The Development of Social Nesting Habits in CuckoosSource: The Scientific Monthly, Vol. 61, No. 1 (Jul., 1945), pp. 72-74Published by: American Association for the Advancement of ScienceStable URL: http://www.jstor.org/stable/18538 .

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72 THE SCIENTIFIC MONTHLY

the soil. In Swiss chard, oxalic acid produc- tion at all fertility levels was in excess of the combined chemically equivalent quantities of calcium and magnesium. The surplus of oxalate was progressively of greater magni- tude as the soil nitrogen supply was in- creased and calcium decreased.

The influence of soil fertility on mineral and vitamin contents and thus its more in- direct effect on the production of plants re- sistant to or unsuitable as food for insect pests was recently noted in a series of strik- ing observations of selective injury on New Zealand spinach by the common greenhouse thrips (Heliothrips haemorrhoidalis). With 320 plants in the experiments, the insects, though free to choose whatever plants they wished, invariably selected as food those low in nitrogen. It was also of significance that when the soil's calcium supply was increased the insect attacks on the low nitrogen groups were less serious.

Many insects have, without question, defi- nite attributes for food selection. This dif- ferentiation has been observed by us among plants of a given variety grown on soils dif- fering in fertility. Similar discriminations are equally obvious between botanical groups. The goosefoot family and New Zealand spin- ach, objectional to man as food sources for calcium, are relatively free from insect at- tacks, while, in contrast, members of the mustard family are commonly infested with numerous parasites. These lower forms of animal life, unbiased by modern dietary propaganda, seemingly demonstrate more judgment in their choice of food than does man in his adherence to the directive of the two green leafy vegetables daily, regardless of kind or nutritional quality.-S. H. WITT- WER, Missouri Agricultural Experiment Sta- tion, Columbia, Missouri.

THE DEVELOPMENT OF SOCIAL NEST- ING HABITS IN CUCKOOS

CUCKOOs have long been famous for an- nouneing the hours of the day, but they have another accomplishment not so commonly known-that of laying eggs in the nests of other birds. Of approximately 200 species in the family about 80 are known to parasi- tize the nests of other birds. Some species

appear to be just starting on the road to such parasitism, and lay eggs only rarely in nests of other birds. Other species have developed the habit further to the extent of never build- ing their own nests but always laying their eggs in the nests of other birds. A few spe- cies have advanced to the point of laying their eggs in the nests of only one other kind of bird.

One small group of cuckoos, instead of developing the habit of social parasitism, has become communistic. The birds associate in flocks of about a dozen individuals, spending the days together and sleeping in the same tree. At the breeding season the members of the flock cooperate in the building of a single nest in which several females may lay and incubate eggs. The males assist in the con- struction of the nest and in the care of the young. After the breeding season the young stay with the flock for a variable length of time, may help in the care of the next brood, and may even breed with the colony the next season.

These communistic breeding habits have developed in a small group of four related kinds of cuckoos called anis living in Central and South America. The most primitive of these communistic birds, Guira guira, live in loosely-knit flocks. The birds form pairs which may separate from the flock to nest or may join with other pairs. The development of communistic habits continues in the bird Crotophaga major. These birds associate in small groups and several pairs join in build- ing one nest. The climax is reached in the two species, Crotophaga ant and C. sulci- rostris. These two birds have somewhat dif- ferent ranges but very similar nesting habits. The marital relations are very flexible. Fre- quently one bird has several mates. In some cases it is certain that a female consorts with several males and in other cases one male mates with several females. All birds assist in building one nest and in caring for the young. All the parents bring in food and give it to the young. Some females which had not laid eggs carried caterpillars to the nestlings. The development of this social nesting is probably the result of several influences in the history of the group.

One important factor in the evolution of

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SCIENCE ON THE MARCH 73

communistic nesting is the modification of territorialism. The concept of territorialism has developed from attempts to interpret various types of fighting behavior in animals. Briefly stated, the idea of territorialism im- plies the defense of a chosen area, usually the nest site and surroundilng land, against members of the same species. For example, many birds defend a plot of land by fighting and singing to threaten possible invaders and drive out interlopers at once. The nest is built in the area and there the young are reared. In all the species of the communis- tic cuckoos the colony owns a territory which consists of two distinct sections. One part is a clump of trees for sleeping and the other is an area of fairly open land for feeding. From the communal territory, in general, other members of the species are expelled.

Intruders are threatened and driven about the territory in furious "dog-fights." The birds use a special call to indicate that an aggressor has entered the territory and then the whole flock attacks and chases the inter- loper, sometimes for as long as 2 days. In some cases the stranger eventually is ac- cepted into the flock. If a stuffed bird is fastened in a tree the owners of the land at- tack and soon demolish the dummy. An amusing experiment is to put a mirror in a conspicuous place and watch the birds attack their own image and attempt to drive the " stranger" away.

Among the kinds of communistic cuckoos, however, certain differences in the mainte- nance of territorialism exist. Guira guira defends its territory only slightly. Numer- ous intrusions are permitted. The impor- tant point is that within this colonial terri- tory one or two pairs may have their own small territories. However, the defense of this small territory is not vigorous and in many cases other individuals of the colony use the nest, thus producing a communal nest. At the next stage of development, as exemplified by Crotophaga major, the birds remain in pairs and all cooperate to build one nest in which several females lay eggs. The flock defends the territory. In the last stage of evolution, shown by Crotophaga ani, each colony defends its territory most ag-

gressively and without exception attacks strangers. The greatest innovation at this climax of development is the disappearance of the habit of pairing. Copulation is quite promiscuous; a female has been observed on several occasions to consort with various males and a male may accompany any fe- male. In these three stages the phylogenetic development of social nesting coincides with the disappearance of territorial defense by the pair and the appearance of territorial defense by the colony.

Several factors probably have contributed to the breakdown of territorialism for the pair. The first is that sexual fighting (fight- ing in reference to the sex-partner) is absent or extremely weak in the whole group. The situation may have permitted freer relations between the various members of the colony and a loosening of the sexual bond. A sec- ond factor which contributed to the' break- down of territorialism is the lack of a song. For most territorial birds song is one of the most effective methods of maintaining a ter- ritory and serves to indicate the boundaries of the plot of land and to threaten intruders. None of the communistic cuckoos has any note with the characteristics of song.

The weakening and eventual disappear- ance of the habit of active defense of a terri- tory by a pair probably contributed to the development of social nesting in these cuc- koos. Another influence of considerable im- portance may be the method of stimulation to lay eggs. Birds, like mammals, may be divided according to spontaneous or non- spontaneous manner of ovulation. In mam- mals the term spontaneous refers to species such as the rat which ovulate at regular in- tervals without external stimulation. Other species (non-spontaneous) such as the rabbit under natural conditions ovulate only after the stimulus of copulation. In birds the simple stimulation of coition has been super- ceded by the more intricate courtship per- formance. In many cases ovulation can be produced by a very slight external stimulus, such for example, as stroking the neck of female pigeons. There is considerable evi- dence that the communistic cuckoos belong to spontaneous type which requires no ex- ternal stimulus to lay an egg. The birds

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74 THE SCIENTIFIC MONTHLY

frequently drop eggs on the ground even far away from the nests and captive birds have dropped eggs promiscuously. The kinds of birds which ovulate spontaneously can dis- pense with courtship performance to stimu- late egg-laying and since the function of courtship behavior is primarily, at least, the stimulation of ovulation, spontaneous ovula- tion and the lack of courtship are compatible. As a further correlative of spontaneous ovu- lation and the lack of courtship may be men- tioned the lack of pair formation. Court- ship, especially the mutual performances carried on during and after laying eggs, serves to bind the mates together and to pre- vent the dissolution of the pair. Thus in the communistic cuckoos the spontaneous method of ovulation made possible the disap- pearance of bonds between the members of the pair and the lack of bonds seems to have permitted communal nesting.

The development of social nesting in these cuckoos has depended largely upon influ- ences closely connected with behavior and with the physiology of reproduction. The type of habitat is a factor of an entirely dif- ferent kind which perhaps encouraged the evolution of communistic nesting habits but the importance of the habitat on the develop- ment of social nesting habits is difficult to assess. Guira inhabits areas of open park- land savanna and sleeps and nests in clumps of dense trees. Since there are few groups of trees in such vegetation, the birds tend to come together in flocks. The original habitat of Guira was probably the Chaco and the Campos of Brazil. These areas are char- acterized by open stretches of grass or marsh with scattered clumps of trees. The birds are able to feed out in the open but are forced to come together to sleep and nest. Although these islands of trees are sometimes very large, nevertheless there is the tendency

for the birds to gather together. The mem- bers of the genus Crotophaga have retained the habit of feeding in open areas and sleep- ing and nesting in clumps of trees. These considerations suggest that the type of habitat in which the species originated forced the birds to come together in groups and to divide the territory into a nesting and a feed- ing habitat. The formation of flocks was conductive to the development of commu- nistic nesting.

This group of birds has developed a unique type of nesting as shown by three develop- mental stages. The evolution of any ana- tomical or behavioristic character procedes in response to a multitude of conditions, none of which can be cited as the entire cause of the development. In the case of the phy- logeny of the habit of nesting communally several factors may be considered as per- mitting the peculiar development. Perhaps the most important aspects are the break- down of maintenance of territory, the habit of spontaneous ovulation, and the type of vegetation. The evolution appears to be guided by psychologic, physiologic, and ecologic influences.

A certain parallelism between the com- munistic cuckoos and humans may be noted in passing. One of the noteworthy differ- ences between man and other mammals is the fact that human offspring remain with the parents for a very long time. Many of our social customs depend upon this intimate relationship. The communistic cuckoos also remain with the parents for a long time, in many cases for another breeding season. Indeed, the parents, the grandparents, and the young of another brood may join in feeding young birds in the nest. These birds have certainly developed social habits similar to ours and perhaps have even gone beyond. -DAViD E. DAVIS.

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