The AOU (American Ornithological Union) maintains a system for describing bird types using four-digit numbers.
These codes have the form "ddd.d". For example, Scrub Jay is given code 478.0, and Colima Warbler is assigned number 647.1.
The current AOU numbers are available from the Bird Banding Lab.
This system is used extensively in some museum collections, especially older egg collections. It is, however, not widely used today, and many users (such as banders and museums) are moving away from it.
The author feels strongly that such arbitrary numeric codes should be avoided, since they can be a significant source of error and extra work in handling bird records. A number doesn't really have any intrinsic meaning, so users must either memorize them or look them up.
The sequence of these numbers approximates a phylogenetic concept roughly a century out of date. The author recommends that code system users avoid trying to freeze any phylogenetic arrangement into numbers. Taxonomy has never been static and probably never will be.