Delayed gratification: A grey parrot (Psittacus erithacus) will wait for a better reward
Journal of Comparative Psychology (2015) - Comment
pubmed: 26214033  doi: 10.1037/a0039553  issn: 1939-2087  issn: 0735-7036 

Adrienne E. Koepke, Suzanne L. Gray, Irene M. Pepperberg