The big boned "fighting" breeds like aseels, shamos, etc., are usually heavy birds with big bones. Thus they generally can't fly well. However, as any Sumatra owner knows, sumatras can fly VERY well. I have had Sumatra mutts that look almost identical to your bird. Does it look like there are any small or flat extra spurs on her legs? Many sumatras have multiple spurs, although they don't always pass on, especially with mutts.