[jammi]

Chickens can perform short flights, but from my limited observations it seems like the stamina is the main limiting factor for them. They've however clearly evolved from fully flight-capable birds, but the now-extinct Moa however might have been a different story: https://en.wikipedia.org/wiki/Moa

[lomnakkus]

Oh, yes, there are evolutionary dead ends, but e.g. the Moa (or even the Dodo[1]) basically evolved for really long without any evolutionary pressure towards avoiding "enemies" and thus was stuck in a local optimum with no way out.

Still ~25 seconds of sustained flight, just about the record for a chicken, I think seems like not-evolutionary-dead-end-enough to my untrained eye. :). YMMV.

[1] AFAIUI, it was humans that ultimately drove it to extiction, but it could have been any invasive species that would kill bird-like thigs. Maybe I'm wrong on that.