Thermal ecology and ecological energetics of California spotted owls

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The Condor




In this study, we used open-circuit respirometry and the doubly labeled water technique (DLW) to examine the thermal ecology and ecological energetics of California Spotted Owls (Strix occidentalis occidentalis). Our physiological and behavioral observations indicated that Spotted Owls are less heat tolerant than typical birds. At temperatures above the thermoneutral zone (18.2–35.2°C), resting metabolic rate increased 1.48 times faster than predicted allometrically, and behavioral responses to heat stress (increased breathing rate, ptiloerection, gaping, and wing drooping) occurred at relatively modest temperatures, 30–34°C. Our data support the hypothesis that Spotted Owls prefer old-growth and late seral stage forests because they provide favorable microclimates. Our metabolic measurements reveal that Spotted Owls have exceptionally low energy requirements. Their basal metabolic rate, 10.13 ± 0.46 J g?1 hr?1, is only 82% of that predicted allometrically for owls. Field metabolic rate (FMR) of five adults provisioning dependent young averaged 249 ± 60 kJ day?1, only 34% of that predicted for comparably sized non-passerine birds. We calculated Spotted Owl prey requirements from our FMR data, laboratory determinations of assimilation efficiency (77%), and the body composition of representative prey types. On average, Spotted Owls feeding young can meet their own energy needs by consuming one northern flying squirrel (Glaucomys sabrinus) every 1.8 days or one woodrat (Neotoma fuscipes) every 3.7 days.