"But why do not cuckoos make nests and sit on their eggs like other birds?" said Jack. Such a question is more easily asked than answered; nevertheless I hope you will always try to discover reasons for things. "It is now," writes a celebrated naturalist, "commonly admitted that the more immediate and final cause of the cuckoo's instinct is, that she lays her eggs, not daily, but at intervals of two or three days; so that if she were to make her own nest and sit on her own eggs, those first laid would have to be left for some time unincubated, or there would be eggs and young birds of different ages in the same nest. If this were the case the process of laying and hatching might be inconveniently long, more especially as she has to migrate at a very early period, and the first hatched young would probably have to be fed by the male alone." The cuckoos come to this country about the middle of April; the male birds arrive before the females. Whether this arrangement is ungallant conduct on the part of the gentlemen birds, who prefer to come alone, or whether, just when the gentleman cuckoo is ready and almost impatient for a start, her ladyship has all at once discovered some important matter that ought to be finished before leaving the country, some adjustment of her dress, some tiresome feather that will ruffle itself up in spite of every effort to keep it smooth, I know not, but the fact remains, that my Lord and Lady Cuckoo do not travel together. Let us suppose that both sexes have arrived in this country, we will say about the 23rd of April. It is natural they want a little time to look about them; at any rate, no egg is ready for being sat upon till some weeks after the arrival of the birds, say the 15th of May. The eggs require fourteen days' setting before they are hatched; this brings the date to the 29th of May. The young ones will require three weeks in the nest and constant feeding all the time; we now arrive at about the 20th of June, when the young ones would be ready to leave the nest. But they want five weeks' more feeding by the parents, after they leave the nest, before they are able to provide for themselves; this would bring the date to about the 25th of July, when there is hardly a parent bird in the country; they have left for other parts of the world. "Oh! but, papa," said Willy, "you said in the lines you told us to remember--