The birds usually return to the rookery in February, and in March, or even earlier if the weather should prove mild, they begin to repair the old nests and build new ones. The nests are placed on the topmost branches of the tree--elm, oak, birch, or fir; but an elm-tree is generally made choice of. The tree to suit the rook must be tall--if possible, the tallest tree in the place--for it is the instinct of the bird not only to have his house far out of reach of all possible terrestrial enemies, but so placed that a wide and uninterrupted view of earth and sky may be obtained from it. As things now are his winged enemies do him little hurt, but it was not always so. In the next place, the branches must afford a suitable foundation to build on: they must be strong, and forked, so as to hold the fabric securely during high winds and sudden violent storms; furthermore, there must be a clear space above or at the side, to enable the bird to approach and leave it without striking against the surrounding boughs. It is a well-known fact that rooks will desert a rookery when the trees are decaying, even when, to a human eye, they appear sound. The most probable explanation which has yet been offered of this fact is, that a considerable amount of pliancy in the branches is necessary for the safety of the nest; for if the branches do not yield and sway to the force of the wind, the nest is in danger of being blown bodily out of its place: in the decaying tree the upper branches become too stiff, from the insufficient supply of sap. The building and repairing time is one of great and incessant excitement in the rookery; and it is curious to note that birds of such a social disposition, and able to live together in concord at all other times, are at this period extremely contentious. As a rule, when one bird is abroad foraging for sticks, his mate remains on guard at the nest. Among these watchers and the birds that are leaving and arriving there is much loud cawing, which sounds like ‘language,’ in the slang sense of the word; and it might appear that they were all at strife, and each one fighting for himself. But it may be observed that a majority of the birds respect each other’s rights, and never come into collision, and that there are others, in most cases a very few, who depart from these traditions, and are, like freebooters, always on the watch to plunder sticks from their neighbours’ nests, instead of going afield to gather them. The presence of these objectionable members of the community may account for some of the curious episodes in the life of the rookery--as, for instance, the fact that all the birds will sometimes combine to persecute one pair, and demolish their nest again and again as fast as it is made. The nest, when completed, is a large structure, two feet or more in diameter, made of sticks, and lined with dry grass. The eggs are four to six in number, and are bluish green, spotted and blotched with greyish purple and dull brown.