Parental Expenditure of Time and Energy in the Abert's Towhee (Pipilo aberti)

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Abstract
Daily activity budgets of male and female Abert's Towhees (Pipilo aberti) were measured in 1980 during the reproductive cycle and then converted into energy budgets to determine the relative demands of different nesting phases. Mean daily energy expenditures (DEEtot) in males reached a seasonal maximum of 126.7 kJ, or 2.16 times basal metabolic rate (BMR), in the prenesting phase. Thermoregulatory demands accounted for higher energy expenses in this phase. The cost for females was also high in the prenesting phase but was exceeded by DEEtot in the egg-laying phase. Average DEEtot was at a seasonal minimum of 103.8 kJ, or 1.8 times BMR, in incubating females. Females conserved energy in the incubation phase by minimizing activity, and 78% of the variation of DEEtot in individual females was explained by mean daily air temperature. Activity costs DEEact differed significantly between males and females during the incubation phase, but DEEtot did not. Mean DEEact and DEEtot were significantly greater in females during the nestling period than during incubation. DEEact explained 58% of the individual variation in female DEEtot in the nestling phase, whereas air temperature explained only 28% of the variation. Interphasic variation in DEEact was wider in females than males, because females closely tracked offspring requirements by attending at the nest. Males did not incubate or brood, and spare time spent "loafing" tended to minimize DEEact. Excluding the egg-laying female, estimates of DEEtot in individual towhees ranged from 1.6 to 2.18 times BMR and compare well with estimates of DEEtot in the Northern Mockingbird (Mimus polyglottos), which is about the same size.