For nearly 100 years ornithologists have considered the genus Aechmophorus to include only one species, the Western Grebe (A. occidentalis). Few ornithologists, especially amateur field ornithologists, have been aware that the Western Grebe has been considered polymorphic, with two distinct phenotypes referred to as dark and light phases (Storer 1965, Mayr and Short 1970).
Recent study of sympatric dark-phase and light-phase populations indicates the polymorphism classification is erroneous and that the forms function as separate species (Ratti 1979). Additional data are needed on dark-phase and light-phase birds, and hopefully this paper will aid in alerting both professional and amateur ornithologists to the identification and distribution of these species.