In many parts of Europe the nest is built in the hole of a tree, but in the south is more generally placed in ruins, under the roofs of houses, in clefts in walls, or on cliffs and steep banks, in which deep holes are excavated for its reception. At the proper season it contains from five to six white eggs, which are brooded in turn by the male and female, and this so assiduously that it is often easy to capture the sitting bird with the hands. In laying white eggs, the roller conforms to the general rule of birds which nest in holes. It is, moreover, an uncleanly bird, and when the nest has contained young for some time, it gives out a most disgusting smell, while the young themselves sit in a mass of filth. Strange that such a beautiful bird should have such dirty habits! As regards its mental powers, the roller is evidently highly developed; but its inherent shyness renders it unsuitable as a cage-bird. In disposition it is, as already mentioned, unsociable, and it will frequently quarrel with other birds and its own fellows. On the other hand, several pairs often nest near together, and its migrations are made in large companies. THE BITTERN (_Botaurus stellaris_) Before drainage and cultivation had driven away so many of the marsh-haunting birds, the boom of the bittern was a familiar sound to the dwellers in the fens of Cambridgeshire and Lincolnshire, and this handsome bird regularly nested not only in those districts, but in the Norfolk Broads, as well as in many other counties possessing situations suitable to its habits. Indeed nests were taken now and then up to the middle of last century, and even later; but at the present day the bittern is nothing more than a casual visitor to the British Islands, in fact, so rare have its visits become, that they are generally considered worthy of special record. A near relative of the heron, the bittern does not expose itself in the open after the fashion of that species, but skulks amid the shelter of reeds and flags, where its presence is made known only by the ordinary raven-like croak, or the loud booming of the male in the breeding-season. From this habit the bittern derives its German name of _rohrdommel_; and with such surroundings its mottled plumage of light and dark brown mingled with black is designed to harmonise, as is also in all probability the greenish hue of its long, spear-like beak, so admirably adapted to seize and hold the unwary fish or frog that may come within striking distance.