Waterfowl Ecology

in Suisun Marsh and the Pacific Flyway

USGS Western Ecological Research Center

April 29, 2022

A flock of ducks with dark heads, white necks, and grey bodies flies in midair.

Introduction

Suisun Marsh and the Central Valley of California contain some of the world’s most important wetlands, providing critical breeding and wintering habitat for many waterfowl species in the Pacific Flyway. Wetlands in the Suisun Marsh and wetland and agricultural habitats in the Central Valley support 10–12 million waterfowl wintering in or migrating through these two regions annually [1].

Despite the importance of wetland habitats in the Central Valley for waterfowl, more than 90 percent of historical wetlands in the region have been modified or destroyed due to agricultural and urban development [2].

Map of the Suisun Marsh Area, with yellow indicating designated wildlife areas and black dashed lines indicating the marsh's rough boundary.

Intensive agriculture, threatened fisheries, and expanding urban interests have resulted in intense competition for limited water resources, especially during drought [3]. Plans for the conversion of managed wetlands to restored tidal wetlands for the benefit of some marsh species will likely reduce the carrying capacity for waterfowl species that prefer managed and seasonal wetland habitats [4]. Natural resource managers require current and reliable scientific data on the availability and usage of existing waterfowl habitats to plan for sufficient waterfowl habitat in the face of these growing challenges. Such information is critical for science-based decision-making on the allocation of limited water resources and wetland restoration and enhancements for waterfowl habitat.

A bird flies away from the viewer over a body of water, as a scientist wearing waders stands on the shore with arms outstretched after releasing the bird. — A USGS researcher releases a duck after study and tagging in Suisun Marsh (credit: USGS).

Biologists at the Dixon Field Station of the U.S. Geological Survey-Western Ecological Research Center (USGS WERC) have been conducting research on migratory waterfowl and their habitats to guide conservation programs in the Pacific Flyway, with particular focus on Suisun Marsh. A wide range of projects investigate the response of waterfowl to a decade of habitat changes in the region, including the influence of tidal marsh restoration, managed wetland enhancements, and changes in habitat management.

Colorful mallard duck takes flight from a log partially submerged in the water — A mallard duck takes flight in Suisun Marsh (credit: Sarah Peterson/USGS).

The USGS WERC waterfowl research program is conducted in collaboration with many federal, state, and local partners, including the California Department of Water Resources , California Department of Fish and Wildlife , University of California, Davis , U.S. Fish and Wildlife Service , Central Valley Joint Venture , U.S. Bureau of Reclamation, Ducks Unlimited , California Waterfowl Association , Canadian Wildlife Service , Suisun Resource Conservation District , and private landowners.

Scroll down or click on the tabs above to learn more about USGS WERC waterfowl research topics, including breeding, wintering, waterfowl movement, and habitat use, as well as the tools and technology USGS WERC scientists use to study waterfowl.

Note: To best enjoy the photos and videos in this Story Map, we recommend viewing on a computer or tablet.

Breeding Ecology

Suisun Marsh is an important site for both wintering and breeding ducks and historically has harbored some of the highest densities of nesting ducks and nests in California and across North America [5]. As a result, there is great interest by land and resource managers to manage and protect this important ecosystem. Many of the dabbling ducks that winter in California, like mallard and gadwall, hatch in upland nesting habitats within the state.

Current data on breeding waterfowl in Suisun Marsh contribute to one of the longest-running datasets on breeding waterfowl in the world, with more than 20,000 nest records in a 35-year period.

Developing and maintaining this long-term dataset has required close partnerships between USGS WERC scientists, California Department of Fish and Wildlife, California Department of Water Resources, California Waterfowl Association, and University of California-Davis. Furthermore, these data contribute to the U.S. Fish and Wildlife Service Central Valley Joint Venture’s Implementation Plan.

Suisun Marsh is also an important site for tidal marsh restoration, which will influence the capacity of the marsh to support waterfowl populations [4]. To better inform local land management actions and examine the effects of tidal marsh restoration, USGS WERC scientists are conducting a multi-year study focused on ducks that nest in Suisun Marsh. Their research examines the factors that shape both nest and duckling survival, two important components that influence the recruitment of new ducks into the population and have a direct effect on California duck hunters' harvest. The findings can be used to shape management of upland and wetland habitats for breeding ducks with the goal of improving nesting and brooding success of California waterfowl.

Click through the slideshow below to see examples of current and recent research in nesting and breeding ecology.

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Locating and Assessing Duck Nests

Researchers search for upland duck nests and assess how nest locations, vegetation composition and structure near nests, and habitat management influence nesting success and nest densities. This information can help managers determine best practices to maximize nesting success and abundance of ducks in preferred habitats.

Long-term trends for nesting waterfowl in the Suisun Marsh and Central Valley ecosystems are examined using an extensive multiyear dataset (since 1985) generated and curated by the USGS and collaborators.

A nest amidst tall grass, with 8 eggs and a round silver object on the middle of the nest. A marker flag stands in the grass next to the nest.

Monitoring Incubation, Hatch, and Departure

USGS monitors incubation, duckling hatch and departure from the nest, predation at the nest, and the role of each of these factors in nest success. Nest temperature recorders and miniature video cameras placed in and near nests record hen behavior.

USGS researchers also monitor the clutch size and hatching success of individual nests. These data are combined with nest density and nest success data to generate annual estimates of duckling productivity, and ultimately, estimates of the carrying capacity of the region and its wetlands for breeding waterfowl.

Six ducklings huddle together in their nest of dry grasses.

Predation of Duck Eggs

Predation is the main source of nest failure in ground nesting waterfowl and can significantly limit recruitment to the population [6]. Using miniature video cameras placed at duck nests, USGS can identify which predators are most detrimental to duck populations.

Nest cameras and temperature recorders are used to build algorithms that identify the precise timing of predation events and how those relate to nest location, vegetation composition at the nest, and predator movements.

A gopher snake curls over a nest of eggs with its mouth around an egg.

Tracking Predators

By attaching GPS collars to striped skunks and raccoons, USGS can quantify how individual raccoons and skunks use different habitats within Suisun Marsh and interact with duck nests. Data on predator activity can help managers better understand the factors that make duck nests vulnerable to predation.

Tracking Ducklings

Researchers attach radio transmitters to young ducklings to study their movements as they travel from upland habitats to nearby wetlands. By tracking individual ducklings, researchers learn how duckling activity and wetland attributes, like water availability and quality, shape brood survival.

Wintering Ecology

Suisun Marsh and the Central Valley are critical wintering areas for migratory waterfowl in the Pacific Flyway, providing habitat for the 10-12 million waterfowl that winter in or migrate through the region each year [1].

USGS WERC researchers integrate studies of movement, habitat use, and body condition to understand waterfowl behavior during the wintering period, providing data to inform management of wetlands used by wintering waterfowl. The abundance of waterfowl in late fall and winter also provides recreational opportunities for hunters and birdwatchers. The USGS collaborates with hunters to collect bands and GPS transmitters from harvested birds, generating data used to inform harvest management in the region.

Click through the slideshow below to see examples of current and recent research in wintering ecology.

Population Dynamics of Ducks and Geese

USGS is evaluating population dynamics of geese in the Central Valley wintering area to understand potential competition with ducks for food and roosting habitat. Goose populations wintering in California have rapidly increased over the recent decade due to high reproduction on northern latitude breeding areas [7].

To understand the potential impact of increasing goose populations in Suisun Marsh and the Central Valley to ducks, researchers are using telemetry to compare goose and duck habitat use. The results will be used to assess competition for food resources among ducks and geese and the impacts of food depletion on waterfowl in Suisun Marsh.

Waterfowl Body Condition and Health

Several USGS WERC research projects are examining links between waterfowl body condition, health, diet, and selenium and mercury contamination.

Body condition is thought to be an important predictor of migration success and breeding success of birds. Body condition metrics such as mass, fat content, and protein content can indicate how waterfowl populations on the wintering grounds are responding to current conditions.

Combining body condition data with concurrent collection of other biological parameters (e.g., movement, habitat use, food availability) provides an unprecedented opportunity to evaluate the linkages between habitat and water management, behavior, and body condition.

Duck with dark head, white neck and breast, and grey body in the water

Aerial Surveys

Wintering populations of dabbling and diving ducks are surveyed aerially from fixed wing aircraft near the start of January. By conducting aerial waterfowl surveys periodically throughout the winter, researchers can determine peak abundance and total winter-use days.

USGS and partners are also developing methods for the use of drone-mounted thermal cameras for the detection, estimation, and monitoring of waterfowl, which will enhance researchers’ ability to estimate the size of broods in flooded habitats in Suisun Marsh. Aerial survey techniques are also used during the breeding period.

Harvesting and Duck Origins

USGS WERC uses web tags and transmitters returned by hunters to understand the movements and origins of harvested waterfowl. Researchers are also using stable isotope analysis to determine the origin of waterfowl that breed in California and the proportion of hunter-harvested mallard and gadwall that originated in California wetlands. These data provide important linkages between wintering and breeding waterfowl populations within the Pacific Flyway.

Found a bird with a USGS web tag or transmitter? Click here to learn how to report it.

Movement Ecology

Animal telemetry--the study of animal movement using tags or transmitters—makes up the core of USGS WERC's waterfowl research program. Telemetry is used to study all aspects of waterfowl ecology, from long distance migration patterns to the within-marsh movements of ducklings and their predators, to behavioral analysis of animal choice and habitat value.

USGS researchers use a combination of high-tech solar-powered GPS-GSM transmitters and VHF transmitters, along with simple web tags and bands. GPS transmitters allow for the collection of frequent animal location data in near real-time, while VHF transmitters are used to detect locations of waterfowl and predators more locally on a daily or semi daily time frame. Both types of transmitters enable researchers to calculate movement patterns and detect mortality. Tags and bands on birds are used to understand movement patterns over a longer timescale and can be deployed on greater numbers of birds.

With location data from transmitters and tags, the USGS can build maps of animal movement that illustrate habitat use and can be used to inform waterfowl and wetland management.

Waterfowl movement data, especially that from GPS transmitters, have allowed USGS waterfowl scientists to build the largest and most robust waterfowl movement database in the world, based on location data collected via GPS transmitters. The database currently contains over 7 million locations from over 1250 individuals of 12 primary species: mallard, gadwall, northern pintail, green-winged teal, cinnamon teal, blue winged teal, American wigeon, northern shoveler, lesser snow goose, Ross's goose, greater white-fronted goose, tule white-fronted goose, as well as a smaller number of records for Eurasian wigeon, canvasback, and greater scaup. The database can serve as a resource for a variety of land management planning needs. For example, the data can indicate where energy infrastructure intersects with waterfowl migratory routes and breeding and nesting habitat.

Click through the slideshow below to see examples of current and recent research in movement ecology.

Daily Movement and Habitat Use

GPS transmitters are used to track fine-scale movements of mallard (Anas platyrhynchos), gadwall (Mareca strepera), and cinnamon teal (Spatula cyanoptera) throughout the year, and winter locations for a variety of other dabbling and diving duck species.

The locations allow researchers to map movement patterns and habitat use for waterfowl . GPS collars on striped skunk (Mephitis mephitis) and raccoon (Procyon lotor) are used to track daily movements, determine habitat use, and quantify interactions with ground nesting waterfowl in upland habitats.

Hands hold out a brown duck with transmitter attached to its back

The Speed of Flight

Researchers have used GPS location data to accurately estimate flight speeds and durations for mallard (A. platyrhynchos), northern pintail (A. acuta), gadwall (M. strepera), American wigeon (M. Americana), cinnamon teal (A. cyanoptera) and northern shoveler (Spatula clypeata).

The updated flight speeds for dabbling duck species can be used to parameterize and validate energetics models, guide management decisions regarding optimal habitat distribution, and, ultimately, improve conservation management of wetlands for waterfowl.

A brown duck with a transmitter on its back flies over a grassy area

Raptors in Suisun Marsh

Researchers use telemetry to study the movement, reproductive, and foraging ecology of several raptors species in Suisun Marsh, including northern harrier (Circus hudsonius), red-tailed hawk (Buteo jamaicensis), and great horned owl (Bubo virginianus) to better understand their effects on waterfowl populations.

Raptors are important apex predators in marsh ecosystems. They serve as indicators of ecosystem health, may affect waterfowl nest success both positively and negatively, and interact with other sensitive prey species, like the endangered salt marsh harvest mouse (Reithrodontomys raviventris).

A person holds up a raptor wearing a transmitter on its back while another person looks on.

Avian Influenza Risk

The USGS is investigating how avian influenza risk varies among waterfowl and if body condition and contaminant burdens affect susceptibility to avian influenza. Using telemetry data of marked birds, USGS is also working to determine real-time risk management of avian influenza transmission to domestic poultry in California’s Central Valley.

Habitat and Land Use

Suisun Marsh and the Central Valley provide critical habitat for waterfowl, but more than 90 percent of historical wetlands have been lost to agriculture and development [2]. Today, the remaining habitat is threatened by further development, drought, and climate change.

Research by the USGS and partners informs wetland management best practices in Suisun Marsh. Using a combination of telemetry, remote sensing, mapping, and on-the-ground monitoring, USGS scientists are learning how waterfowl use different habitats, how waterfowl habitats are changing, and how different wetland management strategies may influence waterfowl populations into the future.

Click through the slideshow below to see examples of current and recent research related to habitat and land use.

Assessing Habitat Management

The Suisun Marsh Plan calls for the restoration of 5,000- 7,000 acres of tidal wetlands. The conversion of managed wetlands to restored tidal wetlands would result in the reduction of available habitat for waterfowl species, since waterfowl are unlikely to utilize restored tidal wetlands to the same extent as managed seasonal wetlands.

Under the Suisun Marsh Plan more 40,000 acres of managed wetlands would be enhanced for waterfowl to offset the reduction in acreage of managed wetlands. Research by the USGS and partners is designed to provide managers with the scientific information needed to accomplish this “no net loss” approach for wetland planning.

Map showing duck locations and percent area covered by managed wetlands (47%), permanent water (34%), tidal marsh (14%) and other (5%).

Monitoring Water Quality

USGS WERC is quantifying water availability and water quality (salinity and temperature) of wetlands and channels in Suisun Marsh to examine their suitability for duckling broods. Duckling survival and growth is understood to decline with increased salinity in brood-rearing habitat, but information on duckling survival within Suisun Marsh is lacking.

Using extensive water sampling and satellite imagery, researchers are creating maps of water availability and quality in the marsh. In combination with data on duckling movement and habitat use, these maps are used to understand effects of salinity on ducklings and how exposure and effects may be mitigated through water management or other strategies.

Duckling brood in wetland, with one duck diving

Assessing Food Types and Wetlands

Research by USGS, UC Davis, and other partners is assessing the availability and composition of winter food resources used by waterfowl in Suisun Marsh and other regions in California.

This information will help determine the carrying capacity of the Suisun Marsh and evaluate impacts of any future changes (e.g., tidal restoration, sea level change, drought, water or habitat management) on the capacity of the Marsh to support target populations of non-breeding waterfowl.

Man in waders stands in the water and holds a long skinny tool in the water to sample for seeds

Ecological Forecasting

USGS is using ecological forecasting to generate long-term projections of flooded cropland and wetland habitat and impacts to waterfowl species. Using remotely sensed Earth Observation data and a water resources simulation, researchers generate maps of long-term projections (100-year) of flooded cropland and wetland habitat under multiple drought, water management, urbanization, and cropland conversion scenarios. The predictions can help land managers understand and plan for future changes to waterfowl habitats.

Tools and Technology

USGS WERC scientists use a wide range of tools to study waterfowl ecology, from simple web tags to solar-powered GPS transmitters that allow for tracking birds in real time. Technology allows researchers to build a comprehensive picture of waterfowl behavior throughout their lifecycles, from measuring what time of day a duckling hatches from an egg to calculating how fast a duck flies during a long-distance migration.

Click through the slideshow below to see examples of tools and technology used by USGS researchers.

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VHF Radiotransmitters

VHF radio transmitters, smaller and less expensive than GPS transmitters, are used to locate ducklings and predators within the marsh. These transmitters emit a signal that can be detected on the ground not far from the tagged animals.

Person holds up young duckling with VHF transmitter attached to its back

GPS-GSM Transmitters

Solar-powered GPS-GSM transmitters track waterfowl and predator movements in real time. These transmitters send information on a bird’s location via cellular networks as often as every minute and can be used to map both local movement and habitat use as well as long-distance migrations.

These transmitters also inform researchers of a suspected bird mortality or transmitter malfunction. Accelerometers within each transmitter provide detailed data on movement and position that can be used to infer a bird’s activities or behavior.

The transmitters are attached to healthy adult animals only, to ducks as "backpacks" and around the neck for geese. The transmitters are made of lightweight materials and weigh only about 1-2% of the bird’s body weight.

Hands hold up a brown duck with a rectangular transmitter on its back

iButtons™

An iButton™ consists of a small computer chip contained in steel casing about the size of a stack of five dimes. iButtons™ are set in a labeled holder and placed inside and nearby nests to record nest temperature and ambient temperature during the incubation period. These temperature records can be used to determine when hens leave the nest or are killed by predators.

Hand holds up a cylindrical object with a small silver circle at the center. It is attached to a rod so it can be stuck into the ground. Numbers are written on the object.

Nest Cameras

Small video cameras placed next to nests allow researchers to study incubation behaviors, predation in the nest (here, a striped skunk eats an egg), and hatch and departure timing.

Drone-Mounted Thermal Cameras

USGS researchers are developing techniques for using thermal cameras mounted on drones to locate nests and ducklings from the air. These techniques could enhance detection, estimation, and monitoring of waterfowl broods in flooded habitats.

Satellite Imagery

Satellite images are used for mapping and assessing wetland habitat at large scales. For example, digitized satellite images are used in combination with water samples to map water availability and salinity across Suisun Marsh.

This example Landsat image, from USGS and NASA, was produced to show winter flooding in California. Healthy green vegetation appears as bright green and water shows as bright blue.

Products

The following is a list of recent products. Click here to download a full list of USGS WERC waterfowl publications.

Ackerman, JT. 2002. Of mice and mallards: positive indirect effects of coexisting prey on waterfowl nest success. Oikos 99:469-480. https://doi.org/10.1034/j.1600-0706.2002.11802.x .

Ackerman, JT, AL Blackmer, and JM Eadie. 2004. Is predation on waterfowl nests density dependent? Tests at three spatial scales. Oikos 107:128-140. https://doi.org/10.1111/j.0030-1299.2004.13226.x .

Ackerman, JT, JM Eadie, DL Loughman, GS Yarris, and MR McLandress. 2003. The influence of partial clutch depredation on duckling production. Journal of Wildlife Management 67:576-587. https://doi.org/10.2193/0022-541X(2006)70[799:EOSDVA]2.0.CO;2 .

Ackerman, JT, JM Eadie, ML Szymanski, JH Caswell, MP Vrtiska, AH Raedeke, JM Checkett, AD Afton, TG Moore, FD Caswell, RA Walters, DD Humburg, and JL Yee. 2006. Effectiveness of spinning-wing decoys varies among dabbling duck species and locations. Journal of Wildlife Management 70:799-804.

Ackerman, JT, JM Eadie, GS Yarris, DL Loughman, and MR McLandress. 2003. Cues for investment: nest desertion in response to partial clutch depredation in dabbling ducks. Animal Behaviour 66:871-883. https://doi.org/10.1006/anbe.2003.2283 .

Ackerman, JT, MP Herzog, GS Yarris, ML Casazza, E Burns, and JM Eadie. 2014. Chapter 5: Waterfowl ecology and management. Pages 103-132 and maps 10 and 11 in Moyle, PB, A Manfree, and PL Fiedler (editors): Suisun Marsh: Ecological History and Possible Futures. University of California Press: Berkeley, California. 239 pages.

Ackerman, JT, JY Takekawa, DL Orthmeyer, JP Fleskes, JL Yee, and KL Kruse. 2006. Spatial use by wintering greater white-fronted geese relative to a decade of habitat change in California’s Central Valley. Journal of Wildlife Management 70:965-976. https://doi.org/10.2193/0022-541X(2006)70[965:SUBWGW]2.0.CO;2 .

Ackerman, JT, MP Herzog, C Brady, JM Eadie, and GS Yarris. 2015. Archiving California’s historical duck nesting data. U.S. Geological Survey Open-File Report 2015-1131, 26 p., https://doi.org/10.3133/ofr20151131 .

Casazza, ML, PS Coates, MR Miller, CT Overton, and DR Yparraguirre. 2012. Hunting influences the diel patterns in habitat selection by northern pintails Anas acuta. Wildlife Biology, 18(1), pp.1-13. https://doi.org/10.2981/09-099 .

Casazza, ML, F McDuie, S Jones, AA Lorenz, CT Overton, J Yee, CL Feldheim, JT Ackerman, and KM Thorne. 2021. Waterfowl use of wetland habitats informs wetland restoration designs for multi‐species benefits. Journal of Applied Ecology. https://doi.org/10.1111/1365-2664.13845 .

Casazza, ML, F McDuie, AA Lorenz, D Keiter, J Yee, CT Overton, SH Peterson, CL Feldheim, and JT Ackerman. 2020. Good prospects: high-resolution telemetry data suggests novel brood site selection behaviour in waterfowl. Animal Behaviour 164: 163-172. https://doi.org/10.1016/j.anbehav.2020.04.013 .

Croston, R, JT Ackerman, MP Herzog, JD Kohl, CA Hartman, SH Peterson, CT Overton, CL Feldheim, and ML Casazza. 2018. Duck nest depredation, predator behavior, and female response using video. Journal of Wildlife Management 82:1014-1025. https://doi.org/10.1002/jwmg.21444 .

Croston, R, CA Hartman, MP Herzog, ML Casazza, CL Feldheim, and JT Ackerman. 2020. Timing, frequency, and duration of incubation recesses in dabbling ducks. Ecology and Evolution 10:2513-2529. https://doi.org/10.1002/ece3.6078 .

Matchett, EL, M Reiter, CT Overton, D Jongsomjit, and ML Casazza. 2021. Using high resolution satellite and telemetry data to track flooded habitats, their use by waterfowl, and evaluate effects of drought on waterfowl and shorebird bioenergetics in California: U.S. Geological Survey Open-File Report 2020–1102, 59 p., https://doi.org/10.3133/ofr20201102 .

McDuie, F, ML Casazza, CT Overton, MP Herzog, CA Hartman, SH Peterson, CL Feldheim, and JT Ackerman. 2019. GPS tracking data reveals daily spatio-temporal movement patterns of waterfowl. Movement Ecology 7:6. https://doi.org/10.1186/s40462-019-0146-8 .

McDuie, F, ML Casazza, D Keiter, CT Overton, MP Herzog, CL Feldheim, and JT Ackerman. 2019. Moving at the speed of flight: dabbling duck-movement rates and the relationship with electronic tracking interval. Wildlife Research 46:533-543. https://doi.org/10.1071/WR19028

Peterson, SP, JT Ackerman, MP Herzog, A Hartman, R Croston, CL Feldheim, and ML Casazza. 2019. Sitting ducklings: Timing of hatch, nest departure, and predation risk for dabbling duck broods. Ecology and Evolution 9:5490-5500. https://doi.org/10.1002/ece3.5146 .

Peterson, SP, JT Ackerman, CA Hartman, ML Casazza, CL Feldheim, and MP Herzog. 2020. Mercury exposure in mammalian mesopredators inhabiting a brackish marsh. Environmental Pollution 273: 115808. https://doi.org/10.1016/j.envpol.2020.115808 .

Partners & Acknowledgments

USGS partners and collaborators include the California Department of Water Resources, U.S. Fish and Wildlife Service, California Department of Fish and Wildlife, University of California, Davis, Central Valley Joint Venture, U.S. Bureau of Reclamation, Ducks Unlimited, California Waterfowl Association, Canadian Wildlife Service, Suisun Resource Conservation District, state wildlife and game agencies for Alaska, Colorado, Idaho, Montana, Nevada, Oregon, Utah, and Washington, and many private landowners.

The use of trade, product, or firm names in the publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Contacts

Josh Ackerman

Research Wildlife Biologist, USGS Western Ecological Research Center

Dixon Field Station

jackerman@usgs.gov

Principal investigator focused on nesting ecology and wildlife health

Michael Casazza

Research Wildlife Biologist

USGS Western Ecological Research Center

Dixon Field Station

mike_casazza@usgs.gov

Principal investigator focused on movement ecology and habitat use

Find a duck with a web tag or transmitter?

Click here to report it

References

[1] Gilmer, DS, Miller, MR, Bauer, RD, and LeDonne, JR. 1982. California's Central Valley Wintering Waterfowl: Concerns and Challenges. US Fish & Wildlife Publications. 41. https://digitalcommons.unl.edu/usfwspubs/41

[2] Frayer, WE, Peters, DD, and Pywell, HR. 1989. Wetlands of the California Central Valley: Status and Trends - 1939 to Mid-1980s. Portland, OR, U.S. Department of the Interior, Fish and Wildlife Service. https://www.fws.gov/wetlands/Documents/Wetlands-of-the-California-Central-Valley-Status-and-Trends-1939-to-mid-1980s.pdf .

[3] Fleskes, JP, Casazza, ML, Overton, CT, Matchett, EL, and Yee, JL. 2018. Changes in the abundance and distribution of waterfowl wintering in the Central Valley of California, 1973–2000, in Trends and traditions: Avifaunal change in western North America (WD Shuford, RE Gill Jr., and CM Handel, eds.), pp. 50–74. Studies of Western Birds 3. Western Field Ornithologists, Camarillo, CA; doi 10.21199/SWB3.2.

[4] Moyle, PB, Manfree, AD, and Fiedler, PL (Eds.). 2014. Suisun Marsh: ecological history and possible futures. Univ of California Press.

[5] McLandress, MR, Yarris, GS, Perkins, AE, Connelly, DP and Raveling, DG, 1996. Nesting biology of mallards in California. The Journal of wildlife management, pp.94-107.

[6] Peterson, SP, JT Ackerman, MP Herzog, A Hartman, R Croston, CL Feldheim, and ML Casazza. 2019. Sitting ducklings: Timing of hatch, nest departure, and predation risk for dabbling duck broods. Ecology and Evolution 9:5490-5500.

[7] U.S. Fish and Wildlife Service. 2020. Waterfowl population status, 2020. U.S. Department of the Interior, Washington, D.C. USA. https://fws.gov/migratorybirds/pdf/surveys-and-data/Population-status/Waterfowl/WaterfowlPopulationStatusReport20.pdf