Up to quite recently, people supposed that the female Cuckoo, when about to lay, watched the nest of some small bird until the owner left it in search of food, when she stealthily sat down and dropped one of her own eggs into the structure. Unfortunately for this theory, it would not hold good in the case of domed nests built by such species as the Common Wren and Willow Warbler, both of which are occasionally victimised. This puzzle has been satisfactorily solved by the discovery of the real facts. The Cuckoo lays her eggs upon the ground, and, picking them up in her bill, deposits them in the homes of birds whose own productions they will to some extent match in colours. [Illustration: YOUNG CUCKOO IN MEADOW PIPIT’S NEST, BIRD’S OWN EGGS THROWN OUT ON TO EDGE OF STRUCTURE.] [Illustration: CUCKOO’S EGG IN MEADOW PIPIT’S NEST.] Cuckoo’s eggs vary very widely in point of coloration. They are usually reddish grey, mottled and spotted closely with darker markings of the same colour, or pale greyish-green marked with spots of a darker hue. I have met with them matching in colours those of the Meadow Pipit, Pied Wagtail, and Reed Warbler so closely that they were scarcely discernible except for their larger size, and a blue specimen has been found in the nest of a Hedge Sparrow. Some naturalists are of opinion that a Cuckoo is able to lay an egg of any colour at will, whilst others favour the opinion that if an egg closely resembles in point of coloration those of the bird, say a Tree Pipit, in whose nest it has been placed, that young Cuckoo’s grandfather and grandmother were also reared by Tree Pipits. [Illustration: ADULT CUCKOO.] [Illustration: “COME AND HEAR THE CUCKOO SING. COME AND BREATHE THE BREATH OF SPRING.”] Another important fact which aids deception when trying to impose upon small birds is that a Cuckoo’s egg is only one-quarter the size of what might reasonably be expected from the dimensions of its layer. It is much heavier, however, than any other egg of its size, and has a thicker shell. Competent naturalists have asserted that the Cuckoo lays as many as five eggs during a season, and although only one specimen as a rule is found in the nest of an intended foster-parent, as many as three may be met with, but whether deposited by the same individual or not, it is, of course, impossible to say. Hedge Sparrows and Robins are the greatest victims, but even the Jay, Wood Pigeon, and Carrion Crow have been successfully imposed upon. [Illustration: YOUNG CUCKOO WAITING FOR ITS FOSTER-MOTHER WITH FOOD.]