BIRD, Reproductive Habits.) Courtship is modified in various ways with reference to these facts. In general, the less the share of the male in the care of eggs and young, the greater the difference between the sexes in plumage, and the more striking the (exclusively male) courtship behaviour. This is owing to the greater need for sober colouring and inconspicu ous behaviour in the female, on whom the whole fate of the brood depends, and the lesser need in the male.
The greater the degree of polygamy, the greater in general the intensity of courtship and the exaggeration of display characters. This is due to the fact that with polygamy the successful males transmit their char acters to many offsprings, while the less successful do not transmit their characters at all. Thus the premium on success in mating is greater.
Per contra, where the male plays a part closely similar to that of the female in incubation and care of the young, both sexes tend to be similar in plumage, and often both develop display-charac ters (crest of crested grebe, plumes of herons), and for the ex clusively male courtship to give place to what may be called mutual courtship, in which both sexes take part.
Mutual courtship is often prolonged until the young are no longer under parental care (divers, egrets, grebes, albatrosses). In such cases it is probable that courtship, in addition to its function in raising the level of sexual emotion, has a secondary function as an emotional bond which helps to keep the pair to gether for the sake of the young.
Then, the greater the need for protection, the less will be the development of bright colours and display-characters. In ex treme cases, as in European warblers, the sexes are practically identical in coloration, although the male alone displays in court ship.
In solitary birds with concealed nests, it is rare for courtship to take place near the nest, for fear of revealing its site. When this danger is not of importance, courtship may occur at the nest (e.g., in the rook). It is especially noticeable in gregarious birds with mutual courtship (e.g., pelicans, herons) when a joint cere mony is often performed when one bird relieves its mate on the nest.
Finally, where food-territory exists, many sexual activities are related with the territorial system, and not, as was originally thought, with true courtship. For instance, the song of all our common singing birds is mainly an advertisement to females of the presence of a male in possession of territory, and a warning notice to other males to keep out.
There is often a recrudescence of courtship in a mild form in early autumn when the birds are released from family cares and are still warm and well fed. In some gregarious forms this ap pears to blend with the remarkable social performances of this season (e.g., stone-curlew). In the oyster-catcher, the same per formance is employed both in courtship and to demonstrate sex ual or territorial jealousy. When these two motives reinforce each other, this performance may be joined in by a number of birds and assumes in a rudimentary way a social character, as of a human dance.
In courtship, grebes and divers present weeds to their mates, penguins stones, herons sticks, warblers twigs or leaves. In all such cases the objects presented constitute nesting material; it appears that some association occurs between the two pleasur able breeding activities of nest-building and courtship.
Occasionally non-sexual actions are incorporated in courtship. Grebes often give a useless (one might say "ritual") imitation of preening their wings during courtship, and swans behave some what similarly. The psychological explanation of this is not easy.
An interesting psychological transference has been noted in the Adelie penguin. The males may present nesting material (stones) not only, as is normal, to the females as part of courtship, but to other organisms which interest them, such as explorers and dogs.
Another type of transference has been observed in a captive male Argus pheasant. The courtship of this species is given to a stationary female. The female with which this male was confined belonged to another species, and would not stand still. The male, after repeated attempts to display before the female, gave up and proceeded to display before his water-trough! Almost as a matter of course, the mode of life is reflected in courtship. As part of courtship, the males of the fast-flying fal cons fly straight at their mates; the golden-eye drake raises a jet of water with his feet; grebes, divers, and various diving ducks use their diving powers to appear from below the surface in strik ing display-poses close to their mates and so forth.
The details of courtship vary enormously from species to species ; a number of special types are figured in the plates. They all have in common two facts. First, the display presents the bird in an unusual aspect ; and presumably stimulates by its very un usualness. Secondly, when bright colours or special plumage exists only in one sex, or only during the breeding season, these are in the great majority of cases made specially conspicuous in courtship. Often such characteristics are normally concealed, but made visible only by display.
A great deal still remains to be discovered about courtship, and amateur naturalists can render considerable service to biology by recording the results of intensive observation of the whole courtship period even of quite common species. (See SEXUAL