In England, spring begins curiously: buyers of the London Times eagerly turn to the letters page, hoping that nature’s confirmation of springtime – the cuckoo’s cock-coo, cock-coo call – has been duly reported. The irony is that many of these keen correspondents and buyers will never have laid eyes on a cuckoo, having detected its presence only by the call. The European cuckoo thus has the distinction to add toot notorious parasitism that its annual cycle impact on human habits in this peculiar way.
The cuckoos are a very diverse family: the sturdy roadrunner of the arid North. American deserts bear precious little resemblance tithe delicate Klaus’s cuckoo of the African bush. Details of internal anatomy, as well as the possession of a foot having two toes pointing forwards and two back (zygodactyls), distinguish cuckoos from the super-facially similar songbirds and relate them tithe parrots and nightjars. This unusual foot structure gives the cuckoos the ability to climb stealthily among slender reed stems or run swiftly over the ground with almost equal poise.
Many species are reminiscent of small hawks, having a distinctly down-curved bill and long tail, and share with these birds the discomfort of “mobbing” attacks by small songbirds. The reason for these unwelcome attentions is, of course, that many cuckoos reproductively parasitize the smaller birds.
About 45 species have no other habit of reproduction than to place their eggs in the nests of another species of bird. The European cuckoo female defends a territory within which she keeps a close eye on the comings and goings of the resident songbirds. She is concerned only with one of her resident species, for her eggs are characteristic in color and will closely match the eggs of only one potential host species. Some of these territorial females also allow a second, subordinate female to use the territory, but perhaps only if these are dependent upon an entirely different host species, thereby avoiding competition for host nests.
When a suitable nest becomes available, usually one in which laying has just begun, the cuckoo flies warily down, takes one host egg in its bill, and quickly deposits a single egg in the nest. The stolen host egg, and the mimicry of the egg color, ensure that the clutch appears untouched when the rightful owner returns. This delicate operation completed, the cuckoo eats the stolen egg as a reward for its stealth!
The cuckoo then begins to climb the inside of the nest, using surprisingly powerful legs until it reaches the rim and ejects its little load into oblivion. Within a short time, it begins again, repeating the task until all other objects have been ejected from the thinnest. What the cuckoo has done, of course. This is to eliminate any possible competition and ensure that its innocent foster parents con-cent rate their reproductive efforts on one thing the raising of the voracious cuckoo!
This pattern is not invariant among the cuckoos. Many species, for instance, the Great spotted cuckoo and the keel, do not show ejection behavior and share the nest with offspring of the host species, in these species usually crows. However, the aim is the same and only the methods are different. The rapidly growing and more active cuckoo nestling either tramples the crow chicks to death underfoot or, more subtly. Monopolizes the food brought to the nest bits foster parents.
In an evolutionary sense, surprisingly, cuckoo nestlings succeed in obtaining parental care from foster parents when selection against this behavior must be very strong indeed (see pp236-237). Mimicry of egg color and size may increase the chances of host acceptance, and appear to be very finely tuned to the resident host population. For example, the Brown babbler is a central African host species that lays clear blue eggs over most of its range, but in one part of northern Nigeria lays pink or mauve eggs.
Incredibly, its cuckoo parasite has evolved to faithfully mimic these changes in color action. The accuracy of this kind of local mimicry is presumably dependent upon the high degree of breeding site tenacity in both migratory and no migratory cuckoos: female which tried to breed other than the closet where she was hatched might find her eggs a discordant mismatch to those of the resident hosts! Even after hatching, the cuckoo nestling must continue the deception to be able ‘to contain food from its foster parents.
The Great spotted cuckoo produces a passable imitation of the begging calls of nestling magpies and its wide open gape is even more vivid than those of the host nestlings. The motivation to respond to these stimuli is so strong in small songbirds that, once it has left the nest, the fledgling European cuckoo may be fed by passing birds that are neither true nor foster parents! This huge adoptee could be quite a hazard to foster parents or others who attempt to feed it.
A cuckoo’s bill is powerful compared with that of a unlock or Reed warbler host and capable of causing injury if the nestling is too eager to seize offered food. For this reason, the acceptance of food in the cuckoos is quite unlike that in other nestlings which close the bill rapidly over that of the parents the food is transferred. The parasite, indifferent to its tiny guardians, keeps its huge gap open until well after the parents have safely departed!
The remarkable behavior of such cuckoos tends to obscure the fact that about two-thirds of the species appear to be nonparasitic in breeding habits, mating monogamously and remaining together while the offspring are reared. The Pheasant council is one such species—pro-dicing one or two grotesque black nestlings which, in common with many cuckoos, excrete a foul liquid when they suspect the presence of a predator. However, our knowledge of most species is extremely poor, and some of these may turn out to be parasitic at least in certain circumstances.
One of the most remarkable things about the cuckoos is that even the nonparasitic members turn out to be unusual in other ways, and the roadrunner is just such a bird. This “outlier” of the cuckoo family, living in the desert chaparral of southwestern North America, was once heavily persecuted in the belief that it was harmful to populations of game birds. It eats an
Rather more firmly established is the fact that this species displays a physiology that is most unusual among birds it is to some extent cold-blooded! When air temperatures become very low, as in deserts during the night.
Most birds need to increase their metabolism to maintain their internal body temperature at a constant, high level. This of course means burning internal food reserves at an increasing rate. The roadrunner takes a more economical course it simply allows its body temperature to fall slightly, turning down the “central heating” with no ill effects, and saving energy costs. The bird does go into a slight torpor and may not be able to respond as quickly to sod-den danger, but for a bird that has few predators, this slight disadvantage may be relatively unimportant.
When the first rays of light break the cool of the desert night, the roadrunner displays a neat trick for warming up. Areas of skin on the back, just between the wings, are darkly pigmented and absorb the energy of sunlight, warming the skin and underlying blood vessels. To hasten this process, the bird fluffs the feathers covering the patches so that the light penetrates more effectively, and with this mechanism, the bird can save up to about 5o percent of the energy it would otherwise use to warm up to a work-in temperature.
Thankfully, and unlike many other groups of fascinating and understudied birds, the cuckoos do not seem at great risk from man’s activities. Many species are characteristic of scrub, secondary forest, and other types of disturbed ground which, if anything, increase under human interference.
For all its weird and wonderful animal species Africa contains only one large family of birds restricted to that continent—the traces. This group is so poorly known that for years it labored under the name of “Plantain-eaters,” until reports filtered through that they rarely or never ate plant taints or bananas. Subsistence through the eons of time would, in any case, have been difficult for plantain-eaters because it was Washman, comparatively recently, who introduced these fruits to Africa!
The traces are, to be sure, committed frugivores, taking a wide variety of fruits, including certain berries highly poisonous to man, as they forage through the dense foliage in parties of up to a dozen birds. The few reports that there are describe the nest-lings also as being fed largely on a fruit diet supplemented by the occasional invertebrate, especially snails. This is unusual among young birds, most of which are fed on a high-protein diet of invertebrates during their growth spurt between hatching and independence.
The trace lifestyle is a halfway house towards that of an even more specialized vegetarian, the hoatzin, and this fact, along with a general resemblance in form, has prompted some students to suggest a common ancestry. There are almost as many ideas about the evolutionary origins of the traces as there are species within the family. Evidence from their feather parasites suggests an affinity with the fowls (Galliformes), while eye-lens proteins appear similar to those of songbirds (Passeriformes), and the structure of the foot places them close to the cuckoos (Cuculiform).
One characteristic that the turacos seem to share with no other living birds is the possession of two vivid feather pigments—a green turacoverdin and a retracing. The latter is known to be a copper compound and colors the crimson wing flashes and head ornamentations found in most trace species. The turacoverdin has not been fully investigated, but may also be a copper compound and produces the rich green body feathers of 14 species.
Most birds’ feathers produce their colors by refracting light with specialized feather structures (iridescence) and turacoverdin is the only green pigment found in birds. The lengthy period of about a year taken by young trace to develop full adult color action may perhaps be related to the difficulty of acquiring the relatively scarce copper fourth pigment.
The common observation that turacos forage in groups might suggest that these birds, like some others in the tropics, are social breeders, organizing themselves so that individuals other than the parents contribute to the nesting chores of incubation, brooding, and feeding the hungry young. Our knowledge of their breeding habits is, however, so poor that this mode of reproduction has been confirmed in only single species. In captivity, traces will nest simple pairs, although their success rates are low, perhaps due to unknown dietary deficiencies. Monogamy seems to be the rule, and the close similarity between the sexes is typical of other monogamous bird species—the only sexual difference seems tube one of bill color.
For several weeks before egg-laying. The male regurgitates gifts of fruit pulp for his female. Once reproduction proper has begun, both birds contribute equally to the incubation, brooding, and feeding of the two or three chicks. These nestlings are covered in a fine down, of varying color and thick nests, and they advertise their hunger withal a large orange-red gape. Parents respond by regurgitating the fruit/insect mixture directly into the throat and, unlike some birds, this operation takes place in silence, perhaps because of the high density of predators in the forest habitat. Yet another affinity with the peculiar hoatzin emerges once the young have recovered from hatching and increased their strength with a few Meals.
The silky nestlings are endowed with tiny claws on their wing joints and can use these, and their adaptable foot structure, to leave the nest and sit on the periphery or even on adjoining twigs. The young leave the nest for good. At an age of about four weeks, several days before they can fly. Independence from parental feeding seems to be gained at about six weeks, although the offspring continue to beg long after this age. If social breeding does occur in turacos might even be expected that, as in other cooperative breeders, some offspring remain within the parental home range for much of the early part of their lives.
The strange unbirdlike hoatzin has defied conventional methods of classification for generations and has traditionally been aligned with the Galliformes an assemblage of “fowl-like” birds whose relationships to each other are not fully understood.
With its stout legs, coarse plumage, and weak flight, it certainly resembles a domestic hen rather more closely than an atypical cuckoo. It has long been put in its own, single-species family, and, in the past, because of its superficial resemblance to reconstructions of the extinct bird Archaeopteryx, the hoatzin was often labeled as a “living fossil.” Probably nothing could be further from the truth.
Early birds, in common with many of their reptilian ancestors, were probably insect eaters, whereas the hoatzin is one of the most refined and specialized herbage eaters in the whole bird class. Many features of its appearance and biology seem to be consequences of its highly specialized way of life, rather than any evolutionary “hang-over” from the distant past. Recent investigations of its biochemistry suggest that the hoatzin has closer affinities with the cuckoos than with any other group of birds.
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