(TOBEC®; Scott et al. 2001) or bioelectrical impedance (BIA; Lichtenbelt
2001), which are related to the amount of body fat. However, TOBEC requires
calibration against dead birds of known fat content, and the result also depends on
the distribution of the fat (Unangst and Merkley 2002). Dual-energy X-ray absorp-
tiometry is a rather new, but expensive, technique that could potentially be used to
estimate fat and lean mass, as well as total bone mineral density (Nagy 2001).
Magnetic resonance imaging (MRI) has recently been used for measuring fat
deposits in White Storks Ciconia ciconia(Berthold et al. 2001). This technique
can also be used on small birds and provides data on the amount of fat and its
spatial distribution (Figure 7.4). MRI is of limited availability and expensive,
however, because most machines are used for human diagnosis.
Ultrasound scanning is also a promising method for estimating the size of
digestive organs and muscles (Starck et al. 2001). It has been used to show how
flight muscle diameter varies in parallel with fuel reserves in Red Knots Calidris
canutus(Lindströmet al. 2000).
7.4.2Energetics
The basal metabolic rate (BMR) is the metabolic rate at rest, during night, in
darkness and in post-absorptive condition. BMR is typically measured in a
respirometry chamber, where the rate of oxygen consumption or carbon dioxide
production is measured. The metabolic scope is often related to BMR (e.g. as
5 BMR), and determines the potential for fuel deposition. The proportion of
fat and protein used can be determined from the respiratory quotient. Another
method to measure energy consumption is the doubly labeled water technique. The
theory and assumptions for this method is explained in Speakman et al. (2001)
and Chapter 9. It requires the bird to be handled at the beginning and end of the
measurement period.
170 |Migration
Fig. 7.4MRI of a sanderling Calidris alba,
showing a 2 mm cross section. The white layer
is fat, the flight muscles are shown as gray
(pm) and the sternum (s) is indicated.pm
pm
Spm
pm
S