The scientist publishing the research says it shows that parrots, whose brains are almost the size of walnuts, compare with chimpanzees and dolphins in having a level of intelligence similar to that of small children.

“Their communication skills are similar to those of a two-year-old child, but their adding and ability with colours and shapes are more like a five or six-year-old,” said Irene Pepperberg, associate professor of psychology at Brandeis University in Waltham, Massachusetts.

Pepperberg’s decades of research have been carried out with African grey parrots. The current group includes Alex, Arthur and Griffin.

Although her work was ridiculed when she began, the depth of Pepperberg’s research has gained the respect of other scientists who increasingly accept her arguments about psittacine intelligence.

Alex Kacelnik, professor of behavioural ecology at Oxford University — whose research group has discovered tool-making abilities in crows — said: “Pepperberg’s work is quite extraordinary. She has worked for many years and has demonstrated in grey parrots abilities that we did not think any bird could have.”

Pepperberg, who is also a visiting professor at the Massachusetts Institute of Technology, will publish her research shortly in the journal Applied Animal Behaviour Science.

Alex, the most accomplished of her birds, is able to name seven colours and five shapes and count or add up to six. He can identify, request and refuse about 100 different objects and uses phrases such as “come here” and “wanna go”. He can also understand concepts such as zero; objects being bigger and smaller than one another; and the same or different.

In one recent experiment, Alex was presented with collections of four, five and six blocks of three different colours and was able to provide an answer, for example, “four blue”. He was also shown various groups of items and was able to identify correctly, for example, “four corks”. Pepperberg concluded: “Overall, his accuracy was above 80%.”

The birds are strongly encouraged to socialise and converse with humans and they are now being taught how to subtract as well as add.

Alex, 29, who has been in the laboratory since 1977, may not be precocious in human terms, but he has astonished scientists with one conceptual leap. When asked which of two equal objects was the bigger, he spontaneously replied “none” without having previously been taught what to say if he saw no difference between two items.

Pepperberg said the results of her research should encourage greater respect for birds. “It has implications for the treatment of parrots in captive situations such as zoos or as human companion animals,” she said.

“We need to encourage a sensitivity to the abilities of non- humans, particularly non-primate and non-mammalian subjects. For far too long, animals in general, and birds in particular, have been denigrated.”

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