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Gadwall Biology in a Hypersaline Environment: Is High Productivity Offset by Postbreeding Mortality?

Abstract The highly herbivorous Gadwall (Anas strepera) is the most salt-tolerant of the North America dabbling ducks and typically breeds in wetlands where salinity does not exceed 15. Small numbers also breed at hypersaline (80-90) Mono Lake, California, which lacks vascular plants. There, Gadwall raise young on the open lake, feed nearly exclusively on alkali flies (Ephydra hians), and minimize osmotic challenges by concentrating activities in areas with access to fresh water. Fledging success is high as a result of abundant food and low predation, but is evidently offset by poor survivorship among postbreeding adults. Failure to complete wing molt and, perhaps, foot disease are implicated. Food in the lake remains abundant well into the autumn. Nevertheless, local birds leave as soon as the young fledge. Many move to nearby freshwater lakes before eventually before continuing to wintering locations in the Central Valley of California. Migrants avoid Mono Lake at this season in favor of nearby freshwater lakes. Overall, Mono Lake’s value for breeding and migrating ducks is low, because of harsh water chemistry, limited fresh water for drinking and marshes for feeding, and apparent low survival among adults.