Roost Selection by Spotted Owls: An Adaptation to Heat Stress

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Abstract
The microclimate at the summer roosts of spotted owls (S. occidentalis) was investigated as a factor contributing to the species'' narrow range of habitat. Habitat features surrounding the roosts were quantified; a dense canopy above the roost and north-facing slopes were characteristic of all sites. The combined physical features created a microclimate that was 1-6.degree. C cooler than that of more open areas. The owls showed signs of heat stress when temperatures reached and exceeded 27-31.degree. C. The microclimate at summer roosts was effective in reducing the extent to which the owls used gular flutter. Winter roosts differed markedly from summer roosts, providing additional evidence supporting the importance of summer roosts to spotted owls. The owls'' apparent intolerance to high temperatures was, at least in part, related to their having plumage as thick as that of boreal-zone owls. Such plumage appears to be an adaptation for enduring periods of winter stress. Selecting cool summer roosts may be a behavioral adaptation to compensate for the owls'' observed inefficiency in dissipating body heat.

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