Illinois Department of Natural Resources
May 2021
May 3, 2021
A least bittern on the nest. Photo courtesy of Stephanie M. Schmidt.

Water is an Important Defense Against Nest Predators

By Stephanie M. Schmidt, Thomas J. Benson, Auriel M. V. Fournier, Joshua M. Osborn
Two juvenile herons resting in their nest of wetland vegetation.
A pair of black-crowned night-heron juveniles in their nest. Photo courtesy of Stephanie M. Schmidt.

Marsh birds are a secretive group of wetland-dependent waterbirds that include bitterns, herons, rails and grebes. They live and nest in dense emergent cattail and hemi-marsh on wetland edges. Marsh birds need this dense wetland vegetation to support their suspended nests as well as provide shelter and protection from predators. Water is also important for marsh birds because it provides access to food, such as fish, frogs, snakes and aquatic insects, and protection from predators. The presence of water below and around a marsh bird nest can aid in its survival and success because some mammalian predators, such as raccoons (Procyon lotor), are limited by deep water.

A cattail nest cradling several brown speckled cream colored medium sided eggs.
Common Gallinule nest. Photo Courtesy of Stephanie M. Schmidt.

In Illinois, marsh bird populations are experiencing widespread declines. Presently, seven marsh bird species are listed as threatened or endangered in Illinois. These declines are fueled by habitat limitation, and possibly predation. By the 1980s, 90 percent of the state’s historic wetland acreage was lost due to agricultural expansion and urbanization. Today, land managers have helped restore the lost and degraded wetland habitat, and one restoration technique used is moist-soil management.

A chart listing seven marsh bird species currently listed as threatened or endangered in Illinois.
A black and white photo of a mink peering into a marsh bird nest.
A mink attempting to raid a least bittern nest. Photo courtesy of the author.

Moist-soil management is a technique that uses water drawdowns to expose new resources and habitats, stop marsh succession to lake habitat, and stimulate the germination of moist-soil plants. It is primarily used to benefit waterfowl who feed on the energy-rich, moist-soil seeds during migration, but this management technique also benefits marsh birds, shorebirds and mammals. Marsh birds use the dense emergent vegetation that grows the year after a drawdown to hold their suspended nests. However, the intensity and timing of water drawdowns for wetland management is a concern because they often occur during the summer months (April to July) when marsh birds are nesting. When water is pulled off after marsh birds have started nesting, the important protection the water provided from some predators may be eliminated or weakened, resulting in greater predator access to nests.

A camera posed above a cattail nest with eggs records the activity of marsh birds.
A camera peering into a least bittern nest. Photo courtesy of Nora Hargett.

Concerns about increased predation sparked our interest in researching if water drawdowns for habitat management are negatively affecting marsh bird reproduction. In 2020, staff from the University of Illinois at Urbana-Champaign Illinois Natural History Survey, including Forbes Biological Station, began researching this question at Emiquon Preserve. Emiquon Preserve is a restored cattail marsh and a RAMSAR wetland along the Illinois River in Fulton County, and it is managed by The Nature Conservancy. Emiquon Preserve was surveyed from May through August for marsh bird nests, and The Nature Conservancy pulled off 4.5 feet of water from the wetland between June and August, after marsh birds started nesting, creating a natural experiment. In 2020, 88 active nests were located and cameras were set up at 52 of the nests to record predation events (Table 1). The cameras helped identify causes of nest loss, including changes in predation associated with drawdowns. Thirty-five total losses were due to predation from mammals, reptiles, and birds, as well as nest failure caused by weather, abandonment and unknown sources (Table 2).

A table indicating active marsh bird nests found in 2020.
A table indicating causes of nest loss in 2020.
Cattails on the edge of a wetland exhibit a mark on the plants of where the water was before it dropped. In the background is a cloudy sky.
A water mark on cattail showing the extent of water depth lost. Photo courtesy of Stephanie M. Schmidt.

Our results suggest that water drawdowns that decrease water depth below nests and move the shoreline closer to the nests are associated with increased predation by raccoons and American mink (Neovision vision; Fig. 1). Raccoons are generalist predators who selectively hunt on wetland edges. As water levels decrease, these edges move closer to marsh bird nests and increase the likelihood of predation. Semi-aquatic mammalian predators, such as mink, are attracted to permanent water sources and also hunt on wetland edges. Therefore, the increased mammalian predation we observed likely resulted from habitat overlap between marsh birds and predators that was created when the water drawdowns moved useable edge habitat for mammalian predators towards the marsh interior.

Two charts showing the daily survival of marsh birds as water depth and distance from the water's edge increased.
Figure 1. The daily survival rate of marsh bird nests suggests mammalian predation risk is greatest at nests closest to the shore and in shallow water.

The results of this study highlight the importance of having water under and around marsh bird nests to protect them from predation. To prevent substantial predation at nests of threatened and endangered marsh birds as a result of water removal during nesting, wetland managers should adjust the timing, duration and intensity of water drawdowns based on the nesting chronology of priority bird species.

Stephanie M. Schmidt is a master’s student associated with the Forbes Biological Station at the Illinois Natural History Survey (INHS) and Department of Natural Resources and Environmental Sciences (NRES) at the University of Illinois at Urbana-Champaign. She earned her bachelor’s degree in Zoology from the University of Wisconsin – Madison in 2018.

Thomas J. Benson is a Senior Wildlife Ecologist at INHS and Research Associate Professor with NRES at the University of Illinois at Urbana-Champaign.

Auriel M. V. Fournier is the Director of the INHS Forbes Biological Station at the University of Illinois at Urbana-Champaign.

Joshua M. Osborn is a Wetland Bird Ecologist at the INHS Forbes Biological Station at the University of Illinois at Urbana-Champaign.