Taxonomy and systematics
Two subspecies are universally accepted:
* Congo African Grey Parrot (Psittacus erithacus erithacus):
This is the nominate subspecies, larger than the Timneh at about 33 cm (13 in) long, with light-grey feathers, cherry-red tails, and an all-black beak. Immature birds of this subspecies have tails with a darker, duller red towards the tip (Juniper and Parr 1999) until their first moult, which occurs by 18 months of age. These birds also initially have grey irises, which change to a pale yellow colour by the time the bird is a year old. The Congo Grey Parrot is found on the islands ...view middle of the document...
Some aviculturalists recognize third and fourth subspecies, but these are not distinguishable in scientific studies.
Illness and Disease
The African Grey Parrot (Psittacus erithacus erithacus) has been known at times to contract a non-infectious inflammatory lung disease called Lipid Pneumonia. Lipid Pneumonia can be classified as exogenous or endogenous depending on whether or not the animal inhaled outside material. A necropsy shows that the lungs of a Grey Parrot with Endogenous Lipid Pneumonia (EnLP) are firm with a diffuse grey discoloration. EnLP is a common illness in other animals as well. The Congo African Grey Parrot is also one of the three parrots that scientists found to commonly suffer from dehydration. They use Plasma Osmolality to find more information about the form of dehydration the African Grey Parrot has. Often, there is need to find replacement fluids for the Parrots. I added the comment that another disease that a lot of the African Grey Parrots get is cardiomyopathy which is a heart disease that usually presents at a young age. It is believed for the heart disease is genetically inherited from parents of the same breed. Some other common symptoms and illnesses in these birds are weakness, coelomic cavity, and retardation.  The African Grey Parrot has been known to contract Beak and feather disease virus (BFDV) which causes a highly contagious and sometimes fatal, psittacine beak and feather disease in parrots.
Behavior and ecology
Like many large parrots, the African Grey is a long-lived bird. The Animal Ageing and Longevity Database states the longest reliably recorded longevity for the species in captivity as 49.7 years. Also acknowledged are claims of captive African Grey parrots reaching the ages of 73 and 93, whereas the World Parrot Trust lists a longevity of 50–60 years for an African Grey in captivity. The Guinness Book of World Records listed a grey parrot that allegedly lived in captivity for 72 years as the longest-lived specimen for the species.
Further information: Talking bird
Unlike other parrots, wild African Greys have been documented imitating the calls of several other species.
Irene Pepperberg's research with captive African Greys, most notably with a bird named Alex, has scientifically demonstrated that African Greys possess the ability to associate simple human words with meanings, and to intelligently apply the abstract concepts of shape, colour, number, zero-sense, etc. According to Pepperberg and other ornithologists, they perform many cognitive tasks at the level of dolphins, chimpanzees, and even human toddlers.  Pepperberg also gave Alex some tutors so he could be able to learn English. Which suggests that African Grey Parrots do not just inherit the speaking ability. Besides Alex...