Thermal ecology and ecological energetics of California Spotted Owls

In this study, we used open-circuit respirometry and the doubly labeled wat er technique (DLW) to examine the thermal ecology and ecological energetics of California Spotted Owls (Strix occidentalis occidentalis). Our physiolo gical and behavioral observations indicated that Spotted Owls are less heat tolerant than typical birds. At temperatures above the thermoneutral zone (18.2-35.2 degreesC), resting metabolic rate increased 1.48 times faster th an predicted allometrically, and behavioral responses to heat stress (incre ased breathing rate, ptiloerection, gaping, and wing drooping) occurred at relatively modest temperatures, 30-34 degreesC. Our data support the hypoth esis that Spotted Owls prefer old-growth and late seral stage forests becau se they provide favorable microclimates. Our metabolic measurements reveal that Spotted Owls have exceptionally low energy requirements. Their basal m etabolic 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 provisi oning dependent young averaged 249 +/- 60 kJ day(-1), only 34% of that pred icted for comparably sized non-passerine birds. We calculated Spotted Owl p rey requirements from our FMR data, laboratory determinations of assimilati on 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.