Strolling across the moor in the sunshine to-day, past the lonely pine where the night-jar sits crooning to his lady-love in the twilight, I came suddenly across his grey mate herself, and saw her flutter up sleepily in dazed surprise from the bare ground where she was sitting. As she flapped her mottled wings and sailed slowly away, like a blinking owl disturbed in the daytime, I noticed that I had lighted unawares upon her nest, or rather, her eggs, for she lays them on the open, without bed of any sort. I left them untouched, for I am no collector. A few minutes later, I came abreast of the low cliff where the sand-martins have established their twittering colony. The soft yellow sandstone that forms the cutting is honey-combed with their tunnels; and as I leaned on my stick and looked, I saw the busy brown birds gliding in and out, with their long curved flight, and carrying back mouthfuls of gnats and mayflies to their fledgelings in the burrows. It was beautiful to watch them swooping in great arcs over the gorse and bracken, and then darting straight, with unerring accuracy, to the mouth of their tunnels. They alight at the very door with all the skill of born pilots, never missing or overshooting the mark by one inch, but steering upon it so truly that they look as though failure or miscalculation were impossible.

These two little episodes coming together set me thinking; ’tis a bad habit one indulges in when one walks too much alone in the open. In towns one doesn’t think, because the shop-windows, and the horses, and the noise, and the people, and the omnibuses distract one; but in the country, one gives way a great deal too readily to what Plato calls the “divine disease” of thinking. I began to philosophize. How curious, I said to myself, that we have but five kinds of bird in England that hawk on the wing after insects in the open; and of all those five, not one builds a proper respectable nest, woven of twigs and straws, like a sparrow or a robin! Every one of them has some peculiar little fancy of his own—goes in for some individual freak of originality. The night-jar, which is the simplest and earliest in type of the group, lays its eggs on the bare ground, and rises superior in its Spartan simplicity to such petty luxuries as beds and bedding. The swift, that ecclesiologically-minded bird, which loves the chief seats in the synagogue, the highest pinnacles of tower or steeple, gums together a soft nest of floating thistle-down and feathers, by means of a sticky secretion from its own mouth, distilled in the last resort from the juices of insects. The swallow and the house-martin, again, make domed mud huts, and line them inside with soft floating material. Finally, the sand-martin excavates with its bill the soft sandstone of cliffs or roadside cuttings, and strews a bed within for its callow young of cotton-grass and dandelion parachutes.

Why this curious variety among themselves, and this equally curious divergence from the common practice of bird-kind in general? Clearly, thought I, it must bear some definite relation to the habits and manners of the birds which exhibit it. Let me think what it means. Aha, aha, eureka! I have found it! The insect-hawking birds are not a natural group; by descent they have nothing at all to do with one another. Closely as the swift resembles the swallow in form, in flight, in shape of bill, in habits and manners, we now know that the swift is a specialized woodpecker, while the swallow and the martins are specialized sparrows. (I use both words, bien entendu, in quite their widest and most Pickwickian evolutionary acceptation.) The swift and the night-jar belong to one great family of birds; the swallow, the house-martin, and the sand-martin t............