The distinct rufous and gray color-morphs of the Eastern Screech-Owl (Megascops asio) have long intrigued ornithologists. Eastern Screech-Owls range across the eastern half of the United States, where the morph ratio varies from >70% rufous birds in the center of the range to <30% rufous at the periphery. Although the adaptive significance of polymorphism is, in general, poorly understood, careful mapping of the morph-frequency and determination of ratio clines is a first step toward understanding this phenomenon. In this paper, I examine the distribution of Eastern Screech-Owl color-morphs in Iowa, which lies in the northwestern part of the species' range. I collected 519 records of Iowa screech-owls from museum curators, wildlife rehabilitators, bird-banders, and bird watchers. For all records combined, 41.0% were rufous morphs, 55.7% were gray morphs, and 3.3% were intermediates. For all birds of known sex, 44.7% (21/47) of males and 57.6% (34/59) of females were rufous; the difference was not statistically significant, but the results are consistent with a trend in other studies suggesting that females are more likely than males to be rufous. The percentage of rufous screech-owls increased clinally from west to east across Iowa, but did not change from north to south. Because annual precipitation exhibits a longitudinal pattern in Iowa and annual temperature exhibits a latitudinal pattern, this suggests that the morph ratio pattern in Iowa is influenced more by precipitation than by temperature. Based on records dating 1880–2003, the percentage of rufous birds decreased from the early to later decades of the 20th century. The range-wide distribution of morph ratio clines fits a model where rufous morphs are better-adapted at the center of the range but gray morphs are selected for toward the periphery.
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Vol. 126 • No. 2