When a bird begins to build her nest, she uses coarse materials first, just as a house builder uses beams and timbers to begin with. The bird and the house builder save all the fine stuff for the last. Look closely at a nest when you find one. Pick up an old last year's nest that has blown down. This year's nests do not belong to you. See how there are, first, large sticks or weeds, or rolls of mud. Between the large sticks or weeds there are small, short ones. You can imagine that these pieces all together are nails and boards, and help to hold the whole nest together. Perhaps these may be all bound together with spiders' web or string, or even paper. We have seen nests made of nothing but one kind of weed; usually a weed that has a strong smell, like wild sage or yarrow, is chosen. We think that the smell of these strong-scented weeds prevents lice or mites from invading the nest. Perhaps the force of habit or taste has led the bird to select this material. Probably her mother before her made the same sort of a nest, and so she thinks that is about the right thing to do. Some birds, as the swallows, make mud houses, after the manner of the Mexicans. We often wonder if these people got their idea of house building from the birds. [Illustration: Barn Swallow] Other birds use sticks and cement, as a man does brick and mortar. Some of the sea birds lay their eggs on a bare, flat rock. Even these do not roll off from the rock, for all eggs are oblong and cannot roll in a straight line. We have never seen a perfectly round egg. If you take an egg of any kind, as a hen's egg, and try to roll it down the floor or lawn, you will see what we mean. Then try a perfectly round ball. You will see that it is better that birds' eggs are oblong or elliptical. [Illustration: Marsh Owl.] The cactus wren[16] makes her nest in the middle of a great barbed cactus in our mountain washes or desert places. The tiny Costa's humming-bird[17] builds its frail nest in the prickly elbow of the low cactus that grows in California all over the barren lowlands. This is probably for safety. A snake could hardly reach a nest which was built in the middle of a cactus whose needles, or thorns, are sometimes an inch long. [16] _Heleodytes brunneicapillus._ [17] _Calypte costæ._ [Illustration: Costa's Humming-Bird] [Illustration] CHAPTER XVIII. SOME OTHER BIRDS AT WORK. Not many of the birds in our yard are quarrelsome. They seem to respect one another's rights, especially at nesting time. It is not so much our business to tell bad or unpleasant things about birds, as to tell what is pleasant and what will make you love them. That is why we spoke a good word for the shrikes and the hawks and the owls.