Many birds species exhibit a resource based mating system where males defend breeding territories. This mating system is thought to have lead to male-biased philopatry as male birds are better able to defend familiar territories. In contrast, female waterfowl are more likely than males to return to their breeding grounds. Waterfowl are the exception because males cannot economically defend breeding territories and a mate-defense system has evolved. However, waterfowl pair in the winter.