CLASS V. Fishes.--Cold-blooded animals which have either scaly or naked skins, but no fur or feathers; which live in the water, breathe it with their gills, and swim in it with fins. CHAPTER V CITIZEN BIRD The apple trees were in full bloom the day that the Doctor again found time to be with the children. It was exactly the kind of a day that birds like. The ground was soft enough to let the earthworms come up to breathe, so that Robins could catch them easily, and the air was full of all kinds of insects newly out from their long winter sleep in their soft cocoon beds, much to the delight of the Swallows and Flycatchers. It was also a beautiful day for House People to watch their bird neighbors; for it was mild but not too bright, and every one knows how it hurts the eyes to look at flying birds with the sun shining in them. Olive, Dodo, and Nat went out first and found Rap waiting. The Doctor followed, carrying something in his hand in a black leather case. When they arrived at the old tree in the orchard, he told them to look up. There was the perch arranged as it had been when he was a boy. Not a perch for birds, but for House People--narrow board seats fitted in between the largest branches and a bar fastened across some of the highest ones, so that it was quite safe to climb up and look out of the top of the tree. The branches had been trimmed away here and there, so that a good view could be had of what was happening elsewhere in the orchard. A scream of surprise and delight came from the group, in which Olive joined. Quickly as the children scrambled into the tree, the Doctor was up there first, laughing and saying that it was thirty years since he had climbed that apple tree; for after he went away to college the old seats had decayed and fallen down. "Give me your hand and I'll help you up," called Nat to Rap, who had dropped his crutch and was looking up at the others. "No, you needn't," said Rap. "I can climb all right. Sometimes it isn't so handy for me, but other times it's easier, for in tight places one leg doesn't take up as much room as two;" and he swung and pushed until he was up as high as the rest. "Here's a nest with eggs in it," whispered Dodo, who had crept out on a limb, where a rather large round nest, made of grass and little sticks plastered together with mud, was saddled on the branch--in fact, a Robin's nest. "Four lovely smooth eggs, not quite blue and not quite green! Please, can I have them? I saw them first." "Think a minute, Dodo," said the Doctor. "A bird will come from each of those eggs. Suppose you take the eggs away from the poor Robins, you will be killing four young birds, besides hurting the feelings of their parents and making them leave the orchard, very likely. You must not take any eggs in the nesting season--not even one. I will tell you what happened once in a field where there were some birds' nests in the bushes.