Functional and Adaptive Significance of Mobbing and Alarm Calls of the Common Crow (Corvus brachyrhynchos)
Functional and Adaptive Significance of Mobbing and Alarm Calls of the Common Crow (Corvus brachyrhynchos)
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Date
1983
Authors
Brown, Eleanor D.
Advisor
Schleidt, Wolfgang M.
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Abstract
This study is an analysis of the functional and adaptive
significance of "caw" calls used by common crows (Corvus
brachyrhynchos) in contexts involving danger. Seven tame
birds, including three siblings and several birds familiar
to the siblings (i.e., within their sight and hearing), were
the subjects of playback experiments. Four types of caws
associated with danger were played back as test calls:
screams, mixed caws, inflected alarm caws, and alert caws.
These four test caws were recorded from each of six
different individuals (senders) and played back to the
experimental subjects (receivers). The vocalizations of the
three sibling receivers were tape recorded during playback
trials, and three types of response caws were scored: mixed
caws, alert caws, and long caws. By counting caws in each
10 s interval during the 1 min before, 20 s during, and 20 s
after playback, the following functional interrelationships
among cawtypes were found. Mixed caw responses were
elicited immediately by, and only by, screams and mixed
caws. These caws are used in harassing a predator, and seem
to function in part to assemble and coordinate a mobbing
group. Alert caw responses were suppressed over the 20 s
during which caws were played back for all types of
presentations except those of alert caws, but reached a peak
just after playback. Alert caws seem to be multifunctional,
probably indicating either mildly threatening objects or
cessation of danger. Long caw responses were suppressed
during the 20 s presentations of all cawtypes, but reached a
peak after scream and mixed caw playbacks. Long caws seem
to indicate either cessation of danger or continuation of
normal activities. By further subdividing numbers of
responses on the basis of social familiarity between senders
and receivers, the effect of social relationship on
responses was examined. The senders were either siblings
of, familiar to (heard and seen only from a distance), or
unknown to the receivers. The receivers did not respond
more to the voices of senders from any particular social
category (G goodness-of-fit tests) although the data were
not conclusive.
The results are discussed in terms of the information
encoded in mobbing and alarm calls, the functions of those
calls, and the sources of natural selection which may have
shaped the evolution of mobbing and alarm vocalizations.