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

Discussion


Daily Energy Expenditure

Critical periods of energy expenditure for wintering pintails included early fall (Aug-Nov), a period of rapidly increasing DERfood; midwinter (Nov-Jan), a period of lowered DERfood and increased DERreserves; late winter (Jan-Feb), another period of increased DERfood; and February-March, a period when DEE and DERfood decreased somewhat in the wet winter and markedly in the dry winter, and DERreserves increased during the dry winter only. The warm August-November period had low thermoregulatory costs (Dawson and O'Connor 1996), and expanding wetlands may have reduced costs of searching for profitable foraging areas. Increasing pintail populations and increased body mass associated with increased food intake by individual pintails (Miller 1986b) contributed to the growing demand for energy. In contrast, midwinter periods were relatively cold, probably at or near lower critical temperatures (Kendeigh et al. 1977: Eq. 5.12), with highest thermoregulatory costs. Extent of wetlands and flooded rice fields and pintail abundance peaked coincidently during midwinters. However, body mass and food intake of individual pintails declined as energy-demanding activities such as courtship increased (Miller 1985, Miller 1986b). Also at this time, pintails relied on body reserves to support DEE, especially in the dry winter. The January-February period was marked by reduced DERreserves and increased food intake and body mass. During February-March, just prior to spring migration, warmer temperatures reduced thermoregulatory costs, pintails reduced food intake (especially in the dry winter), and DERreserves increased (dry winter). Thus, variation in DEE among periods reflected different bioenergetic costs associated with specific activities (Bryant et al. 1985). Variation between winters reflected wet and dry habitat conditions, suggesting that reduced habitat in the dry winter may have limited foraging opportunities in midwinter and prior to spring migration (Miller 1986b).

Mean DEE for pintails in the Sacramento Valley was 818 kJ/day, which exceeded estimates for pintails in Yucatan, Mexico (614 kJ/day; Thompson and Baldassarre 1990), the slightly smaller Eurasian wigeon (Anas penelope) in Britain (630 kJ/day; Mayhew 1988), and the larger American black duck (A. rubripes) in Virginia (682 kJ/day; Morton et al. 1989). Pintails in the Yucatan inhabited a warmer environment than in California, which would have reduced basal and DEE expenditures (Kendeigh et al. 1977, Dawson and O'Connor 1996). Additionally, fecal collection and time budget methods used in these other studies may have underestimated energy budgets (Weathers et al. 1984, Nagy 1989). Time-budget methods assume no growth in body mass, but fat synthesis had a pronounced effect on period DEE of pintails in the Sacramento Valley. Also, mean body mass peaked in February-March, but the late peak in DEE was in January-February, demonstrating the importance of including carcass fat dynamics in bioenergetics models, and that body mass alone did not accurately predict DEE.

Apparent "fasting capacity," the basic survival benefit of stored fat in birds (Rogers and Smith 1993), indexes energy reserves when DEE is known. Pintails encounter short-term (several days) foraging limitations in the Sacramento Valley when wetlands and flooded rice fields freeze. Given 37.66 kJ/g for oxidized fat and a mean DEE of 1,067 kJ/day for males and 943 kJ/day for females during December-February, we estimated that carcass fat of fasting pintails (175 g in males and 205 g in females in Jan; Miller 1986b) would have lasted 6 days for males and 8 days for females in the wet winter of 1981-82, and 4 days for males and 6 days for females (120 g of fat for males and 150 g for females) in the dry winter of 1980-81. These times are remarkably similar to the 5-8 days for pintails in Yucatan, Mexico, which had less carcass fat and lower DEE (Thompson and Baldassarre 1990). Thus, individual benefits of carcass fat to withstand food limitations must be assessed not only relative to maintenance costs (Rogers and Smith 1993, Witter and Cuthill 1993) but also relative to DEE under local environmental conditions.

Independent effects of free-living and allometric error adjustments (i.e., 1.00-1.50 × EM) had moderate influences on model results. However, these additive adjustments had an important combined effect. No empirical estimates of free-living adjustments exist for wintering pintails, and we verified allometric error only with pintail feeding trial data. Kendeigh et al. (1977) suggested a free-living adjustment of 7-10% for small passerines, and 10% was adopted by Reinecke and Krapu (1986) for sandhill cranes (Grus canadensis). Owen (1969) found that EM of wing-clipped blue-winged teal (Anal discors) held captive outdoors was 17% higher than for caged conspecifics. Because flight is the most energetically costly activity (Norberg 1996), free-living costs of wild blue-winged teal would exceed that of wing-clipped ducks. Flight potentially makes the greatest contribution to DEE (Masman et al. 1988), and flight is an important winter activity of pintails (Miller 1985). Cain (1973) increased EM by 25%, largely to account for flight, when modeling free-living energetics of black-bellied whistling ducks (Dendrocygna autumnalis). Additionally, the waddling walk of waterfowl requires more energy than other forms of avian ground locomotion (Robbins 1993:133), and waddling would be minimal in small cages where EM is measured. Clearly, adjustments specific to pintails are needed and, until such refinements are available, our model should be used only where predicted food and habitat estimates are acceptable within relatively broad confidence intervals.

Population Food and Habitat Requirements

Although DEE peaked in September-November and again in January-February, the greatest population demand for food occurred in December-January when pintails were most abundant and body mass was low or moderate. This asynchrony occurred in the fall because although food demand of individuals was high, populations were low; however, in December-January, lower individual energy requirements were masked by the cumulative energy demands of large populations. Thus, DEE did not necessarily predict population energy requirements, and if individual requirements had coincided with peak populations, considerably more energy and habitat would have been required.

In the Sacramento Valley, pintails required 34,000 ha (16.3%) of harvested rice fields in the dry winter of 1980-81 and 41,500 ha (18.6%) in the wet winter of 1981-82. However, farmers may have plowed 20% of the rice, leaving little food (Miller et al. 1989); thus, required areas of rice would actually have been 20% (dry winter) and 23% (wet winter) of unplowed harvested fields. In contrast, pintails used only 5.6% of wetlands in the dry winter and 9.0% in the wet winter. Heitmeyer (1989) calculated Central Valley habitat restoration goals (Central Valley Habitat Joint Venture 1990) so that waterfowl would obtain 78% of DERfood from wetlands. This estimate contrasts with our empirically based model in which pintails obtained >75% of DERfood from rice. Extensive conversion of rice fields to wetlands could potentially reverse this difference, but only if pintails would select wetlands over rice fields for foraging.

As modeled, food resources in the Sacramento Valley adequately maintained wintering pintail populations. However, large populations of other waterfowl, other waterbirds (e.g., American coots [Fulica americana]), and land birds (e.g., blackbirds [Agelaius spp., Euphagus cyanocephalus]) used these habitats and food resources (Cruse and DeHaven 1978, Heitmeyer et al. 1989, Day and Colwell 1998), potentially imposing foraging constraints. Therefore, without bioenergetics analyses involving other wintering avifauna, our results cannot be used by themselves to suggest that Sacramento Valley wetlands and rice fields were adequate to maintain wintering pintail populations, either in 1980-82 or presently.

Additionally, sensitivity analyses emphasized that larger wintering pintail populations and reductions in rice density would have major consequences for food demand. For example, a decrease of 50% in baseline rice density (assuming increased harvest efficiency in the future), accompanying a doubled pintail population (assuming continental population objectives are met), would result in a predicted requirement of about 150,000 ha of rice fields. This estimate is 4-5 times our results and, represents 67-72% of all hectares commercially harvested in 1980-82 (and a similar percentage of the 194,000 ha of rice harvested in 1997-98; Decker 1998). In contrast, only 40,000 ha of rice fields would be required for pintail populations 50% below baseline (Fig. 7), which is similar to pintail abundance in 1997-98 (U.S. Fish and Wildlife Service, unpublished Midwinter Inventory results). In each instance, pintails required markedly less area of wetlands than rice fields, because of higher food density in wetlands and smaller pintail populations in August-October when pintails used only wetlands (specified model input).

Food density had a marked influence on model outcomes. Heitmeyer (1989) and Heitmeyer et al. (1989) estimated that 750-950 kg/ ha of food in wetlands would support wintering waterfowl in California. Spatial and temporal variation in food density is poorly understood in California wetlands, however, so these estimates remain speculative. Waste rice is apparently present at higher densities in California than in rice-growing regions of the Southeast (Reinecke and Loesch 1996). As a result, rice may be more important to pintails in the Sacramento Valley than in the Southeast where wetlands are more extensive (Chabreck et al. 1989, Reinecke et al. 1989).

At night, until winter rains puddled dry fields, pintails fed almost exclusively in rice fields flooded by managers for duck hunting (Miller 1985, 1987). However, flooded rice fields provided only about 15% (31,200-33,500 ha) of total rice land available in 1980-82 (Miller et al. 1989). This area was about 92% of that required to meet pintail population energy requirements in the dry winter of 1980-81 and 81% in the wet winter of 1981-82. Therefore, pintails eventually would have had to forage in dry (1980-81) and rain-puddled (1981-82) rice fields. However, extent of flooded rice and wetlands has increased substantially in the Sacramento Valley in the 1990s, and one-third or more of total harvested rice land may now be flooded annually (Central Valley Habitat Joint Venture. 1996. Hunting success in the Central Valley, California, during 1995-96 compared with previous years, unpublished. Central Valley Habitat Joint Venture Special Technical Report 1, Sacramento, California). Hence, pintails may no longer need to forage in dry or puddled rice fields, but this hypothesis needs to be tested.

Little is known of the relative energetic costs and benefits to pintails of locating and foraging in dry and flooded rice fields, and how this dynamic might affect population energetics during winter. Maximum esophageal capacity of a pintail is about 45 g (seeds; M. R. Miller, U.S. Geological Survey, unpublished data). This amount is less than the estimated daily food requirements after August-September; thus, 2 daily feeding flights to dry fields (Baldassarre and Bolen 1984) or extended foraging in flooded rice may have provided predicted food intake. Pintails foraged in dry rice fields only intermittently in the Sacramento Valley, usually in early fall when few flooded fields existed, and in late winter (Feb) in the absence of rain-filled fields (M. R. Miller, U.S. Geological Survey, unpublished data). Typically, pintails flew to flooded fields after sunset and returned to diurnal roosts on refuges in the morning (Miller 1985). Pintails in the Playa Lakes Region of Texas favored flooded moist-soil foods in wetlands (when available) even though waste corn was available in dry harvested fields nearby (Baldassarre and Bolen 1984).


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