Exactly, if you’re designing a deterrent, focus on frequencies birds actually hear (1–4 kHz), like distress calls. Ultrasonic frequencies are a waste.
To drive the point home: birds hear like us, more or less—not better. Beyond 20 kHz, it’s human silence.
Interesting that some birds produce ultrasonic sounds but can’t hear them, nature really is full of quirks! Maybe that’s about confusing insects or acoustic signaling.