is coded “1,” it becomes “3” between fledging and the year’s end, and turns “5”
on 1 January. A bird of completely unknown age is coded “2.” The same bird
becomes 4 on 1 January (as it could not be in its hatch-year), and “6” the follow-
ing year. Hence, using this system a resident temperate-latitude passerine with
“typical” molt cycle (e.g. a Common Blackbird Turdus merula, Great Tit Parus
major, American Robin Turdus migratoriusor a Golden-crowned Kinglet Regulus
satrapa) would be aged 1 in the nest, 3 or 5 as a first-year (before or after 1 January
respectively), and adults would be aged 4 or 6 (before or after 1 January respec-
tively). The scheme differs somewhat in North America, where 3HY (hatch
year), 5SY (second year), 4AHY (after hatch year), and 6ASY (after
second year). In the tropics and Southern Hemisphere, these codes can make lit-
tle sense because of the different patterns of seasonality which lead many birds to
breed at the turn of the year.
4.6.2Sex
Most species show some degree of sex difference, either in color (sexual dichro-
matism) or size (sexual dimorphism). However, in most sexually dimorphic
species (males typically larger except in raptors and some waders), the statistical
separation on size is only partial, that is, there is some overlap between the sexes.
Thus the ideal species in which to distinguish sex are dichromatic (e.g. ducks,
gamebirds, some passerines). Although many passerines and other species are
technically monochromatic, there is nevertheless a sufficient sex-difference in
the plumage through, for example, its reflectivity (plumage typically more glossy
in males) to “sex” a significant proportion of the birds unequivocally.
For many sexually monochromatic species, it has been possible to produce an
index for sexing based on a Discriminant Function Analysis (DFA) of a combina-
tion of size measures (see below), so that one might be able to say, for example,
that a bird with a wing-length greater than 87 mm, bill greater than 14.0 mm
and tarsus length greater than 35.0 mm has a 0.90 probability of being male.
While this method has better discriminating power than any single measure-
ment, it is rarely foolproof or applicable to all populations. Thus in such cases, if
definite sex identity is essential, molecular methods (based on DNA) must be
used. Thanks to the Polymerase Chain Reaction (Chapter 9) and the fact that
avian erythrocytes are nucleated, birds can now be sexed from minute quantities
of blood (Griffiths and Tiwari 1993, 1995).
In theory it should be possible to determine the sex of birds through examination
of the cloaca since, for example, males may have a pronounced cloacal protruber-
ance, at least when breeding (Chiba and Nakamura 2002, 2003). However, in prac-
tice this is difficult for many species, or when not breeding, although wildfowl,
102 |Birds in the hand