Predator and duck behaviours at depredated nests in wetlands of Great Salt Lake, Utah

Nest depredation is one of the greatest threats posed to ground-nesting ducks. We employed cameras to monitor 164 duck nests (71 cinnamon teal, Spatula cyanoptera, 44 gadwall, Mareca strepera, 38 mallard, Anas platyrhynchos, and 11 nests of unknown species) in the wetlands surrounding Great Salt Lake, from 2015–2021. Of the 164 nests, 21% were successful, 73% were depredated and 7% were abandoned. We observed predators at 99 of the 119 depredated nests; predators at 20 nests went undetected. Raccoons (Procyon lotor, depredated nests) and striped skunks (Mephitis mephitis, ) were the most common of the 99 nest predators recorded. Other predators that depredated nests included long-tailed weasels (Mustela frenata), northern harriers (Circus hudsonius), California gulls (Larus californicus), Sandhill cranes (Antigone canadensis), common ravens (Corvus corax), coyotes (Canis latrans) and red foxes (Vulpes vulpes). Neither the number of eggs removed per depredation event nor the number of eggs remaining varied by predator species. Depredated nests were easier for predators to find than undisturbed, incubated nests, resulting in 68% of depredated nests being visited by multiple predators. All hens detected the approach of a predator and flushed before the predator reached the nest; no hens attempted to defend their nest or attack the predator. Only 21% of hens returned to their depredated nest, and those that did remained off their nest an average of 33 h and 23 h after their nest was depredated by a raccoon or skunk, respectively. Seventeen percent of hens resumed incubation of their depredated nest, but only 1 nest to which a hen returned successfully hatched an egg. Depredation events of raccoons and skunks were not distributed randomly during the 24-hour day, but rather occurred most often during the night and nautical twilight, and rarely during the day. Depredation events of avian predators occurred during the day, rarely during twilight, and none during the night. Depredation events during the night were more likely when the wind was calm but temperature, humidity, and actual moon illumination had no impact. Depredation events by skunks and raccoons occurred more often during the 1st and 4th phases of the moon (new moon) than in the 2nd or 3rd phase.