Their favourite resort is a stream or a pool near trees.  Here they will assemble to the number of some hundreds, living in communities, and working together.  They select, when they can, a stream with a current, because it affords them the means of conveying wood and other materials for their habitations.  They choose such parts as will afford them depth of water sufficient to resist the frost in winter, and prevent it freezing to the bottom.  When, however, they find that there is not depth enough for this purpose, they build a dam across the stream, at a convenient distance below their habitations.  If the current is gentle, the dam is made perfectly straight; but if rapid, it is constructed with a considerable curve, the convex side being towards the upper part of the stream.  The materials employed are drift wood, green willows, birch, and poplar; these are placed horizontally, and kept down by mud and stones.  So strong do these dams become, that they are capable of resisting a considerable force both of water and ice; for generally the wood, taking root, shoots upwards, and forms ultimately a thick hedge. In some cases even trees sprout up, in the branches of which the birds form their nests.