"Aflockalypse Now" -- here's one possible explanation
One possible reason for most -- not all, but most -- of the recent spate of mass deaths of birds and fish is that many birds and fish in the regions where these mass deaths have occurred have been weakened, physiologically speaking, by pesticides, other chemicals, and various non-chemical insults in their environment due to human activities. Up until this winter, the onset of which has been particularly ferocious all across the Northern Hemisphere, it hadn't done much to them, but had made significant inroads on their stamina, endurance, strength, and metabolic vigor that could have been detected by researchers if any such had been looking for it. But once winter hit, in all its fury, and these birds and fish were hit by sudden extreme cold as currents of air or water shifted about and took new directions, the sudden shock of that change in air- or water-temperature, combined with their somewhat weakened constitutions, was enough to kill them. You can stop somebody's heart with sudden shocks like that even when the individual in question seems quite healthy, and a weakened individual of any species is at even greater risk for cardiac failure when hit with such shocks.
As for the remainder of the mass die-offs, such as those in Brazil and a few others here and there, call it honest coincidence. The majority of those deaths could have been due to the sort of shocks described here, because the biosphere has become so contaminated with pollutants of all kinds, from the rare and subtle to the gross and prevalent, that it's likely that almost all eukaryotic organisms on Earth are weakened to at least some extent by continuous exposure to and ingestion of such biological insults, some more than others, but none escaping their impact entirely. Of course, the chemicals and other insults involved for each organism will differ from region to region and even from microregion to microregion, so looking for the chemical pollutant or other environmental insult that did the deed everywhere will not be a fruitful quest. In different areas, different chemicals or other influences will be involved, and researchers should be careful not to overlook subtle things. Check their livers first, their gonads and kidneys second, their brains and hearts third, and look for signs of electromagnetic interference with the nervous system and heart.
I'll be interested to see what happens next. If I'm right, the whole world is poisoned by our activities, and it is getting worse by the day -- and soon the effects of that will be apparent among us, if indeed it isn't already. We, too, are eukaryotes, and like all other eukarya, we can only evolve so quickly to keep up with environmental change of any kind. If such changes take place faster than living creatures can evolve to deal with them, then those creatures will become extinct -- and that applies to us no less than any other form of life.
As for the remainder of the mass die-offs, such as those in Brazil and a few others here and there, call it honest coincidence. The majority of those deaths could have been due to the sort of shocks described here, because the biosphere has become so contaminated with pollutants of all kinds, from the rare and subtle to the gross and prevalent, that it's likely that almost all eukaryotic organisms on Earth are weakened to at least some extent by continuous exposure to and ingestion of such biological insults, some more than others, but none escaping their impact entirely. Of course, the chemicals and other insults involved for each organism will differ from region to region and even from microregion to microregion, so looking for the chemical pollutant or other environmental insult that did the deed everywhere will not be a fruitful quest. In different areas, different chemicals or other influences will be involved, and researchers should be careful not to overlook subtle things. Check their livers first, their gonads and kidneys second, their brains and hearts third, and look for signs of electromagnetic interference with the nervous system and heart.
I'll be interested to see what happens next. If I'm right, the whole world is poisoned by our activities, and it is getting worse by the day -- and soon the effects of that will be apparent among us, if indeed it isn't already. We, too, are eukaryotes, and like all other eukarya, we can only evolve so quickly to keep up with environmental change of any kind. If such changes take place faster than living creatures can evolve to deal with them, then those creatures will become extinct -- and that applies to us no less than any other form of life.