Another feature of this region that influences both the carrying capacity and distribution of migrating and wintering waterfowl is the presence of reservoirs (Ringelman et al. 1989). The semi-arid nature of this conservation region has stimulated construction of many reservoirs to hold water for irrigation and municipal water supply, with flood control, power generation, navigation, and recreation also driving construction. Ringelman et al. (1989) discussed the importance and relationship of reservoirs in this region to migrating and wintering waterfowl. They summarized information from the USFWS that indicate that there are 1,163 reservoirs encompassing 269,974 ha that are of high value to waterfowl, while another 571 encompassing 52,221 ha were available but considered of low value (Ringelman et al. 1989).
Overall, the wetlands of the SGP are of primary importance as migration habitat and they provide significant winter habitat for some species. Wetlands in this conservation region also serve as production habitat, though the number of birds produced is not well documented. The limiting factor in terms of waterfowl using the region is availability of flooded habitat.
Importance to waterfowl
Numbers of waterfowl wintering in the SGP region probably have increased with conversion of grassland to production of cereal crops, particularly with the advent of irrigation that permits corn production in the region, though no historical estimates are available (Bolen et al. 1989). The SGP conservation region is the primary migration corridor for several million ducks and geese in the Central Flyway. Additionally, depending on winter weather severity and wetland conditions, 500,000 to 4 million ducks and 250,000-1 million geese over-winter in this region (Bolen et al. 1989, Ringelman et al. 1989). The playa lakes alone are second in importance only to the Gulf Coast as winter habitat for waterfowl in the Central Flyway (Curtis and Beierman 1980), with estimates of waterfowl ranging from 500,000 to 2.8 million ducks and as many as 750,000 geese (Bolen et al. 1989). Mallards, pintails, green-winged teal and Canada geese are the most common winter residents.
While spring migrants make use of nearly all available habitats in this semi-arid region, the Rainwater Basin wetlands stand out in terms of concentrations of waterfowl. The wetlands in the Rainwater are particularly important spring staging habitat for pintails, mallards, white-fronted geese, Canada geese and snow geese. For many species of ducks, this is the final staging area prior to arrival on prairie nesting areas.
Some waterfowl production occurs on wetlands in the SGP conservation region. Mallards, redheads, blue-winged teal and cinnamon teal are known to successfully nest in playa wetlands, with as many as 25,000 ducks (mostly blue-winged teal and mallards) fledged in some years (Rhodes and Garcia 1981, Simpson et al. 1981). Total waterfowl production in the SGP conservation region has not been well studied.
Non-hunting mortality is the major factor affecting migratory birds in the SGP. Avian diseases, primarily avian cholera and botulism, are the primary source of mortality. Disease outbreaks are related to over-crowding on the limited wetland base in the SGP. Severe disease outbreaks occur in winter and spring when conditions are dry and severe winter weather persists for several days, concentrating birds on smaller areas of open water. However, it also has been suggested that there is a chronic low to moderate rate of disease-related mortality each year in this conservation region. Other factors, including contaminated water, uncovered waste oil pits, and pesticide pollution also contribute to total non-hunting mortality. Wetlands in the region are threatened by sedimentation, pit excavation, overgrazing, land leveling, and other factors. Water availability is a limiting factor in many years. Bolen et al. (1989) summarized disease and other mortality factors that affect birds in this region.