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Body Weight and Juvenile Mortality in Rooks Corvus frugilegus
I. J. Patterson, G. M. Dunnet and S. R. Goodbody
Journal of Animal Ecology
Vol. 57, No. 3 (Oct., 1988), pp. 1041-1052
Published by: British Ecological Society
Stable URL: http://www.jstor.org/stable/5110
Page Count: 12
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(1) Seasonal, year-to-year and other variation in body weight was investigated in a large sample of rooks caught in baited traps in 1964-73 near Aberdeen, Grampian, U.K., as part of a large-scale programme. (2) The aim of this paper is to test whether low body weight in rooks Corvus frugilegus in summer is associated with dry warm weather and with increased population density, both of which might decrease food abundance, and whether low body weight in juvenile rooks is associated with increased mortality. (3) Adult birds were lighter in summer than in spring and juveniles remained lighter than adults from May until August, confirming previous suggestions of food shortage for rooks in summer. (4) Trapped rooks were significantly lighter than those shot in the same study area in winter and spring, possibly because the baited traps tended mainly to catch birds in poor condition, but there was no such difference in summer when all rooks were probably short of food. (5) Year-to-year variations in mean weight in March and June were not related to temperature or rainfall, suggesting that there was no short-term change in weight in relation to food availability, or that these weather variables were not good predictors of rook food supply. (6) Mean weights of adult males and females in June declined similarly as population density increased during the study; recaptured juveniles lost weight over the summer at a rate which also increased with population density, possibly indicating greater competition for food with larger numbers of rooks in the area. (7) Juveniles which survived their first summer were significantly heavier when first caught than those which disappeared, indicating that low body weight was associated with increased mortality.
Journal of Animal Ecology © 1988 British Ecological Society