What might cause a duckling to be hatched with web- less feet if other ducklings in the population all have webbed feet?

A duckling might be hatched with web-less feet due to a genetic mutation or a recessive gene that is present in its parents. If both parents carry this recessive trait, there's a chance that it could be expressed in some of their offspring. Environmental factors during embryonic development, such as temperature or nutrition, could also play a role in abnormal physical traits. Additionally, selective pressure or changes in the environment could lead to variations in traits within a population.