Saddleback Journal of Biology - Saddleback College

Spring 2010 Biology 3B Paper
Studies have found C. americanus to be
morphologically capable of producing a significant
variety of notes, many of which can be used for
assembly or to warn other crows about nearby
predators. Many speculate C. americanus posses its
own language for communication, not implying that
it has developed a system of conversation, but simply
produces a few sounds by which it is able to convey
different emotions and warning to its own specie. The
pitches have a variety of meanings; a high pitch may
be a call, an alarm or to attract a display of attention
(Burns, 1901). Different qualities of caws are specific
to the context: whining caws are given by young
crows soliciting food; high pitched caws are given by
crows on seeing a hawk overhead; low pitched
growling caws are given by crows on close contact
with a predator (Thompson, 1982).
The use of recorded distress calls of corvids
have been documented in the past. The distress calls
that were made when a crow was captured by a
predator were recorded and played back over a
speaker system. Even though these studies have
always been able to replicate a response specific to
the call that was played back, there has always been a
large discrepancy in the number of crows that
respond in each trial. It has been speculated that
external factors, such as weather or the position of
the sun can play a role in the number of responses.
The main objective of this study was to
replicate the assembly call of the C. americanus and
to see if there was a difference in the number of
crows that responded to the calls in the morning vs.
dusk. It was hypothesized that there would not be a
significant difference in the number of responses.
Materials and Methods
Prior to recording the assembly calls,
observations were made on how the time of day and
weather affected the crow’s behavior and activity. All
the trials were executed on clear windless days. This
was to ensure that wind did not affect the sound
distribution. Crows have been known to project their
assembly calls when they see one of their own caught
by a predator. Lorenz reported that tame Jackdaws
(Corvus monedula) attacked his hand when he carried
a pair of black swimming trunks; he went on to
suggest that "dangling black" releases an innate
predator-attacking mechanism in these birds
(Barash,1976). In the current study, a small black
cloth was stuffed with packing material and was
shaken in front of a nearby crow. The response was
recorded using a Sony portable PCM linear recorder.
The recorded audio file was modified to
include just the desired call. The audio file was set on
a two- minute loop with 10 seconds between each
call. The recorded file was played for two- minutes
for each trial Different species of birds respond to
these calls in different ways but gulls and corvids in
particular usually approach the source of the noise
and flew overhead for some time before dispersing
and completely deserting the area (Bremond et al,
1968). If one or more crows showed positive
phonotaxis, responded to the stimulus by flying on a
direct path towards the source and continued by
flying over head then the trial was recorded as
successful. If one or more crows showed moderate
phonotaxis, responded by flying directly toward the
speaker but changed directions prior to reaching the
source, the trial was also recorded as successful. If a
response was not observed, then the trial was
recorded as unsuccessful. The calls were used on
different flocks of birds and were not played in the
same location more than once every 2 days. This was
to ensure that the crows did not get acclimated to the
calls. Calls were played three times in three different
locations during the morning after sunrise. The
number of birds showing positive phonotaxis was
recorded. The nine trials were also conducted at
sunset, at the same locations as the morning trials in
order to minimize any error.
The calls were broadcasted by the audio system
installed in a 1997 Lexus sedan. The stock stereo had
been replaced with a Pioneer head unit, model # Fhp8000BT.
Additional pioneer speaker (4x 270 watt)
and a Pioneer GM-D8400DM, 1200 watt amplifier
had been added. This was to ensure proper sound
levels and frequencies. The vehicles sun roof and
windows were left open to ensure proper sound
distribution. The procedure was to drive along the
campus of Saddleback College on weekends until a
good site for broadcast presented itself. A good site
must have had crows present, been large enough to
permit the observation of approaching birds and far
enough from houses such that the recordings would
not disturb the residents. Parking Lots 1, 5A, and 9
were selected for research.
Results
Positive results were seen in both morning
and afternoon, yet a greater number of crows
responded during the sunset trials. A total of 45 trials
were performed for each set. The number of
successful and unsuccessful trials is shown in Figure
1. The numbers of successful trials were compared to
the number of unsuccessful trials in the morning and
afternoon in a contingency table. There were 7
successful trials in the morning, versus 18 successful
trials at dusk. The Chi-squared showed a statistically
significant association between the morning and the
afternoon trial (p=0.0088, Chi-squared 2x2 two-tailed
contingency table).
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Saddleback Journal of Biology
Spring 2010