ABUNDANCE, DISTRIBUTION, AND BEHAVIOR OF COMMON MERGANSERS WINTERING ON A RESERVOIR IN SOUTHERN NEW-MEXICO

Arrival dares, mean numbers, departure dates, and proportion of adult males differed for Common Mergansers (Mergus merganser) on Caballo Res ervoir between winters of 1992-1993 and 1993-1994. Wintering Common Me rgansers spent daylight hours leafing (58.6%), sleeping (17.5%), flyin g (5.7%), preening (4.0%), stretching (4.0%) and swimming (3.8%). Feed ing accounted for <4% of daily activity. Diet of Common Mergansers con sisted solely of large gizzard shad (Dorosoma cepedianum) during the f irst winter, but was predominantly threadfin shad (D. potenense) durin g the second winter. The change in diet was probably a result of most gizzard shad having grown too large to be eaten by Common Merganser by che second ranter. Fish caught in gill net samples during both winter s were predominately gizzard shad. Other fish netted were: white bass (Morone chrysops), walleye (Stizostedion vitreum), channel catfish (Ic talurus punctatus), and common carp (Cyprinus carpio). we suggest that managers should promote consistent annual recruitment of shad to prov ide a reliable food source for wintering Common Mergansers.