CUCKOO, a well-known bird, distributed during the breeding season over most of Europe and Northern Asia ; in many languages its name is derived from its frequently repeated call. Its abnormal breeding habits have made it the subject of much controversy among ornithologists. The fact that it was parasitic on other birds in the early stages of its life was known as far back as the time of Aelian, yet it is only within the last so years that the details have been gradually worked out and there are still some obscure points.
The common cuckoo, Cuculus canorus, Linn., winters in Africa south of the Sahara, India, the Malay Peninsula, south China and New Guinea. On the south coast of England it usually arrives about the second week of April. On the Continent many pass be yond the Arctic circle to the tree limit in north Finland. The bird itself is far oftener heard than seen. Its bluish colour, barred breast and long tail suggest a likeness to the sparrow-hawk (Accipiter nisus) and it is no uncommon sight to see it mobbed by small birds in the neighbourhood of a nest.
In the British Isles alone 6o species have been recorded as foster parents of the cuckoo and 120 on the Continent. Out of the dozen regular fosterers in England only about six are really common.
A more comprehensive view, however, leads to different con clusions. In vast forested areas of north-eastern Europe the cuckoo is mainly parasitic on the redstart (Phoenicurus phoenicurus), and occasionally other species, all of which lay blue, or bluish-green eggs. Here practically all cuckoos' eggs are blue. In the great reed-beds round some Hungarian lakes the normal host is the great reed warbler (Acrocephalus arundinaceus) and a cuckoo's egg, boldly marked with grey and black, has been evolved which closely matches the host's. Many similar instances could be quoted, but it is evident that mimicry has become general because it is useful in reconciling the fosterer to the presence of the para site's egg.
Any theory must take into account the fact that in some districts mimicry is only occasional. Statistics derived from series of cuckoos' eggs in museums and private collections are misleading, because nests of some species are much more readily found than others. In English collections cuckoos' eggs from nests of the reed warbler and hedge sparrow are common, while those from meadow pipits' nests are less frequently found. Yet the latter is the most numerous host over the greater part of Britain. From the study of the scanty material available we find that in the meadow pipit-cuckoos mimicry is very close, but in those districts where three or f our species are victimized we often meet with a "mixed type" which shares in the characters of the various fosterers, and as regards the hedge sparrow mimicry is almost lacking.
Influence of Selection.—Dismissing this last species for a mo ment, it appears that the fosterers have allowed only those strains of cuckoo to survive which laid eggs harmonizing more or less closely with their own, by elimination (either through desertion of the nest or ejection of the egg) of all eggs in strong contrast with their own. This process has gone on uninterruptedly where the cuckoo is parasitic on one species only, or on two or three which lay similar eggs; but in districts where two or more fosterers which lay eggs of divergent types are victimized, interbreeding between the strains of cuckoo has produced the "mixed type" which passes muster with either, but avoids close mimicry. In the case of the hedge sparrow, experiment confirms the supposition that while other species resent the intrusion of strongly contrasting eggs, this species accepts them with indifference. The intensive study of the cuckoo by Dr. E. Rey and especially by V. Capek has shown that where close mimicry has been thus obtained the percentage of rejections by the fosterer falls to 5 %, but where the host is more exacting or mimicry is less perfect it may rise to to, 3o or even 7 7 %. Yet in all these cases the cuckoo has obtained a foothold and some proportion of young are reared annually. There are, how ever, a few cases in which the cuckoo is apparently struggling with little or no success to impose her eggs on an unwilling host. (Experimental research has been carried on in Germany by Dr. Rensch and in Africa by Swynnerton on these lines.) Method of Egg Deposition.—The method by which the cuckoo inserts her egg in the fosterer's nest is still the subject of keen controversy. The careful work of E. P. Chance, supported by the evidence of films and photographs, has proved that in the case of ground-building species such as the meadow pipit, the cuckoo takes up her position on some commanding site near a selected nest, some time before laying is due. She glides down to the nest, picks up one of the eggs with her bill, settles on the nest for five or ten seconds, during which period the egg is laid, then backs out of the nest and flies away, taking the host's egg with her and usu ally devouring it. The same process also takes place in most open nests in trees or bushes. In the case of small birds which make domed nests with circular openings, or breed in holes of trees or rocks, this method seems at first impracticable, though we have good evidence that the cuckoo can squeeze into a very small open ing, and domed nests often show traces of damage to their super structure by the cuckoo when inserting her egg. Levaillant's suggestion that the egg is carried in the mouth and inserted by the bill provides such a simple explanation that it has been perhaps too hastily adopted, but most of the evidence fails to stand the test of close examination, and definite proof of its adoption is still lacking. The alternative suggestion is that the egg is laid direct into the nest hole while the bird is clinging with outspread wings to the side of the nest, and that a certain measure of direction can be im parted to the egg at the moment of laying by the extrusion of the lower walls of the oviduct. Occasional instances in which a cuckoo's egg has been found lying beneath the entrance to a wren's nest are more readily explained thus than if we accept the suggestion of insertion by the bill.
Rey was the first to assume that the eggs of the cuckoo are usually laid at intervals of about 48 hours, and Chance has proved this. By systematically removing the eggs as laid and pro viding a succession of fosterers' nests at the necessary intervals, Chance has also proved that a single hen may lay up to 25 eggs in a season. This was in 1922, the next highest being 21 from the same bird in 192o. Under natural conditions, nests are not always available and probably the average clutch of the cuckoo consists of five to seven eggs, laid on alternate days, followed by a pause and then by a further clutch of four or five.
The old birds, whose impending departure is heralded by a change of note, begin to leave England in July, and have all gone by the end of August, but the juveniles may be seen even in September.
Economic Status.—Although Collinge estimates that 83% of the food of the cuckoo consists of injurious insects, especially hairy caterpillars, which are rejected by most birds, and regards it as extremely useful, it must be remembered that every cuckoo is reared at the expense of a brood of young insectivorous birds, and monopolizes the attentions of the old birds for some five weeks in the breeding season. Cuckoos also destroy nests with in cubated eggs or young in order to force the parents to lay again.
Great-Spotted Cuckoo.—In another group of cuckoos, which includes the great-spotted (Clamator glandarius) and the crested cuckoos (C. jacobinus) inhabiting southern Europe, Asia and Africa, a different type of parasitism prevails. The former species is parasitic on magpies, crows or ravens, and its eggs show a strik ing resemblance to the Corvine type. When hatched the young parasites (sometimes as many as three to five in one nest) do not eject their foster-brothers, and the whole family lives in harmony. It has only recently been pointed out by the writer that striking mimicry exists between the young cuckoos and their companions. This is also the case with the Indian koel (Eudynanys scolopaceus) which is parasitic in a similar way on crows.
All cuckoos have not adopted the parasitic habit, for the two common species of North America, the black-billed and yellow billed cuckoos (Coccyzus erythrophthalmus) and (C. americanus) both rear their own young, as also do the road runners (Geococcyx) and in the old world, the coucals. Two groups, however, deserve mention on account of their abnormal habits. The ani (Crotophaga ani) nests socially, several females combining to build a large nest, in which they deposit their eggs together and apparently incubate side by side. The Guira cuckoo (Guira guira), of Brazil, Para guay and Argentina, lays extraordinary eggs, blue with a raised network of white chalky matter. The first nest of the season is so flimsy that the eggs fall to the ground and perish, but the second is more substantial.