I agree. Any fish species in a given body of water will have their tolerances adjusted for that body of water. Just as my fish are living at a higher than normal PH for their species (@#$%ing well water) and have gotten used to it and in fact are thriving. Anything that changes the environmental water should kill off more than one species.
Even if the other species are more tolerant of it, the sudden change is more likely to kill them. This is why Crystal Dragon and many other of my trusted sources of fishy lore inform me that as long as my fish are happy at their slightly higher than tolerance PH, leave them alone.
And even if you were to say that one species of fish is more tolerant than another, there should be some of the smaller and weaker members of that species getting killed off as well as the targeted species.
As for poison, I'm doubting that since again there is only one fish species showing up as dead. Especially considering that the reverse is often true. That the Freshwater Drum Fish is often one of the few surviving species left after a spill or other contamination.
The only thing that I can think of that could and frequently does target one species in an environment is some sort of disease be it parasitic, fungal, bacterial, or viral. And that can tie in with the cold since a change in the environment can cause a slight weakness in the fish (not enough to kill them normally) that an infection/infestation can exploit.
The birds could due to the cold. The species hit are normally warmer weather birds who winter in areas that are normally warmer than they have had this past month. God knows it got bitterly cold here in Southern Maryland, an area that normally I survive the winters by wearing a hoodie most of the time as we rarely go below 40.