Hawk has the same clover-shaped mark as Wandle did. Also, their personalities are rather similar, causing Meliodas to mistake Hawk for Wandle. They both have a strong passion for something (Wandle: shiny things, Hawk: scraps), get annoyed by certain habits that Meliodas has (Wandle: littering, Hawk: doing perverted things to Elizabeth), and are talking animals. While a talking parrot isn't completely abnormal, a talking pig is (many people ask Hawk why he can talk). In addition, Wandle and Hawk are the only talking animals we see. Also, Hawk mentions that he has a weird feeling that he used to fly in a previous life (Wandle?)
Combining, all of these facts, I've come to the conclusion that the Demon King used both Wandle and Hawk to keep an eye on Meliodas, and that Hawk is Wandle's reincarnation. Is there any real proof of this, or are we just limited to clues(obvious clues, but they're still clues)? Is it ever explicitly stated that this is true?