ioo THE BOOK OF NATURE STUDY 



of the breeding ground, or, often, to the individual idiosyncrasies 

 of the builder, as in the case of the wild-duck, for example, which 

 normally builds on the ground, but will occasionally lay its 

 eggs in trees. Herons normally build in trees, but where 

 these are wanting they will place their nest upon the ground. 

 Cormorants and gannets, again, nest either on the ground or in 

 trees, as necessity demands ; and many similar instances might 

 be quoted. 



Birds which build cup-shaped nests in trees or shrubs usually 

 place them in the fork of a branch, but some species suspend 

 them from the under side of a bough ; as, for example, in our 

 golden-crested wren and the golden oriole, which occasionally 

 visits these islands. The reed-warbler has adopted a different 

 plan. Here the nest is built between the upright stems of 

 three or four tall reeds in some reed-bed, so that the reeds have 

 the appearance of growing through the sides of the nest. The 

 cup-shaped cavity is very deep, a precaution which prevents 

 the eggs falling out when the reeds are bent low by the wind. 



A considerable number of species lay their eggs in holes of 

 trees or on the ground. Commonly a natural cavity is chosen, 

 or one excavated by some other animal ; but where such ready- 

 made borings are not to be had they will drill these for them- 

 selves. Some species appear always to adopt the latter course ; 

 and one of the most skilful of such tunnel makers is the little 

 sand-martin, a bird which has apparently but the feeblest of 

 tools for such a purpose, the beak and feet being alike small and 

 delicate. Yet this species will drive a tunnel a yard or more 

 in length through a sand-bank, forming a chamber at the end 

 wherein, on a few straws and feathers, the eggs are laid. 



Birds' Eggs. Like the eggs of crocodiles and tortoises, among 

 the reptiles, the eggs of birds are enclosed in a hard, calcareous 

 shell ; but whereas the eggs of all reptiles have a white shell, 

 those of birds are as a rule variously coloured, the significance 

 of which fact will be referred to presently. Birds' eggs further 

 differ from those of reptiles in that they must be incubated by 

 the warmth of the parent's body, a process known as " brooding." 

 During the breeding season the abdomen of the sitting bird 





