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 Gruadaloupe 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 
