cranes and some others can be sexed by eversion of the cloaca since the male has
a distinct phallus (Baker 1993, p29). Breeding females (e.g. passerines) develop a
distinctive brood-patch in which the feathers of the belly are lost, and the skin
becomes hot, highly vascularized and oedematous.
4.6.3Weight
The weight of a bird is a basic measurement that can be used as a measure of size
in cross-species comparisons. However, as it incorporates variation in both size
and condition (e.g. fat reserves), it should not be used alone as either a measure of
size or condition in intra-specific studies. Weight is readily determined either with
a lightweight precision spring-balance (e.g. manufactured in Switzerland by
Pesola®), or on an electronic, digital, pan balance. Again, these are now available
as small, precise, highly portable battery or solar-operated units (e.g. Tanita®
TPK100). Birds of up to 50 g should typically be weighed to a precision of 0.1 g,
though greater resolution is desirable for the smaller hummingbirds. For birds
between 50 and 300 g weight to the nearest gramme should suffice, and for even
heavier birds to the nearest 10 or 100 g as appropriate. The bird should be immob-
ilized in some form of restraint (a polythene funnel or cone weighing c.0.2 g is
ideal for smaller birds), and this should be clipped onto the spring balance or, for
digital balances, closed with a bulldog clip of known weight and laid on the pan of
the digital balance. When weighing in the field, windy conditions can distort
readings, but one way to avoid this problem is to suspend the bird in its cone with-
in a larger windproof container such as a large jar (or a plastic or cardboard tube).
4.6.4Color, for example, UV reflectance
It is often desirable for work on sexual selection and systematics, to record variation
in color traits. The colors of plumage areas can be recorded visually by reference to
a standard color chart such as Küppers (1978). However, this has several problems
due to variation in illumination, observer eyesight, print quality, and fading.
Furthermore, recent work has indicated that some plumage colors are reflected in
the ultraviolet range (400 nm), and so cannot be detected by the human eye. If
you are studying such colors, you can use an electronic color detector (such as the
PS1000 diode array spectrometer available from Ocean Optics, Dunedin USA),
which reflects a standard light source (DS2000 deuterium-halogen) onto a small
area of feather (or any other surface) and analyses the wavelength profile reflected
back in terms of spectral location (color or hue), purity (chroma), and intensity
(brightness). The reflected spectrum can be visualized and analyzed on a micro-
computer (e.g. see Ornborg et al. 2002). The drawback of this equipment is that it
is expensive, cumbersome, and requires a power source.
The bird at close quarters| 103