BILLS OF SKIMMING BIRDS. 231 



feathers of their breast, and then remove with the 

 bill. All the petrels have the nostrils enclosed in 

 separate tubes sometimes single, and sometimes 

 double ; but the use of these in their economy is not 

 known. 



With the storm-petrels one group of sea-birds, 

 classed according to their habits in feeding, and the 

 structure of their feeding organ, may be said to ter- 

 minate ; but there is still another, the commencement 

 of which may again be taken from the shore, and 

 traced to the more extended pastures of the other. 



The group which has now been mentioned as ex- 

 tending from the gannet to the storm-petrel inclusive, 

 may be regarded as having a relation (such a loose 

 relation of mere analogy as can exist between preyers 

 at sea and preyers on land) to the birds of prey the 

 gannets and races which have similar habits to the 

 eagles, the petrels to the vultures, and the storm- 

 petrels to the birds which catch insects on the wing. 

 The analogy is, as has been said, a loose one ; but it is 

 of use in forming a relative estimate of the economy 

 of the sea and ihe land. 



We may, therefore, continue it with the remaining 

 sea-birds, which are principally the lestri or skuas, 

 the gulls, and the terns. These are the omnivorous 

 birds of the ocean ; but the term, as applicable to 

 that element, does not include vegetable food, though 

 many of these sea-birds feed on land during the 

 breeding season, and also when the sea becomes too 

 stormy for them. 



The lestri, though called eagles (the real sea-eagles 

 are land-birds), are the ravens of the deep ; and in 

 their bills, their claws, and the general cast of their 

 bodies, they have a raven-like air. The following is 



