The Different Species of Vasa Parrots
Coracoposis v. vasa/Greater Eastern Vasa
20 inches in length it is the largest of the vasa's and the only greater to have a solid grey eye ring, not seen in captivity as far as I am aware. Wing span, 22-25 inches. Beak color, white to light grey depending on time of year. The eastern vasa is obviously from Eastern Madagascar. It is also darker in color than the western vasa.
Coracoposis v. Drouhardi/Greater Western Vasa
This is the most common vasa seen in the pet trade today. It should be noted that there is a considerable amount of confusion when dealing with the different species and subspecies of vasa parrots. Lesser's are not part of the greaters. In fact they are not even the same species. Because of this confusion, most vasa owners say that they have a Coracoposis v vasa, when in fact they have a Coracoposis v. Drouhardi or greater Western Vasa. Species Pics
18 inches in length and paler in color than the nominate species, with whitish under tail coverts. They do not have grey eye rings. The eye rings will range in color from white to bluish, to yellow depending on the time of the year. Wing span, 20-23 inches. Beak, Horn color to black again depending on season and age. Western Vasa's are again from western madgascar.
Coracoposis v. Comorenis/Comoro Vasa
The most petite of the greaters although in length it is the same as the weastern, 18 inches with a wing span of 20-21 inches. Often confused as a lesser it is not, as there is a lesser subspecies by a similar common name. Slightly paler in color from the previous two mentioned, with the under tail coverts being brown. Not very common in the pet trade at all. Comoros primary live in wooded area and savanna area with trees in the Coromo Islands.
Note: It is typical to see vasa parrots with different head sizes and body sizes from each other. This is thought to be the result of accidental breeding of western vasas with comoro vases.
Reference...Lexicon of Parrots