Time of Day Effects on The Assembly Call of The American Crow (Corvus americanus)
Sherwin Jenabian
Department of Biological Sciences
Saddleback College
Mission Viejo, California 92692
Corvus americanus is found throughout the United States and its vocal behavior has interested ornithologists for many years. The crow’s superior intellect has led to the development of a complex communication system which consists of a definite set of diverse sounds, each of which occurs in a particular situation and seems to have a predictable effect upon the behavior of other crows. In this study I replicated the assembly call of the American crow and examined what effect the time of day had on the number of birds that responded to the call. I recorded the assemblycalls and played them back during the weekends on the Saddleback College campus between March 27th 2010 and April 10th 2010. The calls were played in 2 minute intervals,9 times in the morning right after sunrise on 5 separate days (n=45), and 9 times towards the end of the day right before sunset on 5 separate days (n=45). Crows responded by flying over the source of the call in both cases, but the calls that were played at dusk attracted a greater number of crows compared to that of the morning. These results determine a significant difference in the number of crows that respond to an assembly call between the morning and late afternoon, thus rejecting the null hypothesis.
Introduction
The interest in the common American crow has increased in the past decade among ornithologists due to its superior intellectual properties. Crows have been known to live in complex social groups and have been documented to have many of the same intellectual characteristics as primates. When it comes to brain/body-mass ratio, the corvid brain is among the largest in birds and almost equal to that of apes. Some speculate that this has helped them develop a very complex communication system. This system has been built around a set of different tones, and depending on the intended message being conveyed, other crows respond in the same manner. Even the casual observer of crow behavior can detect changes in pitch and rate of call delivery, an indication of the communicative potential and behavioral complexity of crow vocalizations (Johnston, 1961).
Many studies have been done onCorvus americanusand many have found that the Common Crow to be morphologically capable of producing a significant variety of notes, many of which can be used for assembly or to warn about nearby predators. Many speculate that the common crow posses their own language for communication, not implying that it has developed a system of conversation, but simply by producing a few sounds it is able to convey different emotions and warning to its own species. It has variety of meanings, pitched high it may be a call, an alarm or to attract 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 thatexternal 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 American Crow and to see if there is a difference in the number of crows that respond 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 of the behavior and activity during the time of day and weather conditions. Weather conditions were noted and it was agreed that all the trials would be 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). A small black cloth was stuffed with packing material, shook in front of a nearby crow, and 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 2 minute loop with 10 seconds between each call. The recorded file was played for 2 minutes for each trial.Different species of birds respond to these calls in different ways but gullsand corvids in particular usually approach the source of the noise and fly 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, then 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 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 and to minimize any chance of error. Calls were played 3 times in 3 different locations in the morning after sunrise and the number of birds showingpositive phonotaxis wererecorded. The 9 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 # Fh-p8000BT. Additional pioneer speakers (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 so 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 before sunset. A total of 45 trials were performed for each set, and the number of successful and unsuccessful trials is shown in Figure 1.The number 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 trials versus the afternoon (p=0.0088, Chi-squared 2x2 two-tailed contingency table).
Figure 1. Number of successful responses observed versus the time of day. Total number of trials for each set, n=45. (p=0.0088, Chi-squared 2x2 two tailed contingency test)
Discussion
The purpose of this study was to determine what factors affect the number of responses in the Americancrow, specifically time of day. The results were in accordance with previous research done on the response rate of corvids. Although very few studies have been done to test the correlation between the number of responses and the time of day, our results show the same type of variation found in previous studies. In the past, researches have had results that drastically differon a daily basis.Even though there was a significantly larger amount of responses in the afternoon than the morning, the response rates seemed to also show a variance by location. Three locations were used for research and all three were on the Saddleback College campus. All three locations were presented with the same number of calls and the same number of trials per day, but lot number 1 had ahigher number of responses compared to the other locations. Corvids tend to nest between March and May, so it is speculated that calls closer to nests would have a higher response rate. As a continuation of this study, researchers are encouraged to explore any correlation between nest proximity and response rates.
Literature Cited
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Bremond, J., Gramet, P., Brough, T. and Wright, E. 1968. A Comparison of Some
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Burns, L. 1901. Crow Language. The Wilson Bulletin 13: 5-9
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Thompson, N. S. 1982. A Comparison of Cawing in the European Carrion Crow (Corvus
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