. Bulletin of the British Ornithologists' Club. 7cm-. Figure ia. Grid used for obtaining the index of curvature and index of depth, and the outline of the bill of a crossbill. For further explanation see the text. Figure ib. The grid with the fitted outline of the upper mandible of a crossbill. Distances measured (in mm) and summed to give the index of curvature represented by dotted lines (a-b, c-d, e-f to w-x). ID = distance (in mm) used as index of depth. drawn. For about half the birds it was necessary to reverse the grid as the bill crossed to the other side. From points at 5 mm intervals
. Bulletin of the British Ornithologists' Club. 7cm-. Figure ia. Grid used for obtaining the index of curvature and index of depth, and the outline of the bill of a crossbill. For further explanation see the text. Figure ib. The grid with the fitted outline of the upper mandible of a crossbill. Distances measured (in mm) and summed to give the index of curvature represented by dotted lines (a-b, c-d, e-f to w-x). ID = distance (in mm) used as index of depth. drawn. For about half the birds it was necessary to reverse the grid as the bill crossed to the other side. From points at 5 mm intervals along the 7 cm baseline, starting 1 cm from the 5 cm line, and excluding the final point, the perpendicular distance from the baseline to the outline of the culmen (a-b, c-d, e-f to w-x) was measured in mm. These twelve measurements were then summed to give an index of curvature, 2. figure quantifying the culmen curvature. An index of depth (an indirect measure of upper mandible depth in relation to culmen length) was found by estimating the perpendicular distance of the proximal part of the upper mandibular tomium to the top of the 5 cm line (ID on Fig. ib). From this index of depth, a corrected index of depth (taking into account variations in culmen length) was calculated thus: index of depth X culmen length 70 = corrected index of depth The value of the divisor has no special significance; in this case it is taken from the length of the grid baseline (in mm).. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original British Ornithologists' Club. London : British Ornithologists' Club
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