Almost all birds take great trouble to ensure the sanitation of the nursery, and are diligent in their care of the health of the young. All the excrements are removed from the nest, a task that is rendered easy, as the droppings of the young are enclosed in a white, film-like envelope or capsule. A most careful search is made at the bottom of the nest for these capsules by the parents whenever they come to feed the young. Do they fail to find the expected capsules, one or other of the parents after the feeding will tap, tap on the anus of the young birds as if to remind them of a duty neglected.[42] This is, perhaps, the most extraordinary example of parental care that I have been able to discover. One wonders how far this apparent recognition of the necessity of regular habits and cleanliness is instinctive, or how far we may grant to these parents some direct realisation of the dangers arising to their children from neglect and a dirty nursery. It must not, however, be thought that all birds are good parents. In some species there would seem to have been a revolt against family ties and the duty of caring for the young. The common cuckoo and some other cuckoos are well-known examples. Among them, the mother, as every one knows, always lays the eggs in the nest of some other birds, and the young cuckoo, when it is hatched, would seem to have some knowledge of its precarious position as a stranger. It creeps under the nestlings of its foster-parents, and, by a violent effort, raises them one by one on its hollow back and jerks them out of the nest, so securing undivided attention in its alien nursery. A similar parasitic habit, not yet so firmly established, is found among the cow-birds of the Argentine. Mr. W. H. Hudson has seen the mothers trying to build nests and failing to do this, as if they were struggling to regain a dying instinct. The females flutter about the mud-nests of the oven-birds, and whenever a chance presents itself will dart in and lay their eggs. Other cow-birds make no effort at all in nest-building, and always lay their eggs in the occupied nests of other birds, and, as their eggs develop very quickly, the intruders hatch out before the true children of the nursery and rob them of their parents’ care. What do we learn from this? That neglect on the part of the mother—any shuffling out of her duties, thereby placing the care of her children on the shoulders of other parents, leads to crime and disorder in the social organisation.