4.7.5 Tarsus-and-toe
In some non-passerine groups (e.g. waders, rails) a useful additional size measure
is the tarsus-and-toe, taken by holding tarsal joint at right angles (see tarsus), and
placing the tibiotarsus up against the stop of a wing-rule. The foot and toes are
then stroked out flat onto the rule and measurement taken at the tip (see Baker
1993, p.14).
4.7.6 Bill
Bill-size (length, depth, width) can contribute information toward a size PCA
(see above) but expresses much ecological information in its own right, since
variation in bill-size and shape constrains the diet that the bird can take. Because
of its ecological importance, it is often found to correlate poorly with overall
body size within species. The exact measurement to take depends on the type
of bird because bill-morphology varies too much between taxa to give a single
recommendation. In general, bill-length is taken with ID (inside diameter) dial
or Vernier from the bill’s base at the skull (naso-frontal hinge) to the tip (dertrum)
(Figure 4.6). With practice, this can be taken repeatably in small birds to 0.05 or
0.1 mm. In waders and similar taxa, the measurement should be taken to the
feathering, which is clearly demarcated, rather than to the skull, and in species
with a cere (e.g. raptors) the measurement is taken to this rather than the skull.
Special care must be taken in placing the calliper at the proximal end of the bill
due to its proximity to the bird’s eye. For taxa with complex (e.g. highly curved)
bills it may be necessary to use or devise other measures such as from the tip to
the nostril, and close-up photography may be helpful for taking complex
measurements such as curvature of the culmen (e.g. Gosler 1999).
Bill-depth is ecologically a highly significant trait. It is taken at right-angles to
the cutting edges (tomia) of the mandibles, specifying where along the bill it is
taken. The typical reference point is at the deepest point of the gonys (the distal
portion of the bill where the two lateral arms (rami) of the lower mandible are
fused). In gulls and some seabirds, the bill-depth at the gonys is sexually dimorphic
and a good indicator of body-size, but it is a useful and repeatable measure in
many other taxa. It should be taken with the OD-calliper of a Vernier to 0.05
or 0.1 mm in small birds. Do not apply excessive pressure but take care that the
bill is closed (and the upper mandible not retracted) when taking the metric.
When taking this measurement on waders, remember that the bill surface is
sensitive to touch.
The ratio of bill-depth/bill-length is known as the bill-index. This gives a
useful measure of bill shape (relative bill-depth), which is ecologically relevant in
108 |Birds in the hand