The ducks with green head are called mallards. They are large ducks with bulky bodies, wide and flat bills and rounded heads. Although it is worthy to note that not all mallards have green heads. If we are to be specific on the answer, we can say that the ducks with green head are the male mallards since they are known to have a dark, green head and yellow bill.
A female mallard, on the other hand, has brown head and a combination of both orange and brown for its bill. Mallards can grow between 20 to 26 inches long, they have a wingspan of about 39 inches, their bill is between 1.7 to 2.4 inches long and they weigh between 1.6 to 3.5 pounds.