The young should remain warmly covered by the mother as long as they continue unfledged[56]; that is to say, generally for twelve days: on the thirteenth day they begin to eat alone. In four weeks they may be placed in other cages of a sufficient size; but they must still for some weeks be fed with the above-mentioned paste, conjointly with the food of full-grown birds; for the sudden privation of this nourishment often occasions death, especially when moulting. Experience proves that generally those canaries which are hatched in a large garden aviary, where they enjoy fresh air, and considerable space for the exercise of their wings, are more vigorous, more healthy, and more robust than those which are bred in rooms, and it is easy to understand the reason. I must not omit to mention here an important observation, which has been often made, that if two females are given to one male, and one of them happens to die, the other immediately takes charge of the abandoned eggs, and assumes so completely the duties of foster-mother, that in order rigorously to fulfil them she avoids and even repulses the caresses of her mate. Canaries pair not only among themselves in our aviaries and cages, they also form connexions foreign to their species, and, provided the analogy is not too remote, produce fruitful mules. Serins, citral finches, siskins, goldfinches, or linnets, are the species which succeed best[57]. To succeed, however, it is necessary that the birds should have been brought up from the nest. The custom is to give an old male of one of the above-named species to a female canary, the principal reason being that an old female of one of those species, though she would not object to the union, could never be induced to lay in an artificial nest, like a female canary. The offspring of these mixtures combine the colours of the father and mother, learn well enough if they descend from a linnet or goldfinch, but sing badly if they come from a siskin or lesser redpole. They are easily brought up with the paste mentioned above for canaries. It is asserted that the mules of serins, citral finches, and goldfinches, are fruitful. It is remarked, however, that their first eggs are very small, and the young hatched from them very weak; but the next year the eggs become larger, and the young stronger and more robust.