“The food of small birds falls into two classes, seeds and insects. Certain birds require millet, hemp-seed, pips, and similar seeds of all kinds, while others need grubs, larvæ, insects. The choice of one or the other sort of food is determined by the shape of the beak, just as a mammal’s diet depends on the structure of the animal’s teeth. The molars of the horse and the ox call for forage to grind under their flat, wide crowns; but those of the wolf and the cat, with their sharp edges, need flesh to cut to pieces. In the same manner the bird’s beak, according to whether it is shaped this way or that, whether large or small, thick or slender, strong or weak, requires hard seeds that crack under the mandibles and in opening yield their kernels, or the tender grub that is swallowed without having to be crushed. Show me your teeth, we said to the mammal, and I shall know what you eat. Show me your beak, we might now say to the bird, and I shall know whether you live on insects or seeds. “The beak of the bird that lives on seeds or grain—that is, the granivorous bird—is thick, conical, wide at the base, and strong in proportion to the hardness of the seeds it has to crack open; but the beak of the bird that lives on insects—that is, the insectivorous bird—is thin, slender, delicate, and weak in proportion to the softness of the insects it catches. In our everyday speech we note this difference by applying to the small granivorous birds the general designation grosbeak, while the insect-eaters are often called slender-beaked birds. Let us remember these two expressive words and formulate the general principle thus: Seeds for the grosbeak, worms for the slender beak. “And now without further delay we will put the rule into practice. Here is a bird whose diet is perhaps a matter of uncertainty to you. If I ask you what, to judge from the shape of its beak, is its customary food, shall you be at a loss how to reply?” “That strong beak, so wide at the base, must be meant for crunching the very hardest seeds,” was Jules’s opinion. “Yes,” Emile chimed in, “that bird certainly lives on seeds; it is written all over its big face.” “It is, indeed, a consumer of all kinds of seeds; it is the greenfinch of our copses, greenish underneath and with a yellow border to its tail. The dominant color of its costume, green mixed with yellow, has given it the name of greenfinch. And this one?” “Seeds for the grosbeak, worms for the slender beak,” repeated Emile. “The beak has no strength; it is rather long but thin; the bird is an insect-eater.”