THE PRINCIPAL BIRD GROUPS 87 



dive. Some species (Terns) plunge like Gannets 

 to secure their prey. They feed largely on fish, 

 but may be described, as a group, as practically 

 omnivorous. Their notes are harsh and un- 

 musical. The Skuas obtain much of their food 

 by chasing and robbing other Gulls of their prey. 

 They breed on the ground or rocks, the Gulls 

 and the Skuas making more or less elaborate 

 nests, but the Terns provide little or no ac- 

 commodation for their eggs. These are mostly 

 three in number, sometimes two, and in excep- 

 tional instances four, and richly marked with 

 spots and blotches. In this order the sexes are 

 nearly alike in colour. The prevailing tints are 

 grey and white in the adult Gulls and Terns 

 (often with a black hood or crown), browns and 

 yellow in the adult Skuas. The immature birds 

 are mostly mottled brown of various shades, 

 whilst the young are hatched covered with 

 down. Some hundred and ten species are 

 known. 



Another allied group is the Alciformes or 

 Auks, which, although it contains but a single 

 family (Alcidae), is composed of birds present- 

 ing considerable variation of structure and 

 plumage, especially of the head. The order 

 comprises the Auks (of which the recently 



