The easiest way to tell the difference (and the only way at this age) is the size. Mallards are small enough to virtually be a bantam duck. If they're mallards they'll be smaller than all of the other breeds. If they're rouens they'll be roughly the same size as everyone else.
The one you think is a swede definitely looks like one. I'd lean more toward him being that than a cayuga. Cayygas can be born with yellow on their breasts, but it's usually faint.
The one you think is a swede definitely looks like one. I'd lean more toward him being that than a cayuga. Cayygas can be born with yellow on their breasts, but it's usually faint.