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Journal Article

Daily Allocation of Time and Energy of Western Bluebirds Feeding Nestlings

Patrick J. Mock
The Condor
Vol. 93, No. 3 (Aug., 1991), pp. 598-611
DOI: 10.2307/1368192
Stable URL: http://www.jstor.org/stable/1368192
Page Count: 14
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Daily Allocation of Time and Energy of Western Bluebirds Feeding Nestlings
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Abstract

The daily allocation of time and energy of adult Western Bluebirds (Sialia mexicana) feeding 9-12 day-old nestlings was measured using time-activity budget and doubly-labeled water methods. Components of the bluebird energy budget were estimated by extrapolating laboratory measurements of metabolic heat production to field conditions using a linear heat-transfer model and the operative temperature and wind speed experienced by the free-living bird. This model was validated with the doubly-labeled water method. Adult Western Bluebirds feeding nestlings are neither food-limited nor working maximally. Adults fulfilled their own energy requirements, and those of four to six nestlings, while actively pursuing prey for less than 10% of their active day. The daily energy requirement of a 27.5 g adult bluebird averaged 95 kJ/day (n = 52), which is 2.6 times its nighttime basal metabolic rate. Thermostatic requirements were the most variable components of the energy budget (CV = 41.6%). Differential use of microhabitats by individuals accounted for some of this variability. Activity costs were less variable (CV = 17.6%) and accounted for less than 20% of the total energy budget. Differences in brood size resulted in a minor difference in activity costs (3kJ/day) between broods of four and six. Variation in thermostatic costs overshadowed this small brood-size effect, resulting in no significant brood-size effect on total daily energy expenditure. This study suggests that bluebirds, like several other temperate passerine species, are able to regulate their overall daily energy expenditures within definable limits through differential use of thermal environments and activity budgets. I conclude that potential food limitations on reproductive output are more likely to occur at other stages of the breeding cycle due to greater variability in weather and food conditions compared to the nestling stage.

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