Biometrics of the siskinCarduelis spinus

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Abstract
This paper describes some measurements of Siskins caught during an invasion into the Stroud Valleys, Gloucestershire in the early months of 1984. For each of the seven characters considered (wing, tail and tarsus length, bill depth, width and length, and weight) the mean, standard deviation and range are tabulated for first‐year and adult, males and females. The extent of wing and tail feather abrasion and the number of unmoulted greater coverts in first‐years are also given. A small but significant sexual dimorphism was apparent in six of the seven body measurements (wing length, tail length, bill width and weight in the period 16 March ‐ 15 April, and bill length and bill depth in first‐years only), whereas age dimorphism was appreciable for only wing length, and bill depth in females. The significance of dimorphism in body measurements and their variability is discussed. A marked diurnal variation in weight was found, weight increasing in the first 6–8 h after sunrise at an average rate of 0.24 gh ‐ 1 and maximum weight being at least 15% greater than the minimum. Even more pronounced were the weight gains averaging about 2 g in the period 16 March ‐ 15 April immediately before migration. Together with an additional ca 0.5 g of “reserve” fat which the birds carry in the winter months it is estimated that this would enable the birds to fly 500–1000 km. Scottish breeding birds would, therefore, have been able to return to their breeding grounds in one nonstop flight, whereas birds returning to Fenno‐Scandia or the Baltic States would have needed to stop and refuel en route, probably in either the Low Countries or Scotland.