PROCELLARIID^. 399 



season, when they visit the rocky islets, where they breed, 

 and where they are rarely seen, as they keep in their holes 

 throughout the day, only coming forth to feed at dusk. 

 Although in our arrangement immediately following the 

 Gulls they have little or no connection with those birds, 

 as they differ from them in many respects. First, there is 

 the peculiar position of the nostrils, which, in the Petrels, 

 are placed in bony tubes attached to the ridge of the 

 upper mandible ; another point of dissimilarity is furnished 

 by their eggs, the Petrels laying but one white e^^g, some- 

 times having a zone at one end of small red or violet 

 spots ; they differ also in their young being helpless when 

 hatched, and when the adult birds rest upon the rocks 

 they recline upon the full length of the tarsus instead of 

 standing upright upon their feet, but we have occasionally 

 noticed Gulls doing the same. In their plumage the true 

 Petrels are either black or greyish black, have a white 

 patch above their tails, very slender legs, with webbed feet, 

 and as they fly low above the surface of the sea, rising over 

 the crests of the waves and dropping again into the suc- 

 ceeding trough, they may be seen every now and then to 

 hover, and as it were pat the water with their feet as they 

 are on the look-out for the small marine creatures or the 

 greasy substances thrown overboard from some vessel in 

 whose wake they may be following. Another characteristic 

 of the whole family of Petrels is the quantity of oil they 

 produce, which is contained in their stomachs and used 

 both for the purpose of feeding their young and, in some 

 species, for their own defence, as they immediately vomit 

 it when handled. There are two species of small Petrels, 

 the little Storm-Petrel and the Pork-tailed or Leacli's 

 Petrel, which nest on islands off the Western and North- 

 western coasts of the United Kingdom ; and another of* 

 them, AVilson's Petrel, whose liome is on the Southern 



