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When a bird moves or holds itself in a way that signals information to another bird of the same or different species, it is said to be performing a "display." Thus a pigeon leaping into flight at the sight of a hawk, only coincidentally alerting other members of its flock in the process, is moving but not displaying. In contrast, a male grouse strutting on a lek (a traditional courting ground) is displaying -- passing information about its desirability as a mate to females of its own species. Displays may include vocalizations, as they do in the case of the grouse -- and in the broad sense, vocalizations alone are displays. But because ornithologists often consider vocalizations separately, for our discussion we will define as displays only ritualized movements or postures. Displays are usually classified according to their apparent function: courtship, aggression, begging, greeting, and so on. The strutting male grouse is performing one of the myriad kinds of courtship display seen in birds. Often courtship displays accent a striking feature of the bird's plumage. The conspicuous, labored flight displays of the male Red-winged Blackbird exaggerate its red shoulder patches. The display flight of the male Yellow-headed Blackbird is performed with the body cocked upward so that its prominent yellow head is held high. On the other hand, some male birds do not advertise with physical attributes; they demonstrate skills. Male terns court females by displaying a fresh-caught fish. Courting male European Gray Herons perform ritualized hunting movements, erecting head feathers, pointing their bills downward and clashing their mandibles together. Many male passerines, when courting, also lower their bills as if pecking at something below them. Perhaps, next to singing, the most common component of courtship displays in male songbirds is vibration of the wings; other components include fluffing of the body feathers, bill raising, thrusting the head forward, and running using short steps. Birds also use a great diversity of agonistic displays (those used in threats and actual combat). Male Mourning Doves may bow repeatedly and then lift their heads and coo when defending their territories. Canada Geese often pump their heads up and down just before attacking. Male Gray Catbirds, disputing the boundary between their territories, will fluff their feathers, spread and often lower their tails, and as a last resort, raise their wings. In the same situation Eastern Kingbirds spread their tails to display their white terminal band. Similarly, Tufted Titmice assume a horizontal posture, may open their bills while slightly spreading their wings, and lunge at the intruder. Aggressive geese may rear up and spread their wings when on land; aggressive loons rear up in the water. In established dominance hierarchies, dominant birds often use threat displays against subordinates. Subordinates signal their submission with other displays -- in passerines often by crouching with feathers fluffed and head withdrawn. Quite different from these sexual and agonistic displays are begging displays, which are employed both by chicks to solicit parental feeding and by some females to solicit courtship feeding. Greeting displays are used when one parent relieves the other at the nest, and may serve to prevent aggressive interactions. Pairs of adult Adelie Penguins do bowing displays and exchange vocal greetings at "changings of the guard," thus making large colonies extremely noisy. And, finally, there are social displays that apparently help to keep flocks unified. Displays are, in fact, a major part of the "glue" that binds avian societies together.
How this glue evolved remains a matter of conjecture and dispute. It was the courtship display of the Great Crested Grebe (which is much like that of the Western Grebe illustrated above) that led the pioneering British behaviorist and evolutionist Julian Huxley to develop the concept of "ritualization" -- the gradual evolutionary transformation of an everyday movement into an increasingly effective signal. For instance, preflight movements -- crouching, slight spreading of the wings, and raising of the tail -- have been modified in many birds into signals. When wooing a female, the male Great Cormorant begins an exaggerated takeoff leap, but does not leave the ground. One of the most interesting ideas on the origins of display behavior was developed in the middle of this century by the celebrated ethologist (student of behavior in natural environments) Niko Tinbergen and his colleagues. Tinbergen suggested that internal conflicts between different behavioral systems or tendencies -- such as the desire to threaten and the urge to court -- are responsible for the generation of displays. For instance, after vigorously defending its territory, a male bird may approach a female with ambivalence over whether to attack or woo her. As a result, he may do neither, but instead channel his energies into behavior that is irrelevant to either aggression or mating -- say pulling at a tuft of grass or preening. Such "displacement activities" can then become incorporated into courtship displays, and "emancipated" from their previous functions. Thus when displacement preening becomes part of courtship, it also becomes more conspicuous and stereotyped than normal preening, and becomes useful as a signal, not as an aid in maintaining the bird's feathers. Similarly, males of many species have incorporated components of courtship fighting (battles between rivals during the courtship season) into their courtship displays. For example, among passerines, bill raising is used widely, in some species in fighting, in others in courtship, and in a few species in both. In many finches the preliminary male courtship display is a modified head-forward threat posture. Although some behaviorists think that virtually all displays arise from conflicts between internal tendencies, experiments have so far failed to confirm this. As a result, scientists are now turning away from Tinbergen's approach, and looking at the possible evolution of displays without reference to conflicting "underlying tendencies" in the nervous system. Evolution has modified an enormous variety of activities into displays, from food exchange, as in courtship feeding (originating as food exchange between parent and chick), to stylized fishing movements used in heron courtship. We expect the perspectives of those researchers investigating the overt behavior of the animal and those studying the functioning of its nervous system to gradually converge. That convergence, we hope, will supply a deeper understanding of displays and other behavioral phenomena. SEE: Territoriality; Distraction Displays; Dominance Hierarchies; Shorebird Communication; Duck Displays; American Coots; Redwing Coverable Badges. Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye. |
