McCabe’s system is inadequate for recording the high fat loads of long-distance
migrants. Kaiser’s (1993) system based on both the tracheal pit and abdominal fat
bodies consists of eight main score categories with a further four subcategories in
each giving a total resolution of 32 score classes. An advantage of these scoring
systems is that they are inherently scaled relative to the size of the bird (e.g. a fat
score of 5 means the same whether it is a kinglet or a thrush). A disadvantage is
that, although they tend to be normally distributed, fat scores are bounded between
limits and this can have implications for statistical analysis (Greenwood 1992).
The total fat content of a bird can also be measured by determining the bird’s
total body electrical conductivity (TOBEC). This method uses the fact that the
electrical conductivity of tissues varies systematically with its fat content.
TOBEC machines can be used to assess the fat content of live birds. However,
they are expensive, not readily portable, the bird must be immobilized within the
chamber of the machine during use and, for each size class (e.g. Regulus,Parus,
Turdus, etc.) if not every species, its readings must be calibrated against a series of
birds on which total lipid extractions have been undertaken. Studies indicate
that its precise results are little better than those achieved by non-destructive
fat-scoring methods (Brown 1996; Speakman 2001).
4.8.4Muscle protein
Another useful measure of condition concerns muscle mass, especially of the
pectoral muscles, which constitute 15–21% of total body mass in birds generally.
Muscle mass varies through changes in labile protein reserves and wastage. It can
be assessed from the cross-sectional shape of the pectoral muscles (visually or by
feeling the shape) and recorded on a 4- or 5-point scale (Gosler 1991; Gosler
et al. 1998; Redfern and Clark 2001), or by recording its shape by using fine wire
or dental alginate (Bolton et al. 1991).
4.8.5Physiological measures
Various measures of physiological stress can be obtained from microscopic
and/or chemical analysis of blood components (erythrocytes, plasma etc.) (see
Brown (1996) and Chapter 10 for further details).
4.8.6Molt and plumage
The study of molt is a major line of enquiry in its own right, which in general
must be assessed in the hand (some details of molt of some tracts in some birds
can be determined by observation in the field) or by collecting shed feathers from
roosting sites. Although a detailed assessment of molt would require each feather
tract to be assessed in turn, much information can be gleaned from a quick
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