The Parasitic Habits of Molothrus²

In the “Origin of Species” I adopted the view maintained by some writers, that the cuckoo lays her eggs in others birds’ nests, owing to her habit of laying them at intervals of two or three days; for it could hardly fail to be disadvantageous to her, more especially as she has to migrate at a very early period, to have young birds of different ages and eggs all together in the same nest. Nevertheless this occurs with the non-parasitic North American cuckoo.³ If it had not been for this latter case, it might have been argued that the habit of the common cuckoo to lay her eggs at much longer intervals of time than do most other birds, was an adaptation to give her time to search for foster-parents. The Rhea or South American ostrich is believed likewise to lay her eggs at intervals of two or three days, and several hens deposit their eggs in the same nest on which the male sits; so that one hen may almost be said to be parasitic on another hen.⁴ These facts formerly made me very curious to learn how the several species of Molothrus, which are parasitic on other birds in very varying degrees, laid their eggs;⁵ and I have just received a letter from Mr. W. Nation, dated Lima, September 22, 1881, giving me information on this head. He says that he has there kept in confinement for a long time Molothrus perpurascens, and has likewise observed its habits in a state of nature.⁶ It is a resident species of Western Peru, and lays its eggs exclusively in the nests of a sparrow (Zonotrichia), starling (Sturnella bellicosa), and a pipit (Ænthus chii).⁷ He then proceeds: “The eggs of the sparrow are very much like those of the Molothrus in size and colour. The eggs of the starling are larger and somewhat different in colour; while the eggs of the pipit are very different both in size and colour. Generally one egg of the Molothrus is found in a nest, but I have found as many as six. The young Molothrus does not always eject its foster-brothers; for I have seen a young one nearly fully feathered in a nest with two young starlings. I have also found two young birds of the Molothrus nearly fully feathered in the nest of a starling; but in this instance the young starlings had been ejected from the nest.” He then states that he had long kept in confinement a male and female of this species of Molothrus, which are now six years old. The hen began to lay at the age of two years, and has laid each time six eggs, which is the number laid by Icterus, a near ally of Molothrus. The dates on which the eggs were laid this year are as follows:— February 1, 6, 11, 16, 21, and 26; so that there was an interval of exactly four clear days between the laying of each egg. Later in the season she laid six additional eggs, but at much longer intervals and irregularly, viz. on March 8, April 6 and 13, May 1, 16, and 21. These interesting facts, observed by Mr. Nation in relation to a bird so widely distinct from the cuckoo as is the Molothrus, strongly support the conclusion that there is some close connection between parasitism and the laying of eggs at considerable intervals of time. Mr. Nation adds that in the genus Molothrus, out of every three young birds he has invariably found two to be males; whereas with Sturnella, which lays only three eggs, two of the young birds are, without any exception, females.

Charles Darwin

Down, Beckenham, Kent, November 7