Delayed gratification tests usually involve the promise that if the participant can avoid eating the treat in front of them, they'll get something even better. They've been done on other kinds of birds, most notably corvids (crows and ravens). These birds have been known to wait for up to five minutes for a better offer. However, corvids have the habit of hoarding their food, which might make it easier for them to postpone eating their treats. Cockatoos have no such trait.
Fourteen cockatoos were given the chance to rank three possible treats (pecan, cashew or fried meat). After that, they were taught to exchange less desirable items for more desirable items as shown in the video below.
Notice that the experimenter keeps the more desirable tidbit visible but out of reach in her left hand. The cockatoo is only allowed to exchange for it when the person reopens her right hand after a predetermined delay (in this case, 40 seconds).
All the birds could delay eating the first item for at least a couple of seconds. Remember, they held that morsel in their mouths, unlike the corvids who tended to temporarily discard it. I'm not sure how many of us could wait even that long with a marshmallow in our mouths. Half the birds could wait forty seconds and three waited nearly a minute and a half. When the choice was between one item now and either two or six of the same item later, fewer of the birds were interested in trading, but eight of them did hold out for twenty seconds.
Interestingly, the birds tended to either wait the entire duration or give up immediately. The authors speculate that the cockatoos were able to judge the time duration and decide if the expected benefit was worth the wait. This is reminiscent of the children judging the reliability of the experimenter in the post I mentioned earlier.
Auersperg, A., Laumer, I., & Bugnyar, T. (2013). Goffin cockatoos wait for qualitative and quantitative gains but prefer 'better' to 'more' Biology Letters, 9 (3), 20121092-20121092 DOI: 10.1098/rsbl.2012.1092.