One thing you can watch out for which is quite difficult to explain but once you've seen it you'll never forget is a pigeon not able to use one wing to its full potential. In flight one wing will be fully extended on the downward motion whilst the other doesn't fully complete this action giving the expression the wing is stiff. This can also be noticed inside the loft when a pigeon fly's up to its box or perch, to the trained ear you can even hear the pigeon putting in extra effort to get airborne.