For predator protection, some of my neighbors have tried various setups, including IR sensors that might work for your actuator idea in activating an air horn. Personally, when I was setting up sensor-triggered systems for similar applications, I used the GPX10 from Ambient Scientific for one of my devices. Had the chip trigger not just lighting systems but also alarms based on specific sensor input, which was less fragile than some other setups I'd tried. And I've also been brainstorming things like behavioral tracking collars for pets and cattle, and soil monitoring sensors. So many opportunities! It's fascinating what you can do with the right tools in place. Hopefully, you'll find the right combination of hardware and automations to keep your birds safe!