The house-martin may be distinguished from the swallow by the patch of white upon the upper tail-coverts, and by its feathered toes. Like the swallow, it arrives in this country about the middle of April. The birds soon begin to repair their old nests and to construct new ones under the eaves of houses. The nest is largely composed of clay or mud, which the birds collect from damp places in the road, etc., place in position against the wall, and allowed to harden. The completed nest is somewhat bag-shaped, having only a small hole at the top; it is lined with soft grass. The eggs are white, without spots. It is worthy of notice when a wild bird lays white eggs, they are hidden from sight by the shape or position of the nest, as in the case of the martins, or the nest is built in an inaccessible place. Exposed eggs are usually made more or less inconspicuous by being coloured or spotted. House-sparrows (Fig. 208) have often been known to expel the defenceless martins from their nests, and to lay their own eggs therein. The eggs of the sparrow may be recognised by their grey colour, and by the brown blotches with which they are marked. The structure of the martin, like that of the swallow, is admirably adapted to a life in the air, and to a diet of flying insects—the bird having powerful wings, small and weak feet, and a soft and short but widely-opening beak. House-martins leave us about the beginning of October. The sand-martin is distinctly smaller than either the swallow or house-martin, and it may be further distinguished from them by the “mouse-colour” of its upper parts. It usually arrives in this country about a fortnight in advance of its two relatives, and also departs before them, leaving in August or September. These birds live in colonies in tunnels which they excavate in banks or friable cliffs (Fig. 203)—generally in the near vicinity of water. The excavation may [Pg 316] be three feet in length; it slopes slightly upwards to the nest (which is placed at the end of the tunnel), in order that rain water may not collect in it. The eggs are pure white, and five or six in number. The birds are exclusively insectivorous; they do not sing, but utter a little twitter. The flight is very characteristic, consisting of “odd jerks and vacillations, not unlike the motions of a butterfly.” Fig. 203.—Sand Martins’ Nesting Holes. Fig. 204.—The Skylark. (× ⅓.)