I have a horrible smell coming from my vents could it be a dead bird or other animal? There is a hole where I have seen birds fly in and out of

Yep dead animals will reek up a storm. One time when I was a kid one of my hamsters got into the heating vents and died. Boy did that reek for about a month. You'll have to figure out where the carcass is and remove it or put up with the stench until the animal finally disintegrates and stops smelling.