Abstract
Wing, bill, and mass measurements of eastern (Otus asio) and western (O. kennicottii) screech-owls identify them, their subspecies, and sexes. The eastern screech-owl is the more massive, shorter winged, and longer billed. Females of both species are larger than males. Subspecies follow Marshall (1967), except that O. a. naevius and O. k. macfarlanei are reinstated, and O. k. cardonensis is synonymized with O. k. xantusi. Body size is clinal from large in the north to small in the south in both species. Increasing sexual dimorphism in bill length correlated with decline in mass might reflect increased subdivision of the food niche related to increased competition in more dense populations and among more coexisting insectivorous owls southward. Eastern and western screech-owls coexist in southeastern Colorado, west-central and Trans-Pecos Texas, and northern Chihuahua and Coahuila, where their bill-length differences are 2.3 times greater than among allopatric populations. Interspecific competition is suggested by such character displacement, habitat segregation, and vocal mimicry. The 2 species seem to have mostly separate evolutionary histories, although occasional hybridization indicates continuing speciation.
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