Population trends of falconiforms and strigiforms represented in Illinois Christmas bird counts 1955-1979
Paris, Linda M.
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Christmas Bird Count sightings of falconiforms and strigiforms in Illinois from 1955-1979 were assembled and analyzed to determine winter population trends, to compare the trends with reported trends in previous literature, and to explain the trends in terms of environmental or other factors, Frequency of occurrence and density (birds sighted per party-hour and per party-mile) were used to evaluate population trends. Twenty-eight species of falconiforms and strigiforms were reported in Illinois Christmas Bird Counts from 1955-1979, The Boreal Owl, Hawk-Owl, and Ferruginous Hawk were newly reported species after 1955, The Gyrfalcon and Great Gray Owl were not sighted between 1955-1979. The most commonly occurring species in Illinois were the Red-tailed Hawk, Rough-leqged Hawk, Marsh Hawk, American Kestrel, Screech Owl, Great Horned Owl, and Barred Owl. The Red-tailed Hawk was the most numerous and most frequently sighted species. • Winter species were distributed non-uniformly among the northern, central, and southern sections of the state. Southern Illinois contained the highest density of falconiform and strigiform sightings. Some species showed a clinal frequency of occurrence (Sncwy Owls and Saw-whet Owls). Other species were observed only in one section (Black Vultures, Boreal Owls, Hawk-Owls), Since 1955, the composition of winter species has remained essentially identical to the composition from Christmas Bird Counts for 1903- 1955. Christmas Bird Count data continued to show successively lower peaks of hiqh population densities, although a recovery was apparent in the late 1970's. The general population trends for individual species observed in Christmas Bird Counts from 190 3-19 55 change from 1955-1979, Frequency of occurrence increased 1-63% for most species from 1955-1979 levels and relative abundance shifted among the three sections of the state. Human factors including habitat destruction and pesticide use noted in previous analyses may have continued to be the primary factors contributing to the decline of winter falconiform and strigiform populations in Illinois.