The chaffinch is much more than colourful and attractive than the house of sparrow, yet they are of the same family; they are both finches, and without their feathers they would be identical in appearance. Just as bright plumage gives a bird splendour – thing of the peacock! – so does smart clothing make a person look more impressive then he really is.
The proverb is nearly always used in a sarcastic or ironical sense.
‘Until her husband won the pools, she was the most commonplace, dowdy, illiterate creature imaginable, but now she dresses herself up like a society queen and nearly always remembers the sound her aitches, people are already beginning to forget what she was like before.’
‘Fine feathers make fine birds.’
A rather less cynical meaning of the proverb is that other people do judge us by our appearance, so we are much more likely to make a success of our lives if we dress well than if we are shabby and down-at-heel.