Historic and current status of the American White Pelican breeding in California

摘要

The American White Pelican (Pelecanus eythrorhynchos) formerly bred the length of California on interior lakes and marshes of the Klamath Basin, Modoc Plateau and Great Basin desert of northeastern California, overflow lands of the Sacramento Valley, terminal lakes in the Tulare Basin of the San Joaquin Valley and at the Salton Sea in the Colorado desert. Historic data are sketchy but nesting is well documented for at least eleven sites. Though unknown, the number of pelicans breeding in the late 1800s and early 1900s may have exceeded 20,000 pairs. Decades of declining numbers and range retraction have left the American White Pelican breeding regularly in California at only two sites in the Klamath Basin (Sheepy Lake on Lower Klamath National Wildlife Refuge and Clear Lake National Wildlife Refuge), where very vulnerable to catastrophic losses. Variably obtained counts at these two sites combined appeared to decrease from about 4,500 nests in 1912 to reach an equilibrium, which fluctuated around 1,600-1,700 nests from the 1950s to present. Annual variation is presumably a response in part to prey availability mediated by water levels. Several high nest counts of 2,300-2,500 at Clear Lake since the early 1970s may partly reflect greater accuracy from aerial photo counts. Declines were caused by shooting, human disturbance and, particularly, habitat loss from water diversions and land reclamation for agriculture. The American White Pelican is currently limited in California by the availability of remote, undisturbed nesting sites and rich foraging habitats. Contaminant effects on breeding pelicans have lessened, but large numbers of pelicans have recently died from type C avian botulism outbreaks at the Salton Sea. Management and research should serve the protection and enhancement of breeding conditions in the Klamath Basin.