They also breed more profusely. In their natural range, mallard had to contend with numerous predators, raccoons, foxes, polecats, crows, hawks and ravens. They also had to handle a demanding annual migration, each leg of which would involve a journey of several thousand kilometers. So they responded with a large clutch size and a rapid re-nesting response. In contrast the grey duck, evolving in the more our benign environment, has an average clutch of just eight eggs, three less than the mallard average.
Mallard are sexually more aggressive than the greys.
Mallard drakes pair only briefly. Once the female has laid her eggs and commenced incubation, the male moves on to other females that are still laying and forcefully mates with them. Up to one third of ducklings in any mallard brood are fathered by males other than the primary mate. Murray Williams says that the consequence of this is that we already have an extensive hybrid population and that the creation of a uniquely New Zealand duck, the grallard, is well on the way. International conservation agencies such as the IUCN, The World Conservation Union, view interbreeding very seriously and have already ranked the grey duck in New Zealand as an endangered species.