I recently had a finch(i think) nest underneath my window AC unit and one of the parents caught a cold for about one week. I heard more calls from that specific bird throughout the time, it was much squeakier while making what sounded like normal calls. It made me think that they relay info on frequencies that our ears dont tune into. Just a hypothesis, but was interesting to hear more inbetween sounds that the other birds didn't make. Could also just be a sort of mask for birds, a call that says 'hey Im sick stay away.' Although I think it'd be obvious for them from the change of voice.
Overall it was interesting to listen to them communicate with their young ones. The nest is constantly chirping unless they get warning calls then it gets super quiet until their folks let them know danger has passed.
It was unfortunate when I had to replace the unit, at least I waited for the young ones to start flying. My new unit is half the size and they disliked the new roof I tried building to mimic the size of the old one. I wish them luck. Tough little fellars.
