off ( Jenni and Winkler 1989). This has advantages when skins and live-bird
measurements must be compared, it may be easier to use on larger birds, and may
reduce inter-observer error (although this is not proven—Gosler et al. 1995a),
but is best not used on very small birds.
Length is not the only measure that can be taken of the wing. The relative
lengths and pattern of notching and emargination of the primaries (wing
formula) may be of use in specific identification and taxonomic studies (see Pyle
et al. 1987; Svensson 1994). Wingspan and wing-area can be taken also. These
may be necessary for flight-performance calculations but are not taken as routine
measurements for practical and welfare reasons (but see Pennycuick 1989 if
required). Also they are less repeatable than the more usual measures.
4.7.3 Tail
In general, within species (at least in passerines), tail-length and wing-length are
highly correlated (Gosler et al. 1998). However, especially in studies of sexual
selection, it may be desirable to measure the length and shape of the tail. For over-
all tail-length, slide an unstopped steel rule between the tail feathers (rectrices)
until it stops at the feather bases, and read off the longest feather length from the
rule. It is helpful to hold the bird so that the underside of the tail can be viewed
easily. The difference in length between the longest and shortest tail feathers is
also best measured from under the tail (depth of notch of fork—for example,
Barn Swallows Hirundo rustica).
4.7.4 Tarsus
Another metric that gives a good indication of overall size (and excellent when
combined with wing-length in PCA) is the tarsus-length, strictly the length of
the tarsometatarsus. Earlier methods described in the literature proved to be
highly unrepeatable for live birds but were necessary to measure the “set” legs of
skins. However, the method described here is highly reliable. With the bird in the
ringer’s grip, the tarsus should be held between the thumb and ring-finger (left
hand) so that the tibiotarsus and tarsometatarsus form an acute angle. The foot
is held at right angles to the tarsus and the measurement from these two jointed
right-angle bends taken either with OD (outside diameter) dial or Vernier cal-
lipers, or on a stopped rule. In either case, the measurement (maximum tarsus)
can be read to 0.1 mm in small birds (under 100 g) and to 1 mm in larger birds.
An alternative—the minimum tarsus—is to measure in the same way with
callipers, but into the notch at the tarsal joint (Figure 4.5).
106 |Birds in the hand