I saw these mallard ducks swimming on the water on my pool cover, and thought it odd that two females would be together without any males. Then i noticed that the beaks were different, and did some research to discover that duck with the yellow bill is the male in "eclipse plummage". I went to the website http://10000birds.com/the-eclipse-plumage.htm to get this explanation:

"Different groups of birds have evolved different “moult strategies”: raptors, for example, replace body feathers and wing feathers in a set order and over several years; many passerines (eg thrushes) keep their wing feathers into the second year of life but replace their body feathers in the first year. Ducks, though, shed most of their feathers twice each year, and in mid-summer most male ducks moult from the bright and colourful breeding plumage into a drab, female-like “eclipse plumage”. The reason for this is that ducks have a rapid but complete moult after breeding, dropping both body and wing feathers, and for a few weeks are completely flightless. Temporarily unable to escape predators by flying away, many male ducks have opted to do what the females do for most of the year and merge safely into the background (or, like eg Shelducks, gather in large flocks offshore or in the middle of large lakes.)"