Size| 109(a)(b)Fig. 4.6Measuring bill-length (a) and depth (b) of a Great Tit. The bill-length is
taken with the “ID” calliper, the bill-depth with the “OD” calliper. Both metrics can be
taken reliably (i.e. repeatably) to 0.1 or even 0.05 mm with Vernier or dial callipers.
Variation within and between species in bill-size and shape is ecologically important.
(Photo: Andrew G. Gosler)