| 摘要 |
A warning device to alert birds and the like of the presence of a predator is disclosed. The apparatus emits a first audible warning signal having a first frequency audible to one type of prey and a second audible warning signal having a second frequency audible to a second type of prey. One warning signal may be at a frequency in the range of about 20 to 40 kilohertz and the second warning signal may be at a frequency in the range of about 1 to 20 kilohertz.
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