[concerned about politics]

Birds fly. No other species does that. Maybe they didn't test the air high enough?
If it were food related, other species eat seeds, insects, scraps, etc. They'd be dead, too.
It has to be because of something only birds can do, and that would be flying.

[Fierce Allegiance]

Birds fly. No other species does that.

You sure about that?

[Oberon]

No other species does that.

Except for bats...but that's beside the point.

[Some hope remaining.]

Or maybe there are other animals dying too but, unlike the birds, they aren't flying to areas where they are being found.

[LexBaird]

Birds fly. No other species does that.

Any species can fly. It just takes the proper application of explosives.