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Population Energetics of Northern Pintails
Wintering in the Sacramento Valley, California

Management Implications


General population energetics models (Heitmeyer 1989, Reinecke and Loesch 1996) may be sufficient to assess adequacy of winter habitat for additional representative waterfowl species wintering in the Sacramento Valley until species-specific population energetics analyses are available. These analyses, together with results from our pintail model, would estimate overall food and habitat demand for wintering waterfowl in the Sacramento Valley and would directly benefit habitat conservation efforts of the Central Valley Habitat Joint Venture. To improve predictive capability of all models, researchers should obtain accurate and precise estimates of food density in wetlands. Also, improved estimates are needed of AME for basic winter diets, the relative amounts of food obtained from rice fields and wetlands, and the number of pintails (and other granivorous birds) seasonally using various habitats and foods. Collection of additional body mass data for adult and hatching-year pintails could improve precision of estimates of mass and carcass reserve dynamics. Foraging efficiency (food intake/unit foraging time) should be quantified in wetlands and various rice field treatments (Day and Colwell 1998). To better assess available food supplies, managers need estimates of food density below which pintails and other ducks stop feeding (assumed to be about 50 kg/ha in the Mississippi Alluvial Valley; Reinecke et al. 1989). Sensitivity analyses suggest effects of ambient temperature and carcass fat and protein dynamics have lower research priority.

Our difficulty in finding appropriate adjustments to EM for free-living and allometric error implies that species-specific values are needed to improve population food and habitat predictions for pintails (e.g., Weathers et al. 1984). Alternatively, direct estimation of DEE via doubly-labeled water techniques to measure field metabolic rates have proven useful for free-ranging birds (e.g., Obst and Nagy 1992, Piersma and Morrison 1994, Uttley et al. 1994). Similar investigations of pintails and other waterfowl would simplify and reduce uncertainty of population energetics analyses and facilitate integration with resource management; however, gains in precision need to be balanced against the cost for improvements and the scale of management needs.

In addition to improving and testing model input variables, managers need to field-test model predictions to adaptively guide management of wintering habitats for pintails. Managers could seasonally monitor pintail use of wetlands and rice fields to determine how distribution patterns and behavior change relative to estimated food availability at pintail foraging sites. Conservation programs in the Sacramento Valley (Central Valley Habitat Joint Venture 1990) are adding wetlands and flooded rice fields, so more food may be available than in 1980-82. As food supplies increase, previously documented body mass dynamics (Miller 1986b) may change, and pintails may forage less in dry rice. Managers could easily monitor these possible responses. If the continental pintail breeding population recovers to 1970's levels (U.S. Fish and Wildlife Service and Canadian Wildlife Service 1986), additional habitat may be particularly critical to maintain these populations. Flooded rice fields and wetlands provide primary foraging habitats for pintails in dry winters (Miller 1986b), and shortages of these habitats may contribute to declines in pintail breeding populations (Raveling and Heitmeyer 1989).

Managers must continue to integrate rice fields and wetlands in conservation planning to benefit energetic requirements of wintering pintails in the Sacramento Valley (Heitmeyer et al. 1989, Miller et al. 1989). Without commercial rice as a food source, about 10,000 ha of additional wetlands would be required to support the pintail use-days estimated for the wet winter of 1981-82. The needs of other species would increase this amount of wetlands markedly, because the wetland conservation goals of the Central Valley Habitat Joint Venture (1990) assume that rice lands are available to supplement foraging habitat for ducks. Therefore, managers should monitor economic trends that may affect the rice industry. For example, cotton has recently increased in the Sacramento Valley (Cline 1995), locally replacing rice in some areas, and the new strip-harvest technique (Bennett et al. 1993) directly reduces rice seed available to pintails, and the tall residual vegetation may present a physical barrier to foraging (Miller and Wylie 1997, Day and Colwell 1998).


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