With respect to latin naming, the following rules do apply to day.
If the name of the genus is male, the name of the species and the name of the subspecies get "us" as an ending; e.g. Agapornis canus, Agapornis canus ablectaneus, Agapornis personatus, Agapornis pullarius. If the generic name is female, e.g. the genus Cacatua (Cockatoos), than species and subspecies get "a" as an ending, e.g. Cacatua galerita. For many years people have written cana, pullaria and personata, probably as a result of a mis spelling later on adopted by others.
Dr. Karl Russ (1901) did use the right names at that time and wrote the names of the Agapornis species with "us" as an ending. These days in modern European bird literature the right names are used with regard to the genus Agapornis.
The I.C.Z.N. (International Committee for Zoological Nomenclature) in London keeps record of the scientific names. Agapornis is a male word, therefore the adjectives of this family name such as personata, cana etc. must be written according to the Latin grammar with the male ending "us" instead of "a" (=female) since 1900-1995. This is common practice in German languages.
An exception is taranta: that is a geographical name.
See for more reference:
I.C.Z.N. 1985. International Code of Zoological Nomenclature. 3rd ed.
International Trust for Zoological Nomenclature. London.
Internet: International Committee for Zoological Nomenclature - http://www.iczn.org/
Check also this page - http://www.nrl.fi/pv/Tzlist2000.htm