Winter body mass and measurements of Trumpeter Swans Cygnus buccinator
Morphological measurements and body mass were recorded for Trumpeter Swans captured in Idaho and Montana in the United States during a range restoration programme. Although 887 Trumpeter Swans were captured during four winters (November to mid-January 1990-91 to 1993-94), sample sizes varied for different measurements due to limited time and personnel. Mean body mass varied among sex/age classes and among years. Mean body mass (kg) for adult and cygnet males was 11.9 and 9.9, and for females was 10.3 and 8.7. Annual variation in body mass was directly related to mean air temperature during the four trapping periods. An index of body condition followed the same pattern as body mass, being lower in colder winters. Structural measurements included three bill measurements, tarsus, and mid-toe. ANOVA of the first principal component scores (based on bill 2 and mid-toe length) indicated that structural size varied among sex/age classes, but not among years. A discriminate function based on mid-toe length correctly classified the sex of 67% of females and 79% of males. The rate of infestation with nasal leeches (x=12%) was highest in cold winters and was probably related to body condition; the lightest swans had the highest infestation rate. Interpopulation comparisons of Trumpeter Swans using morphological measurements could be improved by better definition of measurement techniques. Tip of bill to anterior edge of nares (bill 1) measurement _50 mm for adults and _48 mm for cygnets will separate most Trumpeter from Tundra Swans.
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