It is very likely a hoax indeed. The resonant frequency of an object is determined largely by its size, among other material and environmental factors. Also, resonant frequency is probably not enough to kill a chicken, even if it is very loud.
The resonant frequency of the human head is somewhere within the range of 20 to 30 hertz (please correct me on this data). Ideally, an object's resonant frequency will increase as its size decreases, and since a chicken's skull is smaller than that of a human's, the resonant frequency of a chicken's skull must be at least 100 or 200 hertz. Yet again, I would appreciate numbers from anyone who knows such an obscure detail, as I cannot do the field work myself.
Considering that the low B on a 5-string bass resonates at a frequency of 30.8677 Hertz, and all the de-tuning that goes on in modern metal, anti-nu-metal zealots may have something scientific to squawk about now.