The role of Pacific salmon (Oncorhynchus spp.) in the foraging and nesting biology of the American dipper (Cinclus mexicanus) in southeast Alaska

The annual spawning migration of Pacific salmon (Oncorhynchus spp.) results in a large influx of prey that is utilized by a wide variety of predators and scavengers. Predator-scavengers may forage on salmon as live adults, carcasses, rearing juveniles and eggs. In addition, marine-derived nutrients are assimilated by aquatic algae and invertebrates, causing an increase in stream productivity. This study was conducted to determine if the presence of salmon affects various measures of reproductive success of American Dippers (Cinclus mexicanus) inhabiting streams in southeast Alaska, U.S.A. I observed dippers foraging for salmon eggs and invertebrates along three salmon rivers to quantify their feeding rates. I found that dippers foraging in salmon spawning stream reaches ate 1.8 eggs/min versus 0.6 invertebrates/min in non-spawning reaches. I studied the direct and indirect effects of salmon on American Dipper foraging and nesting biology in 1998–1999. I observed adult dippers delivering food-to nestlings and compared dippers nesting on salmon and non-salmon streams. I weighed nestlings three times between the ages of 8–16 days and found that dipper nestlings on salmon streams were 17% (P = 0.04) heavier at the first weighing, than nestlings on non-salmon streams. Brood reduction on non-salmon streams was the result of the smallest nestlings disappearing from the nest and nestlings on salmon streams were 16% more likely to survive to fledging. I found a higher incidence of second broods on salmon streams in 1998. It is concluded that dippers nesting on salmon streams, probably from the use of salmon fry as prey, accrue reproductive benefits via higher nestling weights early in the nestling phase, less brood reduction, and therefore higher nestling fledging rates. The maintenance of abundant healthy salmon stocks may be crucial for sustaining piscivorous predator populations.