This article is from the Birds FAQ, by Lanny Chambers with numerous contributions by others.
If you are keeping your lists for your own purposes, you are free to
establish your own criteria for when you may include a bird on it.
Should you include birds that you identified solely on the basis of
their songs? Even if they're nocturnal? Birds that you saw only in
silhouette? All such choices are up to you. Many birders with a
naturalistic bent apply a stringent criterion: birds may be counted
only if you feel that you've "met" them.
On the other hand, if you intend to submit your list to an
organization of competitive birders, you must abide by their rules.
For instance, the American Birding Association once forbade the
inclusion of "heard-only" birds on North American lists (this
restriction has now been lifted). Another important criterion for ABA
listing is that listed birds must be of species on the official ABA
list. That means that you can't count an escaped parrot, for
instance. Most birders don't count escaped domestic or cage birds
even for informal listing.