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Two classes of individuals-signalers and recipients -xist in all animal communication systems (Wilson 1975, Smith 1977). Signals make information available and recipients use this information when "choosing" a response (e.g., Leger and Owings 1978; Seyfarth et al. 1980a, b). Contextual information, which exists outside signals, is also usually available to recipients and may be appraised by them when selecting a response (Smith 1965, 1977). Consequently, to fully understand the responses of recipients we must identify contextual information in addition to signal-conveyed information.
Vertebrate "alarm" signals (signals emitted in the presence of potential predators) may be multi-functional and appear to vary interspecifically in information content. Some, such as the "hawk alarm" calls of small passerines, may only inform others that a raptor has been detected, but not inform recipients of the hawk's or the caller's locations (Charnov and Krebs 1975). In contrast, the alarm calls of other species, including vervet monkeys (Cercopithecus aethiops) and California ground squirrels (Spermophilus beecheyi), provide extensive information about the type of predator, the location of the caller and even the callers' age and sex (Seyfarth et al. 1980a, b; Leger et al. 1980; Owings and Leger 1980). Regardless of the type and amount of information contained in alarm signals, it would seem beneficial for recipients to appraise as much contextual information as possible, because of the serious danger posed by predators.
An important form of contextual information for alarm call recipients is the individual's vulnerability at the time the signal is detected. In fact, recipients of alarm calls do behave differently when vulnerable than when relatively safe (e.g., beaver [Castor canadensis] [Hodgdon and Larson 1973], vervet monkeys [Seyfarth et al. 1980a, b], California ground squirrels [Leger et al. 1979], hoary marmots [Marmota caligata] [Noyes and Holmes 1979]).
Shorebirds feeding on mudflats adjacent to marshy areas are exposed to attacks by raptors which may use marsh vegetation for concealment during their approach (Rudebeck 1950, 1951; Hunt et al. 1975; Page and Whitacre 1975; Dekker 1980). When they detect predators, shorebirds utter loud calls that usually elicit immediate flock formation and synchronous, erratic flight (Owens and Goss-Custard 1976). "False alarms," i.e., calls occurring in the apparent absence of predators, are also fairly common in shorebirds (Leger, pers. obs.). If most attacks by raptors come from the direction of the marsh vegetation, and if the vision of shorebirds is partly occluded by the vegetation, we would expect that individuals closest to the vegetation would be most vulnerable to attack. Thus, individuals hearing alarms while near the marsh should take immediate anti-predator action. Shorebirds farther from the marsh might be able to look around for the predator, and, not finding one, resume foraging. This study was conducted to determine whether shorebirds vary their responses to alarm calls as a function of their distance from the predator-concealing marsh.