Sequence and names of families and bird species are as used in
James F Clements, The Clements Checklist of Birds of the World, 6th edition.

I have numbered the listings in Clements' 6th edition. Families are numbered from 1 to 203, and I have assigned a 6 digits number to each species. Family and species number are combined to form a speciescode, like this
51006000 Houbara Bustard Chlamydotis undulata
The digits from right to left are: 2 digits to be used for sub-species (00 means "the species, with any sub-species"). The 3rd digit from the right is to be used for future splits, new species, moves. E g if a sub-species of the HB is recognized as a species, it's code will be 51006100 if it is to be placed after the HB or 51005900 if it is to be placed before. The 3 next digits reflects the bird's position in the sequence in it's family. The 1 to 3 next digits is the family number. Thus the HB is the 6th species in the 51st family.
Tip: If you store these numbers like 51006000, you may have your computer program show them like this "51 006 000", or like this "51,006,000", if you change the settings for how to show numbers in the cells or fields they are stored.