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

Results

Daily Energy Expenditure

Over the 2 winters, DEE period means ranged from 794 to 1,180 kJ/day for males and 700 to 1,044 kJ/day for females (Table 1). There was a general pattern of DEE seasonally, with highest values in September-October or October-November and again in January-February, and lowest values in December-January of the dry winter and November-December of the wet winter (Fig. 1). Body reserves contributed more and over a longer period to DEE in the dry than the wet winter (Table 1, Fig. 1). Although DER_reserves increased during November-December, December-January, and February-March in the dry winter, and in November-December of the wet winter, DER_food provided the greatest contribution to DEE during both winters (Fig. 1).

GIF-DEE of ducks in winter in California

Fig. 1. Daily energy expenditure (DEE), daily energy required from food (DER_food; gray-shaded areas), and daily energy required from body reserves (DER_reserves; cross-hatched areas) of adult male and female northern pintails during a dry (1980-81) and a wet (1981-82) winter in the Sacramento Valley, California.

Pintails acquired energy from catabolism of fat reserves during November-December in both winters and again in December-January and February-March of the dry winter (Fig. 2). During mass loss in November-December, carcass fat contributed 4.5-5.6% of DEE for males and 6.1-7.8% for females. Pintails did not need DER_reserves during December-January of the wet winter; however, in the dry winter, fat contributed 11.6% of DEE for males and 6.8% for females. During February-March of the dry winter, fat catabolism contributed 3.9% (males) to 6.2% (females) of DEE, but fat reserves were not required by either sex during this period in the wet winter. Protein catabolism contributed ≤1.1% of DEE; also, costs for protein synthesis were ≤1.6% of DEE (0.3-16.2 kJ/day) compared with up to 18.5-22.4% of DEE for fat synthesis (217.8-223.2 kJ/day). The DEE of males normally exceeded that of females, reflecting males' greater body size and fat synthesis (Appendix A); however, in October-November 1981, a large increase in body fat caused DEE of females to exceed that of males (Table 1).

GIF-Energy costs for ducks in winter in California

Fig. 2. Energy cost for fat synthesis during periods of body mass gain, and energy contributed by catabolism of fat during periods of body mass loss of adult male and female northern pintails during a dry (1980-81) and a wet (1981-82) winter in the Sacramento Valley, California.

Daily Food Intake

Estimated daily food intake of individual pintails averaged 5.9-8.3% of body mass for males and 6.0-8.1% for females, and intake varied by period, reflecting the pattern of variation in DEE (Table 1). Food intake from wetlands and rice fields was highest during September-November and January-February (Fig. 3), ranging up to 82 g/day for males and 71 g/day for females in the dry winter of 1980-81, and 78 g/ day for males and 73 g/day for females in the wet winter of 1981-82 (Table 1). Following the pattern set by input variables, wetlands provided 60-74 g/day during September-October, but pintails obtained 46-77 g/day from rice fields as modeled food intake from wetlands declined (Table 1). Lowest daily food intake occurred during August-September (both winters), December-January (dry winter), and November-December (wet winter). The DER_reserves increased in the dry winter during December-January and February-March, reflecting decreased food consumption (Table 1).

Fig. 3. Patterns of individual and population food intake of northern pintails by monthly periods in the Sacramento Valley, California; years 1980-81 and 1981-82 and sexes combined.

Population Food Intake

The bimodal pattern of food consumption by individual pintails was not reflected by population food intake. Instead, population food intake increased steadily with pintail abundance and peaked in December-January when our model limited most food consumption to rice (>1.4 million kg; Fig. 3, Table 2). In contrast, population food intake from wetlands was greatest during September-October, coinciding with rapid increases in the number of males and low availability of alternative habitats. Population food intake reflected annual variation in use-days and was greatest in the wet winter of 1981-82. On an annual and period basis during October-March, markedly more food was obtained from rice fields than wetlands (Table 2), which reflected model constraints.

Foraging Habitat Required

The area of wetlands and rice fields required for pintail foraging was positively related to population food intake, but this relation was different in early fall when the model specified that most pintail food was obtained from wetlands (Fig. 4). Predicted area of habitat was greater during November-January of the wet than dry winter, and more total habitat was used in the wet (2,100 ha of wetlands, 41,500 ha of rice fields) than dry winter (1,800 ha of wetlands, 34,000 ha of rice fields; Table 2).

GIF-Patterns of food intake and hectares of habitat.

Fig. 4. Patterns of population food intake and hectares of foraging habitat (wetlands and rice combined) required by monthly periods by adult northern pintails in the Sacramento Valley, California; years 1980-81 and 1981-82 and sexes combined.

Sensitivity Analyses

Maximum variation in food density, AME, proportion of food obtained from wetlands and rice fields, and pintail abundance caused a >50% change in area of foraging habitat needed to satisfy population food intake (Fig. 5). Changes in body mass and adjustments for free-living and allometric error, acting independently, only moderately (<30%) influenced predicted extent of wetlands and rice fields. Ambient temperature, energy costs of fat and protein synthesis, and fat and protein catabolism, when varied ą50% in the model, caused a <10% change in area estimates.

GIF-Percent change in area to support DEE

Fig. 5. Percent change in area (ha) of wetlands and rice required by adult pintails (sexes and years combined) to support daily energy expenditure (DEE) relative to changes in model input variables and constants of -50 to 50%.

With pintail use-days held at different levels, predicted area of rice fields and wetlands required to supply food decreased curvilinearly as we increased food density up to 50% in rice fields and wetlands combined (Fig. 6). Also, elevation and slopes of prediction lines decreased proportionately as use-days declined. Percentage change in food density had a much greater effect on area of rice fields than wetlands because of greater food density in wetlands. With food density held constant, increasing pintail use-days caused area of rice fields and wetlands to increase curvilinearly because larger pintail populations occurred later in the winter when rice predominated in modeled diets (Fig. 7).

GIF-Area of wetlands and rice required by pintails.

Fig. 6. Area (ha) of wetlands and rice required by adult northern pintails (sexes and years combined) to support daily energy expenditure (DEE) when pintail use-days were held constant at values of -50, -25, baseline, 25, 50%, and food density was varied from -50 to 50%.

GIF-Area of wetlands and rice required by pintails.

Fig. 7. Area (ha) of wetlands and rice required by adult northern pintails (sexes and years combined) to support daily energy expenditure (DEE) when food density was held constant at values of -50, baseline, and 50%, and pintail use-days were varied from -50 to 100%.

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