Wrens have a curious way of beginning to build nests, and leaving them half finished. These are sometimes supposed to be the work of the male bird alone, and are called cocks' nests; and certainly the cock does not seem to take any part in building the true nest, for he simply sits on a branch close by and sings, while the hen does all the work. Perhaps he is lazy; or perhaps she thinks that she can build much better than he can, and so will not let him help her. And therefore it may be that he makes these cocks' nests just to show her what he can do. But as wrens are very timid birds, and will often desert their nest if one even puts one's finger inside, it seems rather more likely that they are nests which the birds have left unfinished because they thought that some enemy had discovered them. THE DIPPER Not unlike a very big wren with a white throat and breast is the curious and interesting dipper, well known to dwellers in the Rocky Mountains and the ranges west of them. It is never found far from water, and you may often see it perched upon a stone in the shallows of a river, bobbing up and down every now and then just as though it were making a courtesy. And every time that it does so it gives a quick little jerk to its tail, just as the wren does. It also makes a nest of moss, somewhat like that of the wren, which is placed in a hole in the bank of a stream, or often in a crevice of the rocks behind a cascade. It feeds on insects and water-shrimps, etc., and you may often see it busily hunting for the little beetles which are hiding among the moss on the large stones in the bed of a stream, where it actually walks on the bottom. It can swim and dive perfectly well, and keeps itself beneath the surface by flapping with its wings, while it searches for grubs in the mud at the bottom of the water. The dipper has a very bright and gay little song, and always seems happy, and busy, and active. SWALLOWS AND MARTINS Swallows and martins form a very distinct group of small birds well known to everybody, for no one can help noticing them as they sail through the air in swift graceful circles or skim low over the water in constant pursuit of the tiny flies which form their fare, and are so small that vast numbers must be caught. Familiar, too, is their coming in the spring, when they are welcomed as the special sign of returning pleasant weather after the season of cold storms; and in autumn we cannot but notice them gathering in large flocks along the telegraph lines or over the marshes, preparatory to departing to their winter retreat in the tropics. These characteristics, as well as their appearance--slender, long-winged, dark-colored--belong to the swallows and martins all over the world; and they are alike in all countries in their fearless fondness for making close acquaintance with mankind when he dwells in settled homes. COMMON SWALLOWS