Sir H. Davy gives an account of a pair of eagles which he saw on Ben Nevis teaching their young ones to fly; and every one must have observed the same thing among commoner species of birds. The experiments of Spalding, however, have shown that flying is an instinctive faculty; so that when he reared swallows from the nest and liberated them only after they were fully fledged, they flew well immediately on being liberated. Therefore, the 'teaching to fly' by parent birds must be regarded as mere encouragement to develop instinctive powers, which in virtue of this encouragement are probably developed sooner than would otherwise be the case. A few observations may here be offered on some habits which do not fall under any particular heading. 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 concerted 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 consists 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 the mould with his webbed feet. The burrow when finished has several twists and turns in it, and is about ten feet deep. If a rabbit burrow is available, the puffin saves himself the trouble of digging by taking possession of the one already made. The kingfisher and land-martin also make their nests in burrows.