Sixty-six million years ago, as an asteroid rewrote the terms of life on Earth, a small feather passed through the gut of a dinosaur — and stayed there, frozen in stone, waiting to be found. Scientists have now recovered that feather from fossilized feces, and in it they glimpse not just a moment of predation or scavenging, but a possible answer to one of paleontology's oldest questions: why did birds survive when nearly everything else did not. The discovery suggests that dietary flexibility — the willingness to eat what the dying world offered — may have been the quiet engine of avian surviv