Proceedings of the Vertebrate Pest Conference

Bird control often entails the use of sound to disperse birds from croplands or other sites. Little information is available concerning the sound levels produced by noise-making devices or the effective area of coverage. In 1998 and 1999 we measured the sound levels of American crow (Corvus brachyrhynchos) distress calls produced by 2 models of a commercially available broadcast unit. With the units set up at different heights (0.9 to 4.9 m) in an open area or in an almond orchard, we used a sound meter to measure sound levels at distances of 1 m and 15 m, and then at 15-m intervals out to a distance of 90 m from the speakers. Sound levels decreased from about 102 dB at 1 m down to or nearly to background noise levels at a distance of 90 m. Sound levels from a unit set up to broadcast at a 4.9-m height through the orchard canopy were lower than for a unit set up at a 0.9-m height broadcasting under the canopy. Sound levels from a unit set at the 0.9-m height in the open were greater than those from the 0.9-m setup under the canopy only at a distance of 90 m. We set up a broadcast unit at various roadside locations in Yolo County, California, to determine the distance over which crows heard and reacted to the broadcast calls. Upon activation of the unit, we scanned the surrounding area for any crows flying from their perches. When crows reacted to the call we recorded the distance to the site of origin with a laser rangefinder and the number of crows. We also recorded the distance and number as above for any 2nd or 3rd flocks responding. We broadcast the calls on 11 days and on 27 occasions crows responded, typically by flying up from their perches, sometimes flying overhead, and then flying away. The average distance for the 1st flocks was 142 m + 73 SD (range 22 - 275 m). On 7 occasions 2nd flocks responded from an average distance of 174 m + 110 SD (range 71 - 312 m). On all but one occasion the 2nd flocks originated from a more distant location than the associated primary responders. We observed one instance of a 3rd flock responding. Fourteen (51.8%) of the 27 primary responses were from distances >122 m, about the distance at which the sound levels from the broadcast unit drop to background levels. These data indicate crows detected sounds from the broadcast units at distances greater than suggested by the sound meter. Using the average distance of 142 m for the 1st flocks responding, we calculated crows in a 6.3 ha open area could hear the broadcast calls. Using the maximum distances we observed of 275 m for 1st flocks and 312 m for 2nd flocks, we suggest that under some conditions crows within open areas of nearly 24.3 ha and 30.4 ha, respectively, heard the calls or responded to other crows hearing the calls.