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.