Our unusual facet of social behaviour which is found only amongst birds is their habit of depositing their eggs in the nests of other species. After hatching the nestlings are raised by the foster parents often at the cost of their own young.
In Europe, the most typical example of the social parasite is the cuckoo. The main distribution area of the cuckoo is the tropics and not all cuckoos behave like parasites; most build their own nests and incubate the young themselves. Nevertheless, cuckoos have acquired a reputation as intruders into the homes of other birds, even though there are other parasites amongst exotic species.
In all probability, this habit originated as the result of the indiscriminate laying of eggs by two different hens in one and the same nest. Over a period of time, it has evolved into the parasitism of whole populations and, finally, of entire species. In the case of many birds two or more females of the same species lay their eggs in one nest. The eggs are then incubated and the young reared by just one pair of adults. This, however, is not true social parasitism.
Let us take a closer look at the common cuckoo (Cuculuscanorus), the best known of the social parasites. It is found throughout Europe and practically the whole of Asia, up to the tree line at the borders of the taiga, as well as in northwest Africa and in some places south of the Sahara. European cuckoos and those of the species from northwest Africa winter south of the Sahara, Asian populations winter in India and southeast Asia, including the islands of Indonesia. Cuckoos living in Africa, south of the Sahara, are resident.
European cuckoos fly south very early, many leaving their breeding grounds in the second half of July. The main mass flight, however, takes place in August. The older birds al first to leave and young cuckoos depart much later; they are
often seen in central Europe even in late September. With the onset of March, the cuckoos begin the return trip to their northern breeding grounds, usually arriving in central Europe at the end of April or the beginning of May. First to appear at the nesting territory are the males, followed a few days later by the females. The male generally returns to the same place as in previous years, and one bird was observed to do so for thirty-two years.
Females also return as a rule to their old nesting territory and on their arrival, the male attracts the attention of the female with his cries and display, which includes spreading his tail feathers, drooping his wings, ruffing his body feathers and stretching his neck forward while he utters his characteristic cuckoo call. This is usually done early in the morning or towards evening.
The size of the territory depends on the number of songbirds nesting there, for the female cuckoo usually deposits her eggs in their nests. As a rule, the territory embraces an area of 1—1,1/2 kilometres in diameter. Though a regular denizen of the woods, when it comes to selecting a nesting site the cuckoo shows no particular preference for vegetation. The cuckoo may be found wherever there is an abundance of nests and this, apparently, is its main consideration during the breeding season.
As a rule, the female cuckoo lays her eggs in the nests of the species that she herself was reared by. The mere sight of
a partially built nest arouses the laying instinct of the hen, but if there are few such nests in the territory, this instinct is suppressed. The female flies around looking for a suitable nest, preferably one which contains a clutch of eggs. As soon as she finds one, she waits until the adult birds depart, then quickly lays her egg there, removing one or sometimes several of the foster parent’s eggs, either by casting them to the ground or by swallowing them.
The cuckoo generally lays from fifteen to twenty eggs in one breeding season. It has been found that cuckoos deposit their eggs in the nests of 162 different species Of birds, though usually the choice is limited to some twenty species of small songsters. Cuckoo eggs are very small but have a much thicker shell than those of other birds. This is to prevent them from breaking when they dropped from a fair height into the nest or cavity.
Birds that have been ‘selected’ by the cuckoo as foster parents for its young often cannot suffer the cuckoo’s presence in the vicinity of their nest and try to chase it away. If the cuckoo does manage to lay an egg in their nest many birds simply throw it out, while others abandon the nest and build a new one. Warblers and redstarts simply cover the whole clutch, including the cuckoo’s egg, with a new lining and lay a fresh batch of eggs.
Not all birds behave like this; there is a great number who hatch the eggs and rear the young cuckoos. How some birds do and others do not recognize the danger posed to their species by the cuckoo’s egg is something that still remains to be explained adequately. The cuckoo nestling generally hatches sooner than the young of its foster parents since it has a shorter period of incubation than most songbirds.
The young cuckoo is completely naked at birth and hatches with closed eyes. However, some ten to sixteen hours later it instinctively feels the need to remove from the nest anything which gets in its way. The skin of the young cuckoo’s back is extremely sensitive to contact with foreign objects. It puts its head and neck under the object, spreads its feet wide and rests its head against the bottom of the nest. It then lifts the object onto its back, holding it in place with its stumpy wings, and pushes it towards the edge of the nest until it tumbles out.
The process continues until all the eggs or the young nestlings of the foster parents have been cast from the nest. This instinct remains active for three to four days, by which time the young cuckoo is the sole occupant of the nest and receives the foster parents’ full attention. It consumes enough food for a whole family of small songbirds and grows very rapidly, especially during the first two to nine days.
When it hatches the young cuckoo’s cry resembles that of most songbird nestlings, and it is not till the fifth day that it changes to the typical cry of cuckoo nestlings demanding food. Because its gullet is a bright orange colour, its open beak continually implores the foster parents to hasten with the feeding. The cuckoo’s diet consists mainly of fruits, beetles, butterflies and various caterpillars, even the hairy varieties; the hairs of such caterpillars become embedded in the walls of the bird’s stomach and are regurgitated, together with the lining which is ‘shed’ regularly.
The cuckoo nestling remains in the nest for twenty to twenty-three days, after which it climbs out and perches on a branch, though still unable to fly, and is fed by the foster parents for a further three to four weeks.
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