(Møller 1998; Shykoff and Møller 1999). However, observers are themselves
asymmetrical, and typically one side will be preferred as feeling more “natural”
because of the observer’s own left or right-handedness. Normally this does not
matter, but when determining a bird’s symmetry, the fact that we are more profi-
cient in measuring one wing, tarsus etc. than the other can become a problem,
because one side is measured more reliably than the other. This problem needs to
be addressed more frequently than it has in the past (Helm and Albrecht 2001).
4.8.2Relative mass
If it is not possible to measure nutrient reserves directly on the live bird, a
reasonable estimate can be obtained by considering the mass (or weight) relative
to the size of the bird. This is typically done by regressing mass on a size measure
(PC1 or wing-length—see above) and using the residual scores (i.e. a bird above
the regression line is heavy for its size etc.) as a measure of condition. This gives
only an approximation to fat reserves (Gosler et al. 1998) because a bird may be
relatively heavy for various reasons (fat mass, pectoral-muscle mass, fullness of
gut, etc.). Relative mass is widely used and often simply called “condition” or
“condition index,” but for some kinds of analysis it can give misleading results
because body size is measured imperfectly (see e.g. Gosler and Harper 2000). A
useful extension of this method, however (mass/bill-length), using the fact that
the bill grows more or less linearly, can be used to assess the condition of partially-
grown wader chicks (see Beintema 1994).
4.8.3Fat reserves
The characteristic most frequently associated with individual condition is the
quantity of fat carried by the bird. However, the assumption that high and low
fat loads indicate birds in “good” and “bad” condition respectively comes from
our anthropocentric viewpoint as a terrestrial mammal, our knowledge of the
energy requirements of long-distance migrants, and the fact that birds picked up
dead under extreme cold conditions (waders and wildfowl) are usually seen to
have starved. We must be careful how we regard fat loads because they carry a cost
(Witter and Cuthill 1993; Gosler 2001). Nevertheless, the observation of visible
subcutaneous fat in many species (especially passerines) is quick and reliable, and
can give insight into the birds’ biology (Gosler et al. 1995b; Gosler 1996, 2002;
Carrascalet al. 1998).
Most birds deposit fat in discrete depots, which can be assessed by a standard
scoring system (Figure 4.7). One such scale for this was developed by McCabe
(1943) for assessing the fat on museum skins. It subsequently proved reliable for
use on live birds and was adopted by the BTO’s Biometrics Working Group
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