Isolation of Snake River islands and mammalian predation of waterfowl nests

In 1990–1992, we studied predation of waterfowl nests by mammalian predators on 30 islands in a 64-km reach of the Snake River in southwestern Idaho, USA, to identify river flows necessary to protect and enhance migratory bird use of Deer Flat National Wildlife Refuge. We monitored 235–314 Canada goose (Branta canadensis) nests each year and 122 duck nests, primarily mallard (Anas platyrhynchos), in 1991. We calculated island isolation as the flow energy (width × average water velocity [m²/sec]) a predator encountered in crossing to an island. Density of goose nests increased as island isolation increased (P < 0.001). In contrast, visits of terrestrial predators to islands, mammalian predation of goose nests, and variance in predation rates decreased as island isolation increased (P < 0.01). Nests were most frequently depredated by raccoons (Procyon lotor), coyotes (Canis latrans), badgers (Taxidea taxus), and mink (Mustela vison). Multiple regression models explained 67% of the variance in nest density and 48% of variance in predation rates (P < 0.01). Variables of secondary importance relative to island isolation were island size and percentage of farmland on the adjacent mainland. Fewer geese nested (P < 0.01) and predation rates were greater (P = 0.06) on large islands, and more geese nested on islands adjacent to farmland (P = 0.04). We also analyzed rates of nest predation relative to river flows for 21 islands from 1953 to 1992. Mammalian predation of goose nests decreased as island isolation increased with greater river flows (P < 0.01). To identify levels of isolation needed to protect use of islands by nesting waterfowl, we estimated rates of change in nest densities and predation rates with island isolation for a range of regression quantiles. We then calculated levels of island isolation at river flows ranging from 110 to 340 m³/sec. Below flows of 250 m³/sec, nest predation rates increased rapidly. Flows ≥340 m³/sec adequately isolated Snake River islands for nesting waterfowl. Predator visitation and nest predation rates were at a low level, and variability in the rates was at a minimum for these flows.