and the radioactivity remaining in one of the fractions is counted. The amount
of antibody and labeled hormone is constant and the unknown amount of
unlabeled hormone in the sample is calibrated against a standard curve of known
amounts of unlabeled hormone. Steroid hormones are identical in all vertebrates
and so radioactively labeled hormones and antibodies can be readily purchased. In
the case of birds, the volume of blood samples will often be small. Chromatography
can be used to separate different steroid hormones allowing multiple measure-
ments to be made on each sample (Wingfield 1975). Peptide hormones, for
example, luteinizing hormone (LH) or prolactin differ slightly between species
and so purified preparations for the standard curve and for radiolabeling used
to have to be extracted (e.g. Follett et al. 1972) but are now more easily prepared
by recombinant techniques (e.g. Talbot and Sharp 1994). Changes in steroid
hormones can be monitored noninvasively, although less accurately, by measur-
ing the concentration in feces (Cockrem and Rounce 1994; Goymann et al.
2002). This is particularly useful for endangered species (Cockrem and Rounce
1995). An alternative to RIA is ELISA, which uses a colorimetric end-point
rather than radioactive counting.
9.5 Energetics
The determination of metabolic rates and energy expenditure is a key aspect of
many studies on birds. Basal metabolic rate (BMR) is defined as the metabolic rate
of an animal at rest, but not asleep, in a post-absorptive state within the ther-
moneutral zone. This is comparatively easy to assess in humans. With birds it is
obviously impossible to ensure that they remain resting but awake. Other defini-
tions therefore need to be used that define the parameters during which metabolic
rate is assessed: fasting metabolic rate, least observed metabolic rate, and resting
metabolic rate (Blaxter 1989; Speakman 2000). These “basal” metabolic rates are
normally measured by respirometry. The bird is kept in a respirometry chamber in
which temperature is accurately controlled. Air is passed through the chamber at
a known rate. The difference in the concentrations of oxygen and carbon dioxide
in air entering the chamber and air leaving the chamber is used to calculate the
metabolic rate. Nudds and Bryant (2001) provide a recent detailed description of
the methodology.
Animals living in their natural environment will expend energy at a greater
than basal rate almost all of the time. Free-living energy use is defined as daily
energy expenditure (DEE) or field metabolic rate (FMR). This can sometimes be
estimated indirectly and noninvasively by calculating a time and energy budget,
but there are many assumptions and inaccuracies associated with this method.
224 |Techniques in physiology and genetics