While comparative judgements of animal intelligence are always very difficult to make objectively, the African Grey Parrot (Psittacus) is generally regarded as being the most intelligent of birds. African grey parrots are particularly noted for their cognitive abilities, which are believed to have evolved as a consequence of their history of cooperative feeding on the ground in central Africa.

Research with captive African greys has shown that these parrots are capable of associating human words with their meanings, at least to some extent. Ambitious claims of language use have also been made for another African grey N'kisi, who has a vocabulary of over a thousand words and speaks in sentences. However, there is little doubt that Greys and other parrots (especially macaws and cockatoos), along with corvines (Crows, Ravens, and Jays), are highly intelligent in comparison with other birds. 

Link: Wkipedia: African Grey Parrrot