some species. Bill-width is more rarely used than the above metrics because of the
difficulty in defining where, along a tapering object, to take the measurement:
it is usual to do so at the gape (but beware pulli and recently-fledged juveniles
with a gape flange), and at the gonys.
4.7.7 Total-head
The distance from the bill-tip to the center of the back of the skull (known as
total-head or head-and-bill measurement) has been found to give a reliable meas-
ure of overall body size in certain non-passerine taxa (especially waders, some
seabirds, especially gulls, and wildfowl, especially swans). The metric should be
taken with the OD-calliper of a Vernier large enough to accommodate the whole
head comfortably, and apply minimal force when taking the metric. Remember
that some of these birds (waders, wildfowl) have sensitive, highly innervated,
bills. It can be useful to attach a metal plate to the “jaw” of the calliper that goes
at the back of the head, as this helps to standardize its position.
4.7.8 Claw, eye-ring, and other measures
If it is necessary to measure claws, this should be done with the ID-calliper of a
Vernier and the measurement taken from the base of the upper surface to the claw
tip. Older works advise the use of a pair of dividers to take precise measurements,
and then reading the distance off from the dividers with a rule. While fine for skins,
it is inadvisable to use such sharp implements near sensitive areas of a live bird, so
ID-callipers are preferable for any other measures (e.g. eye-ring width etc.).
4.8 Condition
In its present sense, condition means some qualitative assessment (which may be
determined quantitatively) of the bird that has a direct bearing on its fitness.
Although it might be affected by the size of the bird, it is defined such that size is
not, per se, a measure of condition. Condition measures typically reflect the size
of nutrient reserves or the ability to resist parasites or disease, and might be
reflected in attractiveness to a potential mate. Hence color measures (above)
might act as indicators of condition.
4.8.1Asymmetry
The symmetry of an animal (measured as asymmetry) itself gives a measure of the
animal’s condition during development of the trait (fluctuating asymmetry).
Any trait that can be taken on both sides of the bird (wing, tarsus, tail tips, etc.)
can be assessed for symmetry, and used as an indicator of condition and fitness
110 |Birds in the hand