Alex was an African grey parrot who was born in 1976 and was the subject of a thirty-year experiment.
The experiment, which ran from 1977 to 2007, was designed to delve into the bird's ability to process language. The name "Alex" stood for avian language experiment. At the time, it was believed that birds' were not intelligent enough to handle complex problems related to language and understanding, and could not go beyond their common use of communication through mimicking and repetition of sounds.
After training began, it was reported that Alex sometimes corrected his trainers when they made mistakes, and was even observed practising words when he was alone.
In 1999, his portfolio of accomplishments claimed that he could identify 50 different objects and recognize quantities up to six. He could distinguish seven colors and five shapes. He also understood the concept of size. Alex was also observed expressing surprise and even anger when an unfamiliar or unexpected object during tests.
More interesting details on Alex's and the study's accomplishments can be found on Wikipedia.
At the age of 31, Alex unexpectedly died. There was no obvious cause of death, but it was attributed to "a sudden, unexpected event associated with arteriosclerosis".
Reportedly, Alex's last words to Irene Pepperberg, the animal psychologist in charge of the study, were: "You be good, see you tomorrow. I love you."