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Breeding Season Time and Energy Budgets of the Polyandrous Spotted Sandpiper

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The study was conducted from early May to early August 1975-77 on a 1.6 ha island in north-central Minnesota. All breeding spotted sandpipers were colour-ringed. Time budgets were determined for 10 females and 23 males by instantaneous sampling (at 15-second intervals) of 10 predetermined behaviours. A total of 1,899 30-minute samples (2 birds each) yielded 308,817 data points when birds were visible. Terrestrial arthropod abundance was sampled by cylindrical sticky traps at 12 locations for 48 hours per week. Temperature, wind direction and velocity, and cloud cover were recorded at the time instantaneous samples were conducted. Basal metabolic rates were calculated separately for each sex as a function of body weight. Energy budgets were estimated by extrapolation from time budgets. Caloric and calcium content of principal food items and sandpiper eggs was determined with a Parr adiabatic oxygen calorimeter and a Perkin-Elmer atomic absorption spectrophotometer. During prelaying, females foraged more than males, preened less and flew less. During laying, females foraged, rested, and engaged in agonistic activity more than males. Males spent more time preening, flying, nest building and incubating than their mates. All behaviours differed between sexes during incubation as females spent more time than males in all activities except nest building and incubation. Sex differences were the result of differential strategies to maximize reproductive success and differing energy requirements due to size dimorphism and the energy costs of egg production. Time spent in all behaviours varied among stages of the breeding cycle due primarily to changes in foraging and incubation requirements. Foraging varied by time of day but the pattern of this variation changed among stages of the breeding cycle due principally to diurnal variation in incubation and brooding time. With the exception of the incubation period when only courtship and agonistic behaviour did not vary diurnally, most behaviours did not show diurnal variation. When females helped their mates, nests were incubated a greater proportion of the time than when males incubated alone. Terrestrial arthropod abundance exhibited 2 major peaks each year about 4 weeks apart. Each year a different territory produced the most food over the season as a whole. Time budgets varied among clutches, among territories, and among years but these dif ferences were largely explicable by changes in food abundance. Food abundance consistently influenced foraging time for both sexes. Other activities correlated less consistently with food abundance. Decreases in foraging time, due to higher food levels, were compensated for primarily by increasing time spent in maintenance activities. Temperature, wind, and cloud cover had less influence on time budgets than food abundance, especially for females. Eggs averaged 5.6 Kcal/g dry weight and 1.3 Kcal/g fresh weight. Each egg cost a female 17.8 Kcal to produce. On days of peak egg formation costs, this represented an increase of 94 % (prelaying) to 102 % (laying) in the daily energy expenditure (DEE) of females. For activities other than egg formation, foraging received the most energy expenditure during all stages of the breeding cycle except for males during incubation. Both sexes had minimal DEE's during the incubation period. A clutch of 4 eggs totaled 1.7 times a female's total body calcium. Analysis of principal food items indicated that it was unlikely that females totally relied on these organisms as a calcium source. Energy expended in foraging was typically lower during periods of food abundance than periods of food scarcity. DEE usually increased as food changed from abundant to scarce. Required foraging efficiencies (RFE) decreased as food abundance decreased, indicating that birds had to expend greater amounts of energy to obtain a given quantity of food as food levels dropped. For polyandry to evolve, males must assume most or all of the parental duties. Data of the present study support the hypothesis of EMLEN & ORING (1977) that a male increases his individual fitness by conducting most parental care because the low success rate of individual nesting attempts (due to factors such as high predation rates) places a high priority on the female's ability to provide replacement clutches for the male. A female can produce replacement clutches faster if freed from incubation duties, as this allows her more time to forage. Once male parental care has evolved, sequential polyandry can readily evolve where fewer replacement clutches are required (e.g., where predation rates are less than average) and "surplus" males are available.

Affiliations: 1: Department of Biology, University of North Dakota, Grand Forks, North Dakota, U.S.A.

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