SWALLOW PETRELS. 321 



and attentively scanning its surface for their diminutive prey of 

 marine insects and small mollusca. They venture out at all times 

 of the day in quest of their accidental fare, and follow the wakes of 

 vessels partly for the animal productions which are thus whirled to 

 the surface, and not less for the fat and other animal matters which 

 are occasionally ejected from the decks. In stormy weather they 

 easily find shelter from the blast by skimming through the valleys 

 of the mountain waves. They are often seen tripping upon the 

 surface of the water, while eagerly engaged in seizing their food, 

 balancing themselves with singular lightness, by gently flapping and 

 fanning their expanded wings. At such times they often dip their 

 heads beneath the water, and though they swim and rest upon that 

 element at night and in fine weather, they are incapable of diving. 

 Their voice is low, guttural, and somewhat chattering, particularly at 

 night and during calm weather. They breed in society near the sea, 

 selecting for their nests the holes and cavities of rocks, which they 

 sometimes burrow out for themselves, but often make use of the 

 deserted resorts of other hiding animals ; the eggs are 1 or 2, and 

 they feed their young by disgorging food ; at these times, and on 

 other occasions, they are observed to hide themselves by day, and 

 sally out only towards twilight in pursuit of their prey. They are, 

 however, by no means nocturnal when at sea>, and are seen alike in 

 fair or foul weather, but scarcely follow vessels but in breezes, as 

 their own ordinary resources for obtaining food are equally produc- 

 tive in calm weather. 



A small and natural genus, consisting of about 5 nearly allieqi 

 species, 



