Is it normal for mallards to lay eggs on grass and decking then just leave them?

It is not typical for mallards to lay eggs on grass or decking and then leave them. Mallards typically build their nests in concealed locations such as tall grass or under bushes, and they will defend their nests and eggs aggressively. If you have found mallard eggs on grass or decking, it is possible that the nest has been abandoned, or that the eggs were laid by a sick or injured bird. If you are concerned about the eggs, you can contact a wildlife rehabilitator for assistance.