Except that when it comes to mass animal deaths, the rate of it being caused by human activity isn't anywhere near as low as noise. And mass die offs simply don

Except that when it comes to mass animal deaths, the rate of it being caused by human activity isn't anywhere near as low as noise. And mass die offs simply don't happen "all the time". Canaries die of non-gas-related causes far more often than entire flocks of birds and schools of fish die within feet and seconds of one another. Essentially, if a mass die off doesn't correlate with a particularly sudden weather event, or known human activity (fireworks displays) it's safe to assume it is us, from something we (tautologically) didn't know was happening or didn't expect to cause these problems. And at that point, it's just straight-up good science to investigate the remaining cases to better understand our environment and how our activity is affecting it.