THE PRINCIPAL BIRD GROUPS 61 



of the Divers are loud and unearthly screams, 

 mostly heard during the nesting season, but 

 those of the Grebes are more feeble and of a 

 croaking description. 



Another archaic and fairly distinct order is the 

 Procellariiformes, which includes the Petrels, 

 the Fulmars, the Shearwaters and the Alba- 

 trosses. They are all pelagic birds, and remark- 

 able for their great power of wing. Their chief 

 external characteristic is the tubular nostril, a 

 feature that serves to identify them at a glance. 

 They vary in size from about that of a Sparrow 

 (the Storm Petrel, or " Mother Carey's Chicken," 

 the smallest web-footed bird known) to that of 

 a big Goose (the Albatross Diomedea exulans, 

 one of the largest of volant birds, and with an ex- 

 panse of wing of upwards of twelve feet). All 

 the species have the anterior toes connected by 

 webs ; all swim well and buoyantly, and many 

 of the smaller species possess the habit of patter- 

 ing the surface of the waves with their feet, 

 running as it were over the water, hence the 

 name of Petrel derived from the Apostle Peter ; 

 but their most usual province is the air. They 

 are the feathered nomads of the wide ocean 

 wastes, and rarely visit the shore unless to breed, 

 or when driven to do so by exceptionally violent 



