a series of life-history stages, with each stage largely devoted to a particular
activity at the optimal time of year, for example, breeding, molt, migration, and
overwintering. In birds, as in other vertebrates, hormones regulate morphology,
physiology, and behavior appropriately (Jacobs 2000; Wingfield et al. 2002).
Because hormones, by definition, are transported in blood, assessing their
concentration in blood samples offers a convenient nondestructive approach to
investigate endocrine control mechanisms. Investigating the roles of particular
hormones, for example, gonadal steroid hormones, such as testosterone, can be
achieved by monitoring seasonal changes (e.g. Wingfield and Farner 1978a,b;
Dawson 1983), implanting the hormone (e.g. Ketterson et al. 1996, 2002) or
removing the testes (e.g. Dawson and Goldsmith 1984).
In recent years, the response of adrenocortical hormones to a standardized
stressor has been used to study adaptation to environment and to monitor
species in potentially disturbed habitats (e.g. Wingfield et al. 1994). To do this
requires holding the individual for a period of 30–60 min and collecting a small
blood sample at intervals for measurement of hormones. The standard stress is
simply capture, handling, and restraint—it is assumed that all individuals of all
species will regard capture and handling as stressful. Between samples, birds can
be held in cloth bags, which allow adequate ventilation but prevent injury if the
bird struggles. These should be placed in a secure place in the shade and sheltered
from direct effects of weather. This stress series protocol provides useful infor-
mation on hormonal changes in response to stress and birds are released
unharmed. Care should be taken to ensure that breeding birds are not withheld
from their nests for too long. At other times the 30–60-min holding period is not
a problem, unless the individual becomes separated from a flock, or could poten-
tially lose a territory. Investigator discretion is required. Caution should be exer-
cised in interpreting the results. The protocol shows the magnitude of the stress
response. Often, the first sample is assumed to represent a “base-line” value. This
will not normally be the case because the sampling procedure takes longer than
the stress response (Dawson and Howe 1983).
For basic information on vertebrate endocrinology, see Norris (1997) and for
further information on current topics in avian endocrinology see Dawson and
Chaturvedi (2000), Harvey and Etches (1997), and Sharp (1993).
The most important tool in endocrinology is the measurement of hormone
concentrations in serum or plasma by radioimmunoassy (RIA). See Chard
(1995) for details of the methodology. Briefly, this involves competition between
radioactively labeled hormone and unlabeled hormone for a limited number of
binding sites on an antibody specific to the hormone being assayed. After reach-
ing equilibrium, the bound and unbound fractions of hormone are separated
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