Clinal variation in the relative proportions of red and gray plum- age phases in Screech Owls (Otus asio) was analyzed by Owen (1963) and Marshall (1967). This variation was well known prior to Owen's work, but was misinterpreted (Baird, et al. 1874, Hasbrouck 1893, Allen 1893).]

Laurel VanCamp and Charles Henny (MS) have 30 years of data on a northern Ohio Screech Owl population. They observed an over- winter decline (from about 25% to 15%) in the proportion of red phase birds in the winter of 1951-52. This decline was correlated with a severe winter of above normal snowfall and below average temperatures. They examined banding and recovery data and found overwinter survival of red and gray birds to be the same except for this one severe winter when 44% more red phase birds were lost than grays (VanCamp and Henny MS). Differential mortality was reported by Gullion and Marshall (1968) for red and gray phase Ruffed Grouse (Bonasa umbellus) where snow conditions for roosting is apparently the critical factor for grouse overwinter survival and is related to predation. Snow- roosting has not, to our knowledge, been observed in Screech Owls. VanCamp and Henny (MS) discuss the observations of Ruffed Grouse and Screech Owls and suggest that possible thermoregulatory differences between red and gray phase birds could account for differential overwinter survival.

Our objective was to test for differences between color phase in oxygen uptake at several ambient temperatures. We hypothesized that oxygen uptake would be greater by red phase birds, especially at lower temperatures.