_The art of sewing among birds._--There are birds which have succeeded in solving a remarkable difficulty. Sewing seems so ingenious an art that it must be reserved for the human species alone. Yet the Tailor Bird, the _Orthotomus longicauda_, and other species possess the elements of it. They place their nests in a large leaf which they prepare to this end. With their beaks they pierce two rows of holes along the two edges of the leaf; they then pass a stout thread from one side to the other alternately. With this leaf, at first flat, they form a horn in which they weave their nest with cotton or hair. (Fig. 31.) These labours of weaving and sewing are preceded by the spinning of the thread. The bird makes it itself by twisting in its beak spiders' webs, bits of cotton, and little ends of wool. Sykes found that the threads used for sewing were knotted at the ends.[98] It is impossible not to admire animals who have skilfully triumphed over all the obstacles met with in the course of these complicated operations.[99] [98] _Catalogue of Birds, etc._, p. 16. [99] Tristram, "On the Ornithology of Northern Africa," _Isis_, 1859-60. [Illustration: FIG. 31.] Certain Spiders, while they do not actually sew in the sense that they perforate the leaves they use to build their nest, and draw the thread through them, yet subject the leaves to an operation which cannot well be called anything else but sewing it.[100] [100] McCook describes, and gives good illustrations of, these nests in various stages of progress, _American Spiders_, vol i. p. 302. _Modifications of dwellings according to season and climate._--A certain number of facts show that these various industries are not fixed and immutable instincts imposed on the species. Certain Birds change the form of their dwelling according to the climate, or according to the season in which they inhabit it. For example, the Crossbill, _Loxia taenioptera_ (Fig. 32), does not build its nest according to the same rules in Sweden as in France. It builds in every season. The winter shelter is spherical, constructed with very dry lichens, and it is very large. A very narrow opening, just sufficient for the passage of the owner, prevents the external cold from penetrating within. The summer nests are much smaller, in consequence of a reduction in the thickness of the walls. There is no longer need to fear that the cold will come through them, and the animal gives itself no superfluous trouble. [Illustration: FIG. 32.]