting may be associated with the relative 

 rarity of suitable sites, as may be illustrated by our sand- 

 martins, or by the sociable grosbeaks, which join nest to 

 nest until the tree sometimes breaks down. An eloquent 

 testimony to the advantageous inaccessibility of certain 

 sites is afforded by the huge numbers of birds which some- 

 times possess them, as is well seen on " bird-bergs " like 

 Ailsa Crag and the Bass Rock, where gannets are so abun- 

 dant, or, in a different way, in the marshy ground, where 

 thousands of black-headed gulls often nest , 



A SERIES OF NESTS 



Let us work up the series of nests from the simplest to 

 the most complex, following, almost necessarily, the article 

 " Nidification " in Newton's Dictionary of Birds. We 

 naturally begin with those birds in which the nest-building 

 which comes as a serious practical task after the nuptial 

 song or the nuptial dance is " scamped " or shirked. 



The tern makes no more than a mere scraping on the 

 gravelly sand. It is the same with divers, thick-knees, 

 and sand-grouse. The stone-curlew and the night-jar 

 make no nest, nor any preparation of the soil, yet year after 

 year they select the same spot. In the case of guillemots 

 and razorbills, the egg is laid on a bare ledge of rock, and the 

 top-like shape is to some extent a safeguard against being 

 blown over or knocked over. 



Many of the gulls, sandpipers, and plovers simply lay 

 their eggs in shallow hollows in the ground, adding a breast- 

 work of stems and leaves as incubation proceeds. The 

 ducks are mostly about the same level of nest-making, but 

 their depression is lined with down. The ringed plover 



