Shorebird Communication

photograph of a Spotted Sandpiper by Rohan Kamath
Communication in shorebirds, as in other birds, can be very confusing to the observer. Their displays are extremely varied and often complex, and, above all, the meaning of both displays and calls appears to depend upon the ecological and social contexts in which they are given. During winter, shorebirds are often found in flocks in estuarine mud flats and other open habitats. When breeding they usually nest on the ground, most often in the open. Thus highly visible, they are especially suitable subjects for studies of avian visual communication. Indeed, plumage color and pattern are perhaps the simplest bases of shorebird communication, and in the breeding season usually suffice to signal a bird's gender.

While their acoustic communication is not as well studied as that of passerines,
shorebirds also signal extensively with sound. Shorebird vocalizations tend to match the context of their open habitats. Loud, low-frequency, repeated sounds are used during aerial displays, as territorial signals and to attract mates. Oystercatchers and others, for example, have evolved loud, piercing calls that carry over the crashing surf of rocky coasts. Such sounds will remain recognizable over greater distances than more complex, high-frequency songs -- they are less attenuated by distance and their repetitiousness helps to differentiate them from background noise. Exchanges between parents and offspring at the nest and other short-distance acoustic communication are not, of course, subject to the same constraints.

The sounds produced by shorebirds vary greatly, both from individual to individual and in the same individual in different situations. For instance, many species of calidridine sandpipers (members of the tribe Calidridini, which includes our smallest sandpipers) produce two distinct types of call: a trill lasting almost a second, and a much shortened, frequency modulated call. Edward H. Miller, who has studied acoustic communication in shorebirds extensively, reports that one male Least Sandpiper rarely trilled, while another male almost never gave the short call. Furthermore, the mix of trills and short calls uttered by a single sandpiper would change as the individual changed its direction of flight toward or away from an intruder.

Shorebirds also communicate with a wide range of visual displays, some aerial, some on the ground. Aerial displays are especially suited to communication in the open habitats, since the display can be viewed without obstruction over relatively long distances (woodland birds make much less use of aerial displays). The aerial display of an old-world shorebird, the Northern Lapwing (found casually in our area), has been studied in great detail. The male Lapwing starts a typical display sequence with "butterfly flight" (slow, deep wingbeats), followed by a zigzag flight in which the body is rotated from side to side around its long axis. During that phase, the bird produces a humming sound with its specially modified outer primary feathers. Then the Lapwing flies low and soundlessly with slow, shallow wingbeats, finishing that phase with a steep climb. At the end of the climb there is a period of straight flight during which two distinct "motifs" of its song are sung, and a third started. The third motif is finished during a steep bank and vertical dive. The bird may then repeat some of the earlier display components.

Other shorebirds have similar components in their aerial displays. As examples, oystercatchers may perform butterfly flights, Solitary Sandpipers do low display flights, and Common Snipe include dives in their high display flights. Aerial displays are very diverse, with variation in height, direction (straight, circling, undulating, etc.), wingbeat amplitude (shallow, deep), wingbeat frequency (rapid to glide), patterns of plumage display (especially tails or wings spread or flashed), calling during flight, and post-landing behavior (deliberately exaggerated wing-folding, strutting). Descent from flight displays, with the frequent production of nonvocal sounds, is thought to convey a great deal of information about the signaler's change of behavior.

As previously mentioned, the problem with interpreting shorebird aerial displays is that they are used in a wide variety of situations. For example, the Lapwing display may occur in response to predators, when a male returns to its territory, in response to other males, in response to females, and "spontaneously" (when there is no obvious triggering stimulus). Furthermore, the precise form of the display varies with the stage of the breeding cycle, the time of day, the weather, and the audience.

Context seems to play a similar role in giving meaning to the ground displays of shorebirds. For instance, five different displays used by Black-tailed Godwits in aggressive encounters have been studied in detail. They include two "upright" displays in which the legs are stretched, one with the back plumage smooth or slightly ruffled, the other with it ruffled. In each the wings, tail, and bill are in different positions. The nonupright displays are "forward" (body almost horizontal, legs not stretched, neck extended, plumage very ruffled, bill usually down); "crouch" (body horizontal, legs deeply bent, neck withdrawn, plumage ruffled or smoothed, bill in various positions); and "tilt" (body slanted forward with breast near ground, legs deeply bent, neck withdrawn, plumage smooth, bill forward).

Analysis of these five displays and their variations revealed no simple association with attack or retreat behavior. Neither was any component of the displays (e.g., ruffling of plumage, lowering of bill) clearly a sign of attack or retreat. Again it appears that details of the context in which the displays occur (rather than the attack-retreat dichotomy) are required for their interpretation. Indeed, attacking Black-tailed Godwits within six feet of their opponents showed a different array of displays from those of godwits separated by a dozen feet or more.

Often the same general elements are used in both aggressive and sexual displays, so that their interpretation depends entirely on the audience or the stage of the breeding cycle. Male Least Sandpipers use a forward tilt, elevated tail, and wing-up display when approaching other individuals of either sex. Courtship displays are differentiated from antagonistic ones only by being more slowly paced and more stereotyped (showing less variation). Male Killdeer may do a "forward-tipped, neck-extended" display during which the legs are often kicked backward (scraping), when defending their territories against other males, then advertising for mates, during nest building, and as a precopulatory display.

Certain kinds of display show constancy of context not only within but also between species. For example, Semipalmated, Mountain, and Little Ringed Plovers (the latter a European species similar to the Semipalmated), among many others, all show a similar threat posture in which the body is kept horizontal, the flank feathers are spread over the closed wings, and the head withdrawn, giving the individual a flattened appearance.

To make sense of communication in shorebirds, careful notes should be made of the detailed form of the communication itself as well as of the context in which it occurs. Keep records of the physical environment (season, time of day, weather, illumination, background noise, etc.), the state of the communicating bird itself (sex, maturity, breeding condition, activity before, during, and after communicating -- feeding, flying, nest building, resting, etc.), and the identity of birds of the same species, other bird species, or other animals to whom the communication might be directed (for instance, conspecifics of the same or opposite sex, individuals of closely related species that might compete for resources, potential predators). Pay particular attention to subtle movements, such as bobbing of the head, a slight change in the angle at which the body is held, or an extension of the wings before they are folded after landing. These may seem insignificant, but they may carry important messages for other birds.
SEE: Visual Displays; Duck Displays.
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.