In order to understand how we represent the kinds of bird seen, see A system for representing bird taxonomy, which describes a system of six-letter codes for describing kinds of birds.
That system allows for single kinds of birds,
“species pairs” (e.g., we think it was
either a Hammond's or Dusky flycatcher), or hybrids
(e.g., it looked like a hybrid of Blue-winged and
Cinnamon teal). In general, the representation is a
form is the first
or only six-letter code. The
rel value is the relationship code, blank
for single forms, “×” for hybrids;
/” for species pairs.
If the relationship code is not blank, the
altForm value is the second form's
six-letter code. If there are two codes, we stipulate
altForm, so we don't have to search twice
to pick up a given hybrid or pair.
Additionally, this database sometimes represents the sighting's age category (adult, immature, or female/immature) or sex category (male or female).