Origin of Species Variorum

It has been objected by some authors that I have not noticed other related instincts and points of structure in the cuckoo, which are falsely spoken of as necessarily co-ordinated. But in all cases, speculation on any instinct or character known in only a single species, is useless, for we have no facts to guide us. Until quite recently the instincts of the European and of the non-parasitic American cuckoo alone were known; now, owing to Mr. E. Ramsay's observations, we know something about three Australian species, which lay their eggs in other birds' nests. The chief points referred to are three: firstly, that the cuckoo, with rare exceptions, lays only one egg in a nest, so that the large and voracious young cuckoo receives ample food. Secondly, that the egg is so remarkably small, that it does not exceed in size that of the skylark,—a a bird not more than one-fourth of the size of the cuckoo; that this is a real case of adaptation we may infer from the fact of the non-parasitic American cuckoo laying eggs of full size proportionally with her body. Thirdly and lastly, that the young cuckoo, soon after birth, has the instinct, the strength, and a properly shaped back for ejecting its foster-brothers, which then perish from cold and hunger. This, it has been boldly maintained, is beneficently designed, in order that the young cuckoo may get sufficient food, and that its foster-brothers may perish, before, as it is supposed, they have acquired much feeling!

Turning now to the Australian species; though these birds generally lay only one egg in a nest, it is not rare to find two and even three eggs of the same species of cuckoo in the same nest. In the Bronze cuckoo the eggs vary greatly in size, from eight to ten lines in length. Now if it had been of any advantage to this species to have laid eggs even smaller than those now