XI.—Some Reflections on the Breeding‐habits of the Cuckoo (Cuculus canorus)
Authors: Meiklejohn R. F. 
Published: April 2008

Summary.Having, then, dealt with the main theories and conclusions concerning the breeding‐habits of the Cuckoo, the results I arrive at are as follows, those based on theory being marked with an asterisk:—

*The Cuckoo is probably polygamous.
The eggs vary more in colour and markings than those of any known species.
Their main distinctive features are the comparatively greater weight, thickness, and strength of the shell.
Most Cuckoos' eggs resemble the normal type of those of a certain number of the species which are usually selected as foster‐parents; others are of a “mixed type,” intermediate between two such species or of a “distinctive type” of their own. The percentage of eggs exactly resembling those of the clutch in which they are found is only about 13 per cent., while those resembling the general type of the species in whose nest they are found amount to some 45 per cent.
The resemblance occurs most often in the cases of the Common Redstart, Brambling, Whitethroat, Garden‐Warbler, Reed‐Warbler, Marsh‐Warbler, Sedge‐Warbler, and Meadow‐Pipit. In all other species it is much more occasional, and it never occurs in the case of the Wren, Willow‐Warbler, Wood‐Warbler, Chiffchaff, and Hedge‐Sparrow; yet, of these latter, the first four invariably refuse to hatch the egg, while the Hedge‐Sparrow willingly accepts it—a strange fact which is hard to fit in to any theory.
In some localities the percentage of eggs resembling the type of the species in whose nests they are deposited is very high, nearly all being similar; in others it is very low. The high percentage usually occurs where one species is mainly used as foster‐parent, and the low percentage where several are used.
*The explanation of the resemblance of Cuckoos' eggs to the normal type of those of the foster‐parents is probably that the nest is selected after the egg is laid, and that the female Cuckoo is influenced in her choice mainly by the colour of the eggs in the nest and the resemblance they have to her own. Quite possibly she may also be influenced to some extent by the nest being similar to that in which she herself was reared, especially when she cannot find eggs which match her own. Also she would normally return in spring to the locality in which she herself was reared. Further, when she had once laid, she would know the colour of her eggs and could select nests beforehand if she came across them.
*The prevalence of certain distinctive types of eggs in specific localities can be assigned to the law of adaptation. The strain of Cuckoos laying distinctive eggs, e.g. blue or very rufous types, would have a large number destroyed, since many species would refuse to accept them and thus this strain would gradually die out except in localities where many nests with similar eggs were to be found. Ou the other hand, the strain of Cuckoos laying more neutral‐coloured eggs would have more young reared, since their eggs would be more easily accepted and thus would flourish in most districts.
*Most female Cuckoos return to the same locality as that in which they were themselves reared, and as it may be assumed that their ancestors selected nests whose eggs resembled their own, and that egg‐coloration is more or less hereditary, they find suitable nests in such localities. Hence, where several foster‐parents are available in one locality, Cuckoos laying eggs of different types will collect, and there is more likely to be a shortage of suitable nests and consequently a lower percentage of resemblance.
*The Hedge‐Sparrow, which is singularly ready to adopt eggs of any colour, is the favourite “makeshift” foster‐parent.
*The number of eggs laid yearly is probably nine to twelve.
*Laying appears to take place at irregular intervals.
*The date of the laying period varies in different localities and corresponds with that of the species selected as foster‐parents.
*Most Cuckoos lay in the same rather restricted locality.
Each female lays eggs of the same type all her life.
*The eggs are probably laid on the ground amongst vegetation or in the forks of trees, and at times may be left there with a second or even third egg until suitable nests are found.
Only one egg is placed in any nest, and if two or more are found in the same nest they belong to different females.
When placing its egg in a nest the Cuckoo usually removes one or more of those of the foster‐parent. Occasionally this is done the day before, and at times other eggs are removed subsequently. In the case of the Cow‐birds, the eggs of the foster‐parents are either ejected or have a minute puncture made in them, either by the beak or sharp claws, so as to prevent their hatching.
At times a vigorous resistance is met with from the owners of the nest, resulting in the Cuckoo's egg being destroyed, but usually it is accepted and the young Cuckoo reared with extraordinary devotion by its foster‐parents. In this connection it is interesting to learn that some species, such as the Indigo Bunting, will often desert their nests if the Cow‐bird's egg be removed, but do not appear to mind the loss of one or two of their own eggs.
*Probably the Cuckoo inserts its egg, whenever possible, in the absence of the lawful owners of the nest.
Neither the ovary nor the egg‐development of the Cuckoo presents any abnormality in comparison to other species.
Shortly after being hatched the young Cuckoo ejects all its companions from the nest, and if by chance two young Cuckoos are hatched in the same nest the weaker is ejected. Exceptions occasionally occur, due probably to the nest being in a deep hole, etc., and ejection not being possible.
*The female Cuckoo appears, at least at times, to take some interest in the future development of her eggs, subsequently removing other eggs from the nest, and assisting on occasions in the process of ejection and even in feeding her young.

In conclusion, I wish to state clearly that this paper has not been written with any idea of minimising the important results achieved by Dr. Key and others, who devoted endless labour to trying to throw some further light on this involved question and to whose efforts much of our knowledge is due. My object has been to endeavour to show that the problem is still unsolved and to try to interest others in the subject. The difficulties in obtaining reliable information are so great that it seems that success can only be achieved by united efforts and by ensuring, so far as possible, against any chance clues being lost.