Table of Contents
Environmental & Conservation Balance Sheet for The California Rice Industry
Chapter 6: Wildlife in Relation to Rice Farming
The purpose of this chapter is to document the wildlife- related issues associated with rice production in California. The background material for this chapter was the white paper Water Use for Rice Farming in California (CH2M HILL, 1992). New information pertaining to recent initiatives, regulations, monitoring results, and other wildlife-related issues are reviewed and documented in this chapter.
This chapter provides a description of the naturally occurring habitats available to wildlife in the Central Valley, historical changes in wetland habitat, wildlife in the Central Valley, and benefits of rice management to wildlife.
Background
Rice farming takes place in an environment that is rich in wildlife species. These species exist in rice fields or associated fallow lands, marshlands, waterways, banks, and levees.
Rice farming is an economically beneficial land use, and it is compatible with the natural condition of land used for its cultivation. The environment in and around rice fields supports large numbers of native and introduced species. The extent to which rice farming is compatible with wildlife habitat requirements is partly fortuitous and partly by design. Many farmers are outdoors people with a keen interest in environmental stewardship and preserving native wildlife. Like any land or water users, rice farmers face an ever- increasing challenge to ensure their operations are more compatible with fish and wildlife, especially threatened and endangered species.
One of the main concerns for California wildlife managers is the rapid depletion of habitat as a result of development. This is especially true of waterfowl habitat within the Central Valley. The Central Valley is critical to migratory waterfowl (ducks, geese, and swans) in the Pacific Flyway. Wetland habitat, and to some extent the adjoining upland habitat, is most critical for waterfowl. Wetlands provide cover, a place to rest, freedom from disturbance, food, and specialized nesting habitat, all of which are necessary for waterfowl to thrive (Sather and Smith, 1984). Of all the agricultural practices in the Central Valley, rice production appears to be the most favorable to waterfowl by providing artificial wetlands that compensate, to some degree, for the extensive historic loss of native wetland habitat in this region. Rice production, as described in previous chapters, is concentrated in the northern region of the Central Valley known as the Sacramento Valley.
Although rice fields cannot replace all of the habitat contributions of native wetlands, they can nevertheless be outstanding habitat in their own right. Recent radical changes in rice field management have further improved their habitat value. Most notable is the advent of winter flooding, which restores the historic, wet winter condition of much of the land where rice is farmed. Because the waterfowl depend heavily on Central Valley habitat during winter, this practice greatly increases the habitat value of rice fields.
Naturally Occurring Wetland Habitat Types
Some critical natural wetland habitats occurring in the Central Valley have been rapidly depleted since 1939, including vernal pools, marsh, and riparian forests. These habitat types are discussed further in the following sections.
Vernal Pools
Vernal pools are a low, open mixture of semi-aquatic annual broadleaves and grasses. Germination and growth begins with winter or fall rains, continuing while plants are submerged until standing water is evaporated in the spring. Rising spring temperatures evaporate the pools, leaving concentric bands of vegetation that colorfully encircle the drying pool (Holland, 1986).
Marsh
In the Central Valley, two types of marsh are dominant: coastal and valley freshwater marsh, and vernal marsh. Coastal and valley freshwater marsh is dominated by perennial, emergent monocots about 12 to 15 feet tall, often forming completely closed canopies. These marshes are quiet sites that are permanently flooded by freshwater (rather than brackish, alkaline, or water of variable salinity). Vernal marshes are mostly low growth, primarily annual broadleaves, and are marshy with standing water following the winter rains. Their extent is greatly reduced or they become completely dry by summer. Historically, both types of marsh were extensive throughout portions of the Central Valley. However, because of development, this type of natural wetland habitat is currently reduced in area through its entire range (Holland, 1986).
Riparian Forest
In the Central Valley, two types of dense, broadleaved, winter deciduous riparian forest are prevalent: cottonwood and mixed. Both types of riparian forests have understories that are dense, with abundant vegetative reproduction of canopy dominants. Scattered seedlings and saplings of shade-tolerant species may also be found. These sites are composed of fine- grained alluvial soils near perennial or nearly perennial streams that provide subsurface irrigation even when the channel is dry, and are inundated yearly during spring. The cottonwood and mixed riparian forests were extensive in the Central Valley, but now are reduced to scattered, isolated remnants or young stands because of flood control, water diversion, agricultural development, and urban expansion (Holland, 1986).
Historical Changes in Wetland Habitat in the Central Valley When California became a state in 1850, it was estimated that about 5 million acres of wetlands existed within the state, 4 million of which were located within the Central Valley (Frayer et al., 1989). By 1939, the wetland acreage in the Central Valley had decreased by 85 percent as a result of land use conversion, primarily urban development. Wetland and deepwater habitats in the Central Valley had dwindled to 794,000 acres. By the mid-1980s, the wetland and deepwater habitat had further decreased by about 32 percent from the 1939 levels. During this 135-year period 86 percent, or almost 3.5 million acres of the Central Valley's wetland and deepwater habitats, was converted to other uses. Detailed land use data are not available for years prior to 1939. Table 6-1 lists some general land use changes that occurred from 1939 to the mid-1980s in the Central Valley.
Table 6-1:
Land Use Changes in California's Central Valley from 1939 to
the Mid-1980s
| Land Use | 1939 (acres) | Mid-1980s (acres) | Net Change (acres) | Change (%) |
|---|---|---|---|---|
| Urban | 151,200 | 1,130,10 | + 978,900 | + 647 |
| Crops Other than Rice | 6,943,10 | 7,916,10 | + 973,000 | + 14 |
| Rice | 434,500 | 658,600 | + 224,100 | + 52 |
| Wetlands including Deepwater Habitat | 794,800 | 544,600 | - 250,200 | - 32 |
| Other | 5,107,70 | 3,181,80 | - 1,925,900 | - 38 |
* Source: Frayer et al., 1989.
Planted rice actually averaged 405,000 acres in California from 1985 to 1994 (California Department of Food and Agriculture, 1995). The acreage shown in Table 6-1 for rice in the mid-1980s signifies land that is dedicated to rice farming, not land actually planted with rice in any given year.
The most notable change shown by these data is the growth of urban and agricultural land use. Rice cultivation accounted for approximately 18 percent of the growth in agricultural acreage from 1939 to 1985, and it constituted about 7.7 percent of total agricultural acreage in the Central Valley in 1985.
Land development prior to 1939 resulted in the net conversion of large areas of wetlands to agriculture (including rice farming), urban, and various other uses. However, conversions of wetlands to and from rice acreage since 1939 present a different history. The develop-ment of rice acreage has resulted in the net gain of 11,100 acres of wetlands and deepwater habitats. This gain in wetlands does not reflect classification of rice acreage as wetlands; it reflects the actual gain of wetlands, primarily palustrine emergent wetlands (Frayer et al., 1989). Palustrine emergent wetlands are more commonly known as marshes, prairie potholes, and sloughs (Cowardin et al., 1979). This type of habitat is immensely valuable to waterfowl, particularly puddle ducks such as mallards. Over 90 percent of the net gain in rice farmland between 1939 and the mid-1980s was the conversion of existing agricultural acreage to rice. The development of agricultural lands other than rice farmland, along with urban development, resulted in the net loss of approximately 270,000 acres of wetlands and deepwater habitats over the same time period.
Future land use changes in the Central Valley may reduce critical wetland habitat for waterfowl. Recent droughts prompted proposals to reduce the amount of rice acreage in California. However, the U.S. Fish and Wildlife Service stated in 1978 that if the elimination of rice farmland in the Central Valley should occur, fully half of the available waterfowl habitat would be lost. It is abundantly clear that rice production plays an integral part in maintaining the state's waterfowl populations.
Wildlife in the Central Valley
As previously mentioned, the Central Valley is a key area for migratory species of the Pacific Flyway. A broad range of waterbirds, mammals, and reptiles can be found in the Central Valley. The most numerous of the species known to use the rice fields are waterfowl. After fields have been harvested in the fall and winter, literally millions of ducks, geese, and swans arrive to rest and feed (Ducks Unlimited, Inc., 1995). The importance of this migration and their use of rice fields will be discussed in the Rice Water Management Benefits to Wildlife section.
The Central Valley supports nearly 250 species of birds. This vast resource can be characterized into four groups on the basis of annual occurrences: residents, winter residents, summer residents, and transients. These seasonal patterns of bird movement in the Central Valley are a key to wildlife- friendly farm management activities.
Permanent residents are those species that contain populations that spend their entire annual life cycle in the Central Valley. Ninety-two species of permanent resident birds occur in the Central Valley. During the past 100 years, agricultural lands have become an integral component of land use for several species of resident birds. Open space settings of agricultural fields, in association with wind break plantings around farm houses, provide habitat for red-tailed hawk, American kestrel, barn owl, and yellow-billed magpie. Pastures provide hunting habitats for herons and raptors.
Numerous species of waterfowl breed in the Central Valley, but four species are widespread permanent residents. The mallard is the most common breeding species found throughout the Central Valley. It prefers marshes, ponds, lakes, and irrigation ditches, and requires uplands within 1 mile of their permanent water body. Cinnamon teal and gadwall are also common, breeding in semipermanent and permanent marshes, ditches, and small ponds. Both breed closer to water than do mallards. The wood duck is the fourth species. Wood ducks nest in tree cavities near permanent rivers, ponds, and lakes. Many landowners and wildlife areas place artificial nest-boxes for wood ducks. Wood ducks are most common in the Delta and the Sacramento Valley.
Summer residents breed in the Central Valley in summer and migrate to southern climates for winter. Many of these species are called neotropical migrants because they winter in Central and South America (the neotropics). Examples of summer visitors include cinnamon teal, Swainson's hawk, American avocet, western kingbird, blue grosbeak, and northern oriole. The Central Valley supports 39 species of summer residents.
Summer residents begin to arrive in the Central Valley by mid- February to establish nesting territories. By mid-summer, the number of bird species residing in the Valley totals 131 (permanent residents plus summer residents).
Winter residents breed in latitudes north of the Central Valley or in montane habitats. They migrate to the Central Valley during fall to spend the winter months. They depart in spring for their breeding grounds. The Central Valley supports 75 winter resident species. During winter, the Valley realizes its highest diversity of birdlife with nearly 170 species (Ducks Unlimited, 1995c).
Transients are bird species that migrate through the Central Valley enroute to northern breeding grounds or southern wintering grounds. These species typically nest in more northern habitats and winter in the tropics. They are present in the Valley for a short period of time in spring (March through May) and fall (late August through October) (Ducks Unlimited, 1995d).
The maintenance of bird diversity in the Central Valley will require the protection of remaining natural communities. Agricultural lands play a critical role in this effort. As primary habitats and buffers to wildlife areas, agricultural lands provide resources for numerous breeding species. If explored, opportunities for habitat enhancement on agricultural lands can add to the natural community protection being accomplished by public agencies and wildlife conservation organizations on refuges (Ducks Unlimited, 1995d).
Of the wildlife species in the Central Valley, approximately 21 have been given "special status" by federal and state officials. Some are listed under state and federal Endangered Species Acts as endangered or threatened (Ducks Unlimited, Inc., 1995). Special-status wildlife species include taxa that are: (1) designated as threatened or endangered by the state or federal governments (i.e., "listed species"); or (2) are proposed or petitioned for federal threatened or endangered status; and/or (3) are state or federal candidates for threatened or endangered status; and/or (4) are identified by the Department as "Species of Special Concern." Table 6-2 provides a listing of the special-status wildlife species that potentially occur in the Sacramento Valley.
Table 6-2
Potential Special-Status Wildlife Species In The Sacramento Valley
aStatus Definitions:
Federal
|
State
|
bSpecies that use ricelands for breeding activities
| Common Name | Scientific Name | StatusaFed/CA |
|---|---|---|
| Invertebrates | ||
| Conservancy fairy shrimp | Branchinecta conservatio | FE/- |
| Vernal pool fairy shrimp | Branchinecta lynchi | FE/- |
| Vernal pool tadpole shrimp | Lepidurus packardi | FT/- |
| Valley elderberry | Desmocerus californicus | FT/- |
| longhorn beetle | dimorphus | |
| Amphibians | ||
| Western spadefoot toad | Scaphiopus hammondii | C2/CSC |
| California red-legged frog | Rana aurora draytonni | FPE/CSC |
| Foothill yellow-legged frog | Rana boylii | C2/CSC |
| Reptiles | ||
| Northwestern pond turtle | Clemmys marmorata marmorata | C2/CSC |
| Giant garter snake | Thamnophis gigas | FT/CT |
| Birds | ||
| White-faced ibis | Plegadis chihi | C2/CSC |
| Aleutian Canada goose | Branta chanadensis leucopareia | FT/- |
| Cooper's hawk | Accipiter cooperi | -/CSC |
| Sharp-shinned hawk | Accipiter striatus | -/CSC |
| Golden eagle | Aquila chrysaetos | -/CSC |
| Ferruginous hawk | Buteo regalis | C2/CSC |
| Swainson's hawk | Buteo swainsoni | -/CT |
| Northern harrierb | Circus cyaneus | -/CSC |
| Bald eagle | Haliaeetus leucocephalus | FT/CE |
| Osprey | Pandion haliaetus | -/CSC |
| Merlin | Falco columbarius | -/CSC |
| Prairie falcon | Falco mexicanus | -/CSC |
| American peregrine falcon | Falco peregrinus anatum | FE/CE |
| Greater sandhill crane | Grus canadensis tabida | -/CT |
| Long-billed curlew | Numenius americanus | -/CSC |
| Short-eared owlb | Asio flammeus | -/CSC |
| Long-eared owl | Asio otus | -/CSC |
| Burrowing owl | Speotyto cunicularia | C2/CSC |
| Willow flycatcher | Empidonax traillii | -/CE |
| Purple martin | Progne subis | -/CSC |
| Bank swallow | Riparia riparia | -/CT |
| Loggerhead shrike | Lanius ludovicianus | C2/CSC |
| Tricolored blackbirdb | Agelaius tricolor | C2/CSC |
| Yellow warbler | Dendroica petechia brewsteri | -/CSC |
| Yellow-breasted chat | Icteria virens | -/CSC |
| Mammals | ||
| Pallid bat | Antrozous pallidus | -/CSC |
| Spotted bat | Euderma maculatum | C2/CSC |
| Little brown myotis | Myotis lucifugus occultus | C2/CSC |
| Yuma myotis | Myotis yumanensis | C2/- |
| Townsend's big-eared bat | Plecotus townsendii | C2/CSC |
Rice Water Management Benefits to Wildlife
The rice industry's water management practices for rice fields are directly and indirectly benefiting wildlife throughout the Sacramento Valley. Because fields are generally flooded in the spring and summer, they function similarly to wetlands during this period and provide wetland habitat, even in the driest summers. Fields that are flooded after the harvest, whether through the interest of hunting clubs or for straw management, provide much needed winter wetland habitat, food source, and potential disease control for waterfowl populations.
Traditionally, about 40,000 acres of rice land has been flooded during the winter, principally for hunting clubs and wildlife refuge. In 1994, an additional 80,000 to 100,000 acres were flooded during winter for straw decomposition (Ducks Unlimited and California Department of Water Resources, as cited by Jim Hill, personal communication). In 1995, an additional 40,000 acres appears to have been flooded (Jim Hill, personal communication).
Winter Habitat
Habitat is most critical for waterfowl during the breeding season and the winter. While some species of waterfowl do breed in the Central Valley, the primary habitat value to waterfowl is as a haven and food source during the winter migrations. Between 3 million and 6 million waterfowl, or about 20 percent of waterfowl wintering in North America and 60 percent of those wintering in the Pacific Flyway, winter in the Central Valley each year (Reid and Heitmeyer, 1995). In some years, 10 million to 12 million ducks may spend some portion of the winter within the Central Valley (California Valley Habitat Joint Venture [CVHJV], 1990). The Central Valley has occasionally wintered about 25 percent of all ducks counted in the United States by annual mid-winter census studies (CVHJV, 1990). Some species, such as the northern pintail, depend heavily on Central Valley habitat in their winter migration (Ducks Unlimited, 1990).
One important response to the need for winter habitat has been the creation of wildlife refuges in the region. In many cases, these are located on retired rice farmland or adjacent to rice farms, and rice is sometimes cultivated on the refuges as a wildlife food source. The refuges generally share water delivery facilities with rice farms. Unfortunately, the extent of these refuges cannot satisfy the need for winter habitat, so wildlife managers are working in partnership with rice farmers to increase the value of ricefield habitat to wildlife.
Water management in the rice industry is changing to accommodate environmental concerns within the Central Valley. The most notable of these changes is the flooding of rice fields in the wintertime. This is one alternative to burning stubble left after the fall harvest, and it is now being practiced on about 160,000 acres, or about 40 percent of the long-term average rice acreage.
Rice fields that are not flooded also provide valuable habitat for upland birds. Also, wintering arctic geese, such as white- fronted geese, feed and rest in these fields. Following winter rains these fields provide good habitat for shorebirds and browsing geese such as Ross' geese and Canada geese.
Food Source
Rice fields that are flooded after harvest are used by more waterfowl than those that are burned and left unflooded (Heitmeyer, 1989). Harvest losses available in unburned fields are about 300 to 350 pounds/acre, depending on the harvest method used (Brouder and Hill, 1995). When rice is harvested, some falls to the ground and is lost to the farmer but remains available as feed. The amount of rice lost depends on harvesting methods. New methods, such as the stripper header, result in less rice loss during harvest, and therefore reduce the residual rice available to feed wildlife. The impact of this technology is currently being examined by rice production and wildlife experts. In addition, about 250 pounds/acre of other food (principally invertebrates) is available. The critical importance of this food source to waterfowl is discussed in detail by WESCO (1991) and Reid and Heitmeyer (1995). Extending this practice to more rice farmland would further benefit waterfowl.
A variety of rollers are used to flatten rice straw against the soil to speed decomposition. This method provides easy access to the residual food because it is left on top of the soil. Even after grain and weed seeds are depleted, waterfowl and shorebirds remain in the fields to feed on insects and snails residing in the soil and decaying straw (Ducks Unlimited, Inc., 1995).
Principal Health Risks to Birds
Avian botulism is one of the most deadly diseases for waterfowl. Botulism results from the ingestion of toxins that are produced by certain bacteria. It commonly occurs in the western United States and is often fatal. Of all of the diseases that affect waterfowl, none has resulted in more losses than botulism (Bellrose, 1980). An example of this was the death of more than 250,000 waterfowl from a botulism outbreak in the Central Valley in 1969. Botulism results from bacterial growth within the carcasses of invertebrates, primarily fly maggots, which have been feeding on dead animal matter (Bellrose, 1980). The bacteria require a warm, anaerobic environment which is provided by the invertebrate carcass.
Several criteria favorable to botulism outbreaks have been discussed in the literature (Bellrose, 1980; Locke and Friend, 1987) and are summarized as follows:
- Invertebrates drowning through flooding of dry land during warm weather
- Mud flats being exposed, resulting in death of aquatic invertebrates through receding or fluctuating water levels that produce "feather edge" shorelines
- Rotting vegetation
- Acidity falling within the range of pH 5.7 to 6.2
- Absence of oxygen in the growth medium
- Rising ambient temperatures
- Waterfowl overcrowding that stresses birds and exposes more individuals to outbreaks
Botulism outbreaks are not considered a serious problem for waterfowl that use rice fields (T. Rocke, USFWS, pers. comm.). Even though these areas are heavily used by waterfowl, the only record of a botulism outbreak in rice fields was concurrent with an avian cholera die-off. (The botulism outbreak could have been a secondary result of avian cholera mortality that occurred under conditions favorable for botulism toxin production.)
This lack of botulism occurrence in rice fields may be related to the way rice operations are managed. Rice fields are typically flooded during the spring when ambient temperatures are relatively cool. The bacteria require warm ambient tempera tures to thrive. In addition, water levels in rice fields are kept relatively constant and the dikes that surround the fields are relatively steep, which prevents drying edge waters throughout the summer.
One important aspect in controlling botulism is to provide a well-aerated water source to have a healthy invertebrate population within the wetland (B. Clark, DFG, pers. comm.). Many rice farmers are converting to recirculating, static, and other irrigation systems. The extent to which these systems avoid stagnant water may be an indication of their potential effect on wildlife. Reducing the amount of stagnant water in their fields can significantly lessen the chances of botulism outbreaks (B. Clark, DFG, pers. comm.).
Selenium toxicity problems in the 1980s at Kesterson Wildlife Refuge have fueled concerns about trace element toxicity. This occurs when naturally occurring elements become concentrated to toxic levels in wildlife habitat. Typically, these elements have been weathered from native soils and sediments, and they are concentrated by evaporation and transpiration (water use by plants).
There is no indication that this has or will occur in the principal rice-growing region of California, the Sacramento Valley. Although similar geologic material exists on the west side of the Sacramento Valley, widespread irrigation for nearly a century has not resulted in elevated trace element concentrations in shallow groundwater or subsurface drainage. Drainage from rice fields is mainly off of field surfaces and is generally of very good quality (see Chapter 3, Water Quality).
Conclusions
The following sections present the justification for ratings of the rice industry's performance relative to the environmental value of wildlife.
Land Preparation
The method a farmer chooses to dispose of rice straw greatly affects the value of his rice field as wildlife habitat. The primary objective of rice farming is to produce an economic yield of rice. However, rice farmers have made habitat enhancement by careful management of straw a reality by replacing burning with techniques like rolling of rice straw. Also, recycling of rice straw has the potential to replace raw energy and fiber materials, indirectly benefitting other (e.g., forest) habitat.
Other land preparation activities, while somewhat disruptive to wildlife habitat, are minimal when compared with urban land uses, and similar to those practiced for alternative crops. These are therefore scored at zero.
Irrigation
Flood maintenance during the growing season is of value to wildlife. Rice fields and ditches are home to a wide variety of species. No feasible alternative land use would provide the same benefit. This benefit is expected to continue at its present level. The principal investment by the industry is its major contribution to maintenance of levees and canals that control water flow.
Winter flooding has steadily climbed from a traditional 40,000 acres of hunting clubs and wildlife refuges, to an estimated 180,000 acres in 1996, or about 40 percent of the rice acreage. Most of the increase resulted from farmers flooding their fields for straw decomposition. Approximately 86 percent of the estimated 4 million acres of native wetlands in the Central Valley was converted to urban and agricultural uses between 1850 and 1985. From 3 million to 6 million waterfowl, or 20 percent of those wintering in North America, winter in the Central Valley. In some years, 12 million ducks have spent some portion of the year in the Central Valley. Wetland habitat has also been shown to benefit upland wildlife species by providing a rich food source while birds are still young. Flooded rice fields also provide approximately 300 pounds/acre of macro invertibrates. Therefore, winter flooded fields provide critical wetland habitat for wildlife, especially migratory waterfowl that pass through the Sacramento Valley during this period. Winter flooding is probably one of the most outstanding positive environmental effects of rice farming, and would not be achievable under any feasible alternative land use. It is projected to increase during coming years, and it is the product of investment by the industry and its collaborators in on-farm research and development of the technology.
Pest Control
Animal pest control, as practiced in rice fields, has not been shown to have adverse impacts on wildlife. It compares favorably with pest control activities of feasible alternative land uses, since the level of animal pest control required in rice fields is relatively low when compared with other agricultural or urban land uses. These practices are not expected to change greatly. Development of efficient and environmentally acceptable techniques has required substantial industry investment.
Harvest
Rice harvest leaves behind about 350 pounds/acre of grain. This grain can provide abundant food for wildlife, particularly migratory waterfowl, if fields are flooded during winter. However, more efficient harvest technology, such as the stripper header, may reduce grain available as feed. Flooded rice fields also provide approximately 300 pounds/acre of macro invertibrates. Also, rolling of rice straw onto the field surface for decomposition (a method developed by the rice industry to save plowing energy) leaves more grain available than straw incorporation by plowing. Although other cereal crops also leave residual grain for wildlife, rice provides more benefit because of the flooded habitat in which the grain can be provided. Harvest and post-harvest practices that increase the habitat value of rice fields are increasing as the industry continues to invest in this type of innovation.
References
Bellrose, F. C. 1980. Ducks, Geese, and Swans of North America. Stackpole Books. Harrisburg, Pennsylvania. 68-73.
California Department of Food and Agriculture, 1995. California Field Crop Statistics for 1994, Sacramento, California.
California Valley Habitat Joint Venture Implementation Plan. 1990. A Component of the North American Waterfowl Plan. U.S. Fish and Wildlife Service, Portland, Oregon.
Cowardin, L. M., V. Carter, F. C. Golet, and E. T. LaRoe. 1979. Classification of Wetlands and Deepwater Habitats of the United States. U.S. Fish and Wildlife Service. FWS/OBS-79/31.
Ducks Unlimited, Inc. 1995. "Rice Straw Decomposition and Development of Seasonal Waterbird Habitat on Rice Fields. Valley Habitiats. Number 1.
Ducks Unlimited, Inc. 1990. Population Recovery Strategy for the Northern Pintail. Long Grove, Illinois.
Frayer, W. E., D. D. Peters, and H. R. Pywell. 1989. Wetlands of the California Central Valley: Status and Trends - 1939 to Mid-1980's. U.S. Fish and Wildlife Service. Region 1, Portland, Oregon.
Heitmeyer, M. 1990. Opportunities for Enhancement of Rice Lands in the Central Valley of California. J. Payne, editor. In Proceedings of Waterfowl and Wetland Management of Private Land Conference. Texas Agricultural Extension Service. Corpus Christi. 16-25.
Locke, L. N. and M. Friend. "Avian Botulism," M. Friend, editor. Field Guide to Wildlife Diseases. U.S. Fish and Wildlife Service. Resource Publication 167, Washington D.C. 83- 90.
Sather, J. H. and R. D. Smith. 1984. An Overview of Major Wetland Functions and Values. U.S. Fish and Wildlife Service. FWS/OBS-84/18.
WESCO (Western Ecological Services Company, Inc.). 1991. Environmental Attributes of Rice Cultivation in California. California Rice Commission. Yuba City, California.
Personal Communications Bill Clark, California Department of Fish and Game, Region 2, Rancho Cordova.
Toni Rocke, U.S. Fish and Wildlife Service, National Wildlife Health Research Center, Madison, Wisconsin.
