BIRDS NIDIFICATION. 291 



habits which do not fall under any particular head- 

 ing. 



The habit which many small birds display of mobbing 

 carnivorous ones is probably due to a desire to drive off the 

 enemy, and perhaps also to warn friends by the hubbub. 

 It may therefore perhaps be regarded as a display of con- 

 certed action, of which, however, we shall have better 

 evidence further on. I have seen a flock of common terns 

 mob a pirate tern, which shows that this combined action 

 may be directed as much against robbery as against 

 murder. Couch says he has seen blackbirds mobbing a 

 cat which was concealed in a bush, and here the motive 

 would seem to be that of warning friends rather than that 

 of driving away the enemy. 



I have observed among the sea-gulls at the Zoological 

 Gardens a curious habit, or mode of challenge. This con- 

 sists in ostentatiously picking up a small twig or piece of 

 wood, and throwing it down before the bird challenged, in 

 the way that a glove used to be thrown down by the old 

 knights. I observed this action performed repeatedly by 

 several individuals of the glaucous and black-back species 

 in the early spring-time of the year, and so it probably 

 has some remote connection with the instinct of nest- 

 building. 



Nidification. 



In connection with the habits and instincts peculiar to 

 certain species of birds, I may give a short account of the 

 more remarkable kinds of nidification that are met with in 

 this class of animals. As the account must necessarily be 

 brief, I shall only mention the more interesting of the 

 usual types. 



Petrels and puffins make their nests in burrows which 

 they excavate in the earth. The great sulphur mountain 

 in Guadaloupe is described by Wasser as * all bored like a 

 rabbit warren with the holes that these imps (i.e. petrels) 

 excavate.' In the case of the puffin it is the male that 

 does the work of burrowing. He throws himself upon 

 his back in the tunnel which he has made, and digs it 

 longer and longer with his broad bill, while casting out 



