The truth is, your molecular structure is still in the process of being transformed by your contact with the alien biochemical agent. But whatever it is you’re turning into, one thing’s for sure: it isn’t any species of bird.
You hit the pavement like a sack of meat, and a portion of your anatomy melts into purple goo, ready to spring back into its natural form none the worse for wear. The bulk of you, though, just gets kind of broken and bloody the way a regular person would after leaping from a five-story building.
The overall effect is extra gross for the guy whose job it is to hose your remains off the sidewalk.