Changes in the abundance and distribution of waterfowl wintering in the Central Valley of California, 1973–2000

Abstract

The Central Valley of California is one of the most important areas for wintering waterfowl in the world and the focus of extensive conservation efforts to mitigate for historical losses and counter continuing stressors to habitats. To guide conservation, we analyzed trends in the abundance and distribution (spatiotemporal abundance patterns) of waterfowl and their habitats in the Central Valley and its major subregions (Sacramento Valley, Suisun Marsh, Delta, San Joaquin Valley), from 1973 through 2000. We used existing databases, satellite imagery, and aerial photography to measure habitat area, and aerial surveys and radio telemetry to track the abundance and distribution of wintering waterfowl. Wetlands increased throughout the Central Valley, but agricultural fields flooded after harvest increased greatly to the north in the Sacramento Valley and decreased to the south in the San Joaquin Valley, resulting in an overall increase in the relative availability of winter habitat in the former region. Reflecting the continental decline of the most abundant wintering species (Northern Pintail, Anas acuta), the overall abundance of wintering waterfowl in the Central Valley declined during our study. By contrast, numbers of the American Wigeon (A. americana), Mallard (A. platyrhynchos), and Northern Shoveler (A. clypeata) were stable, and numbers of the Green-winged Teal (A. crecca), Gadwall (A. strepera), diving ducks, and geese increased from 1973–1982 to 1998–2000. The areas of greatest abundance of wintering waterfowl within the Central Valley shifted northward as many species responded to changes in the distribution of habitats. Wintering waterfowl migrated earlier in fall and winter from the San Joaquin Valley, Suisun Marsh, and Delta to the Sacramento Valley, and fewer waterfowl emigrated from the Sacramento Valley to other parts of the Central Valley. Because changes in waterfowl distribution were primarily a response to the increase of a beneficial agricultural practice (i.e., the flooding of rice after harvest) in the Sacramento Valley, changing agro-economics, reduction of water supplies, or other factors that reduce this practice could change the abundance and distribution of wintering waterfowl in the Central Valley rapidly. Thus to maintain abundant suitable habitat and restore the historical distribution of wintering waterfowl, our results suggest a continuing need for the conservation of wetlands and other waterfowl habitats with secure water supplies throughout the Central Valley. Despite our findings, achieving goals for winter waterfowl populations in the Central Valley likely will depend on a combination of factors including some acting in breeding ranges farther north or elsewhere outside of the valley.

Wintering Areas and Migration Characteristics of Swainson’s Hawks That Breed in the Central Valley of California

“…Perhaps CV Swainson's Hawks are gradually adapting to this conversion of native vegetation by wintering in increasing numbers in agricultural and pasture lands north of former wintering areas that formerly were not suitable. A similar change in migratory pattern to winter north of historical wintering areas in response to changes in agricultural land use has been documented for Snow Geese (Chen caerulescens) in central North America (Davis et al 1989, Alisauskas 1998) and for wintering waterfowl in the CV (Fleskes et al 2018).…”

The benefits of crops and field management practices to wintering waterbirds in the Sacramento–San Joaquin River Delta of California

“…The Central Valley is one of the most important regions in the Pacific Flyway of North America for wintering and migratory waterfowl 15 , shorebirds 16 and other waterbirds 17 . Within the Central Valley, the Sacramento–San Joaquin River Delta (Delta) is particularly important to wintering sandhill cranes ( Grus canadensis ), including the state threatened greater sandhill crane ( G. c. tabida ) 18 , 19 .…”

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“…It is possible that some of the patterns Pandolfino and Handel (2018) described from Central Valley CBCs may in part reflect changes in distribution rather than in abundance. Fleskes et al (2018) increased greatly to the north in the Sacramento Valley but decreased to the south in the San Joaquin Valley. The result was a pronounced shift in the availability of winter waterfowl habitat, and of the waterfowl themselves, to the north.…”