They were training a younger parrot and trying to get the younger parrot to count to two by tapping twice.
Alex overheard the training and got impatient with the other bird. He yelled out “two” and then after two more taps “four” and then “six”.
The trainers were just expecting “two” each time.
It was this book: https://www.amazon.com/Are-Smart-Enough-Know-Animals/dp/0393...
The book is interesting and goes into how humans need to set up experiments properly to actually test non-human animals in ways that make sense (rather than just in some biased human way).
One quick example was testing tool use, the original experimenters left branches on the ground for the monkeys to use, but the monkeys can’t pick stuff up that’s flat on the ground since they’re normally in trees (their hands don’t have thumbs that move that way). When he redid the experiment with the tool raised they were able to grab and use it.
Same author also wrote Chimpanzee Politics and did this great video experiment: https://youtu.be/meiU6TxysCg
Interestingly, about 30% or so of the book is kind of a rant against other animal behaviorists who have an "animals are stupid, and no amount of proof to the contrary will shake that belief" attitude. But that yields some pretty entertaining stories (and interesting experiments) as well :-)
[0] https://www.amazon.com/Are-Smart-Enough-Know-Animals/dp/0393...
“Are we smart enough to know how smart animals are?”
https://www.amazon.com/Are-Smart-Enough-Know-Animals/dp/0393...
Here is a great review of this book http://www.theatlantic.com/magazine/archive/2016/05/how-anim...
the passage that got me hooked: “a better way to think about other creatures would be to ask ourselves how different species have developed different kinds of minds to solve different adaptive problems. Children and chimps and crows and octopuses are ultimately so interesting not because they are mini-mes, but because they are aliens—not because they are smart like us, but because they are smart in ways we haven’t even considered"
for example different species have different approaches to problem solving: Chimpanzees try to comprehend/model a problem while Monkeys are solving tool building problems by trial and error (that's from the book)
https://www.amazon.com/Are-Smart-Enough-Know-Animals/dp/0393...