The Behavioral and Physical Development of Barred Owl (Strix Varia) Nestlings in Illinois

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Varchmin, Thomas Edward Michael
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Western Illinois University
The behavioral and physical development of 22 Barred Owl (Strix varia Barton) nestlings in 9 nests in McDonough County, Illinois, was studied from March to May in 1973 and 1974. Five eggs and one embryo were measured and weighed. Hatching, fledging, and mortality information was recorded. Physical development was studied by comparative measurements of culmen, claw 3, a central rectrix, primaries 9 and 10, and weight, and by noting change in egg tooth and development of body plumage. Eighteen nestlings were measured to within 3 days before fledging, and 7 owls were captured by hand up to 10 days after fledging. Weight in relation to fledging for 9 owls was analyzed. Behavioral development was observed by climbing to the nest cavity or by watching at a distance from a blind. Sonograms of 3 nestling calls were made. Food habits were determined by analysis of collected prey remains and regurgitated pellets. In 1973, 13 eggs hatched between 6 April and 9 April, and 12 owls fledged between 3 May and 10 May. In 1974, 9 eggs hatched between 28 March and 3 April, and 6 owls fledged between 26 April and 4 May. Twelve out of 18 owls fledged 31 days after hatching. Five owls died from various causes. An important behavioral development was the acquisition of fear and its gradual replacement by active hostility. The period in the nest just prior to fledging involved much exercise and activity. A positive relationship existed between development of the juvenal feathers and weight. The correlation coefficient of the relationship was 0.82 and was significant at the 0.001 level. Seven owls captured up to 10 days after fledging had an average weight loss of 27 g. The diet of the nestlings consisted of 47.9 percent birds, 29.3 percent mammals, 14.2 percent invertebrates, 7.9 percent fish, and 0.7 percent amphibians.