I wonder why I almost never see dead animals or birds when I go out in the woods. I guess scavengers get to the dead bodies asap. A bird croaking mid flight would drop like a rock, not dive down fully in control of its wings and direction.
The only time Im sure I caught a bird having a heart attack is when I owned a parakeet some years ago. I was sitting in the living room with my laptop, birdie was chillen on his perch as usual. Then I hear a loud squawk and found him on his back either dying or involuntarily twitching. He pissed himself and stopped moving 10 minutes later. All was quiet in the house.