The other day an old friend visited the workshop whilst I was in the middle of mixing or melting something that was letting off a hint of noxious fumes. I keep the shop well-ventilated and usually have a fan set up to extract fumes from where I'm working. When I'm using things that are particularly likely to shuffle off my mortal coil, I have a variety of exotic and expensive respirators to choose from. Still he posed the question: "Aren't you worried about what all of these fumes are doing to you?"
In answer, I explained that there's a pair of swallows that have a nest in the rafters and I use them like canaries in a coal mine. As long as they continue to chirp, I figure I'm okay. When they fall over dead, I need to relocate to fresh air on the way to seeking medical attention. It was all chuckleworthy until he asked the next question with a hint of concern: "Where are they?"
A cursory search of the workshop revealed no bird carcasses and later I found a long-abandoned nest. Initially I was worried, but after a bit more discussion, we figured the missing swallows were not a cause for concern.