Sources

Alex and Me by Dr. Irene M. Pepperberg

“Grey Parrot (Psittacus erithacus) Numerical Abilities: Addition and Further Experiments on a Zero-Like Concept” by Dr. Irene M. Pepperberg

http://www.youtube.com/watch?v=R6KvPN_Wt8I

http://www.edge.org/3rd_culture/pepperberg03/pepperberg_index.html

http://animalreview.wordpress.com/2008/06/09/parrot/