4.7 Size
4.7.1 Body size
There are many reasons in ecological and evolutionary research for measuring
the size of individual birds. For comparative studies, where most variance under
consideration is among (rather than within) taxa, weight gives a reasonable
approximation. However, because this reflects condition (e.g. weight changes
through the day) while size cannot, weight alone is an inappropriate measure to
study intraspecific variation. Thus some measure of overall body size is required.
No single measure is ideal for what is really a surrogate for the overall skeletal size.
Traditionally, total bird length measured by straightening the bird out laid on
its back on a rule (see Svensson 1994), is frequently given in field-guides.
However, in practice this is difficult to do with a live bird, and is neither highly
repeatable nor a reliable indicator of body size since it includes independently
variable features such as bill- and tail-length. The most direct measure of body size
is the length of the sternal keel, as taken by Bryant and Jones (1995) to measure
Sand Martins Riparia riparia. Again however, this measure is difficult to take and
not very repeatable.
A more meaningful, reliable, and now widespread method, suggested by Rising
and Somers (1989), is to take several measures such as wing, tail, tarsus, and bill, and
to use the first Principal Component from a Principal Components Analysis (PCA)
since this represents the component of variance in each which correlates with the
other measures largely through the shared correlate of overall size. Although PCA is
the ideal, it may not be practicable. If, for example, time constraints mean that only
one measure of size can be taken, then wing-length is recommended because it gives
the best approximation to PC1 in full-grown individuals of most species studied
(Gosler et al. 1998). If chicks are to be included in sampling, tarsus-length might be
preferred because in many species it is full-grown earlier than the wing.
4.7.2 Wing
Wing-length, defined as the distance from the carpal joint to the tip of the
longest primary on the closed wing, is the single measure of size most commonly
recorded in birds. It is measured on a stopped stainless-steel rule (e.g. BTO wing-
rule) with the wing held in its natural closed position or as near to it as possible
(i.e. do not pull the wing out) (Figure 4.4). With the bird in the ringer’s grip (left
hand if right-handed) with its back adjacent to the palm, gently slide the rule
under the wing and draw it down until the carpal joint abuts the end stop. With
the left thumb securing the carpal at this point, draw down the primaries with
the right thumb to straighten them and read off the rule (proximal side to the
104 |Birds in the hand