Louisiana detective and a cockfight

The Louisiana State Police received reports of illegal cock fights being held in the area around Lafayette, and duly dispatched the infamous Detective Desormeaux to investigate. Desormeaux reported to his sergeant the next morning.
"Dey is tree main groups in dis cock fightin," Desormeaux began.
"Good work Desormeaux! Who are they?" the sergeant asked.
Desormeaux replied confidently, "De Aggies, de Cajuns, and de Mafia."
Puzzled, the sergeant asked, "How did you find that out in one night?"
"Well," said Desormeaux, "I went down and done seed at cock fight, I knowed de Aggies was involved whan a duck was entered in de fight."
The sergeant nodded. "Ok, I'll buy that, but what about the others?"
Desormeaux intoned knowingly, "Well, I knowed de cajuns was involved wen sumbody bet on de duck."
"Ah," sighed the sergeant. "And how did you deduce that the mafia was involved?"
"De duck won."