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Heating eggs during incubation may be relatively energetically costly, affecting the outcome or number of breeding attempts. We determined the effect of reduced egg heating costs on nest attendance, change in body mass, and daily energy expenditure (DEE using the doubly labeled water technique) by heating nests of pectoral sandpipers. We also considered ground temperature, which may influence overall incubation costs, and mass reserves and stage of incubation, which may influence an individual's ability to respond to changes in overall incubation cost. The total proportion of time spent in attending the eggs was significantly greater in nests that were experimentally heated (3.6% or 52 min daily), and this effect was significantly greater at low ground temperatures (14.7% or 211.7 min daily). Mass change was independent of experimental heating when controlling for attendance, although mass loss rate was greater for birds that attended more (for every 10% increase in daily proportion of attendance 0.12 extra grams of body mass were lost per hour), and overall daily attendance increased by 0.5% for every extra 1 g of body mass. DEE was greater for birds that had the higher rates of mass gain (for every 0.1 g of mass gained per hour, DEE increased by 20.5 kJ per day) but was independent of experimental heating when controlling for attendance. Overall, the results suggest that females are constrained from attending more by their energy reserve levels being depleted at least partly by the costs of egg heating, but these costs probably do not determine DEE, as costs off the nest may far exceed those incurred while sitting. Breeding in the arctic is clearly energetically demanding: pectoral sandpipers had an average DEE of 361.1  8.9 kjd-super- - 1, a mean power output of 4.1 W, equivalent to 6.1 times basal metabolic rate (n = 24 birds). Copyright 2004.daily energy expenditure; incubation; nest attendance; pectoral sandpiper

The energetic costs of egg heating constrain incubation attendance but do not determine daily energy expenditure in the pectoral sandpiper