THE PRINCIPAL BIRD GROUPS 85 



are known to perch in trees. Their food is 

 partly animal and partly vegetable substances. 

 They usually nest upon the ground, making a 

 slight provision for the eggs, which are generally 

 four in number. These as a rule are handsomely 

 marked and pyriform in shape, but the Crab 

 Plover is said to lay a white egg and to breed 

 in a burrow. The young, almost without ex- 

 ception, are hatched clothed in down, and 

 able to run about soon after leaving the shell. 

 We may also mention that many species are 

 migratory. 



Following the Charadriiformes, and certainly 

 closely related to them, come the Lariformes or 

 Gulls, Terns, Skuas, and Skimmers Here again 

 we have a remarkably homogeneous assemblage 

 of birds, possessing external characteristics 

 which prevent them from being confused with 

 any others. This order may be conveniently 

 subdivided into four well-marked sections, only 

 one of which, however, is worthy of more than 

 sub-family rank. These are the Gulls (Larinae), 

 the Terns (Sterninae), the Skuas (Stercorariinae) 

 and the Skimmers (Rhynchopinae). Possibly 

 the Skuas form a distinct family, as will be seen 

 below. The birds in this order have the an- 

 terior toes united by a web. The bill in the 



