Characters of Birds' -e(jys. 357 



coufiiiecl to the surface, but it also occurs at various depths 

 in the substance of the shell. 



Then follow portions of shell of tlie eggs of y^pyornis, 

 Ostrich, and Duck, in section, to show their relative 

 thickness. 



Next to these is placed the egg of a Tern [Stei'na can- 

 tiaca), showing various shades of markings produced by the 

 same coloured pigment, deposited either on the surface or at 

 different depths in the substance of the shell. 



Number. — The eggs laid in one nest, which are sat upon 

 together and hatched about the same time, are called a clutch. 

 Their number, though tolerably uniform in each species, 

 varies greatly in different species. Some have only one, 

 others two, the majority of species four or five. Higher 

 numbers are less common, but eight to twelve eggs in a clutch 

 are frequent among Ducks and Rails, and even more among 

 some game-birds. 



The examples given are : — For single eggs, the Manx 

 Shearwater and Razorbill ; for two eggs, the Black Guillemot, 

 Swift, and Ring-Dove ; for three eggs, the Oyster-catcher 

 (perhaps one of the Sand- Grouses would have been better) ; 

 for four eggs, the Golden Plover and Common Sandpiper ; 

 for five eggs, the Kestrel and Robin ; for higher numbers, 

 the Long-tailed Tit, with nine, and the Red-legged Partridge, 

 with twelve in a clutch. 



Form. — In'form, eggs may vary from almost spherical to 

 different modifications of ellijitical or oval. The latter form, 

 in which one end is smaller and more pointed than the other, 

 though not universal, is the most frequent, and distinguishes 

 the eggs of birds from those of reptiles. If there are many 

 eggs in the nest it is obvious that the conical form makes close 

 packing more easy. Where only two eggs are laid they are 

 seldom conical. Eggs having a pyriforra shape, or narrowing 

 very rapidly towards the smaller end, are mostly those of 

 wading birds {Lhnicolce) , which lay four in a nest, and are 

 large in proportion to the size of the bird. Their pointed 

 ends being turned inwards, they occupy as little space as 

 possible, and are thus more easily covered by the brooding 



