This, because the French word for hawk is "faucon" (falcon), which sounds like "false cunt". Hence Duchamp's conclusion... 
Where may be seen that by doing this, he added a bit to "This is not a pipe" by Magritte. 
As a matter of fact, Duchamp's conclusion adds perspective to what Magritte fingers. Because Courbet's work, of course, is not the true cunt either. So that the hawk says "false cunt" just as Courbet's work should say "false cunt" to any viewer. And so that ordinary perspective itself  is also fingered as a lie. 
But the movement introduced by Duchamp's play on words fingers that our representation is, as regards reality, in the same relationship as words in a play on words. That is, not much more than a chance, or more precisely said, in the best cases, a coincidence.