The sound frequency that most birds hate to hear and will take off away from or stay away from is between 1,000 Hertz to 4,000 Hertz.
Although some birds may hear at higher frequencies or lower frequencies but most birds will fly away or stay away from a sound frequency of between 1,000 Hertz to 4,000 Hertz.
Those bird deterrent devices that work using high frequency sounds work at around frequencies of between the 1,000 Hertz and 4,000 Hertz range.
I have a couple of those bird deterrent high frequency sound devices that the birds can hear and it does keep birds away.
So they do work for some birds although some other types of birds may not be deterred by them but for some birds they do work.