Humphrey–Parkes terminology is a system of nomenclature for the plumage of birds.
However, as this system did not always work correctly, the new Humphrey–Parkes came into use to rectify this error.
[1] Under the Humphrey–Parkes nomenclature, the main adult plumage, especially when it is produced by a complete molt, is called basic plumage.
For birds that do not completely molt into full adult plumage the first time, a numbering system is used to signify which plumage it is in.
The numbers are dropped after a bird achieves its full adult plumage.