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Predation of eggs is the main cause of nest failure for birds, and ground nesting ducks are particularly vulnerable to predation by mammals, such as raccoons and skunks. Habitat management actions that minimize predator interactions with nests could potentially decrease rates of predation and thus increase waterfowl nest success. However, different predator species may behave and interact with habitat features and nests in distinct ways. For example, if raccoons are attracted to human structures and striped skunks are not, duck nests near human structures will be more vulnerable to raccoons but less vulnerable to skunks. In the absence of predator-specific behavior information, it can be difficult to determine where and why duck nests are most vulnerable to predation and what habitat management actions are likely to be most effective.

To better understand where duck nests may be most vulnerable to different predators, USGS researchers and partners used GPS collars to track 25 raccoons and 16 striped skunks, two of the most widespread mammalian predators of waterfowl eggs in North America. The study was conducted over a 4-year period in Suisun Marsh, California, an important nesting area for the Pacific Flyway. During the same time period, the researchers monitored approximately 2000 duck nests to determine which nests survived and document evidence of predation. The researchers then used the duck nest locations and predator tracks to determine what habitat features, such as the presence of trees, wetlands, or human structures, made nests vulnerable to each type of predator.

At the landscape scale, the farther duck nests were located from invasive phragmites patches, large shrubs, canals, telephone poles, and other human structures, the less vulnerable they were to predation. Phragmites and shrub patches provided denning and day resting sites for mammalian predators in the study, increasing predation risk to ducks nesting nearby. However, findings at the landscape scale may obscure strong interactions between specific predators and duck nests. The researchers found marked predator-specific differences in the likelihood of a raccoon or skunk encounter (defined as a predator coming within 25 m of a nest) relative to habitat features within upland nesting habitat areas. Proximity to canals, wetlands, trees, levees, roads, human structures, shrubs, and telephone poles increased the likelihood of a nest being encountered by a collared raccoon. Nests located near to canals, trees, and shrubs were also more likely to be encountered by a collared skunk as they were with raccoons, but proximity to seasonal wetlands and human structures actually made nests less vulnerable to skunks.

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Most predator encounters with duck nests were attributable to a few individuals that more frequently used upland habitat: 29% of raccoons and 39% of skunks were responsible for 96% of total nest encounters by collared predators. It is possible that dominant male raccoons with stable

boundaries could deter other male raccoons in upland habitat and decrease access to duck eggs as a resource. Although individual raccoons encountered duck nests more often than skunks, a higher percent of the nests encountered by skunks were depredated (52%) than those encountered by raccoons (22%).

The researchers also found that collared predators most likely encountered duck nests opportunistically: 96% of the time collared predators did not locate active nests even when they were within 25 meters. Because duck eggs were most likely consumed as raccoons and skunks happened to discover nests, managing the habitat features those predators most strongly associated with could potentially reduce rates of egg predation.

Select Management Implications:

· Habitat management near nesting habitat may be most effective if it focuses on habitat features associated with predators, such as removing invasive phragmites patches that predators use for denning and resting.

· Since raccoons were located in close proximity to canals and wetland edges, managing water availability immediately adjacent to duck nests could reduce the likelihood of raccoons opportunistically encountering and depredating duck nests.

· Some habitat management strategies may work to reduce encounters with some predator species but not others. Because the subset of striped skunks that spend most of the duck nesting season within upland habitat spend less time than raccoons near wetland habitats and human structures, manipulating these features may not reduce skunk encounters with duck nests.

· Removal of individual predators may not reduce predation rates on duck nests because territoriality may reduce predation by other individuals.

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This research spotlight refers to the following publication and data release:

Peterson, S.H., Ackerman, J.T., Keating, M.P., Schacter, C.R., Hartman, C.A., Casazza, M.L., and Herzog, M.P., 2022. Predator movements in relation to habitat features reveal vulnerability of duck nests to predation. Ecology and Evolution, 12(9), e9329. https://doi.org/10.1002/ece3.9329

Peterson, S.H., and Ackerman, J.T., 2022, Predator movements and duck nests in relation to habitat features in Suisun Marsh, CA (2016-2019): U.S. Geological Survey data release, https://doi.org/10.5066/P9W12VVX.

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