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324 the same hole and cling there without quarrelling; but once I saw a bird in a hole attacked by another, who flew suddenly down upon it with a little twittering scream.

Though each pair of birds excavate their own tunnel, yet the whole community, or, at any rate, a large proportion of it, will sometimes work together, sweeping on to the pit's face in a body, clinging there and burrowing, with a constant twittering, then darting off silently in a cloud and sailing and circling round in the pit's amphitheatre, making, when the sky is blue and the sun bright, a warm and delicious picture such as the Greeks must have loved to gaze on.

As each bird, however, only works at his own and his partner's hole, it is evident that this kind of social working is not the same as that of ants or bees and other such insect communities, though it has something of that appearance. Sometimes, for a short time, all the birds will keep fluttering round in small circles that only extend a little beyond the face of the cliff, not rising to a greater height than their own tunnels in it, which they almost touch each time, as they come round. They look like eddies in a stream beneath the bank, but are not so silent, for all are twittering excitedly. This is an interesting thing to see, a kind of aerial manoeuvres the special cause of which, if there be one, is not obvious.

But we will suppose that the birds are now all working, either inside their tunnels or clinging to the face of the cliff. All at once, either at or about the same instant of time, they all fly off, darting away, and disseminate themselves in the sky, not one being left either in or about the pit. In a few minutes