Just how smart are birds? Until recently, birds were thought of as being at the low end of the intelligence scale - hence the uncomplimentary term 'birdbrain'. It turns out, however, that our feathered friends are far from stupid. They can actually be counted amongst the smartest creatures on the planet. One woman who knows this better than most is famed animal psychologist, Dr Irene Pepperberg.

Pepperberg was born in 1949 in New York City. An only child, she kept parakeets as pets and taught them to speak. While studying for a PhD in Chemistry at Harvard University, Pepperberg happened to see a documentary about animal intelligence. Fascinated, she immediately decided she wanted to change fields, but her professors discouraged her, so she continued her Chemistry studies. Nonetheless, in her spare time, Pepperberg began reading everything she could about animal intelligence.

In 1976, after completing her degree, Pepperberg walked into a pet shop and purchased a one-year-old African Grey parrot with the idea of studying him. She called the parrot Alex, for 'Avian Learning Experiment. For the next three decades, until Alex's premature death in 2007, Pepperberg dedicated herself to seeing just what Alex was capable of learning.

Before Pepperberg's work, it was widely believed in the scientific community that a large primate brain was required to handle complex problems related to language and understanding. Pepperberg proved otherwise. She showed Alex could understand and use English on his own initiative. Alex learnt to use phrases along the lines of, "I want X" or "I want to go to Y" and clearly meant them to express genuine desires.

Alex also grasped the concept of certain categories, including bigger and smaller, and same and different. He could identify fifty different objects, recognise numbers up to six, and distinguish seven colours and five shapes. Alex even understood the concept of zero - if asked the difference between two identical objects, he would answer "none". His vocabulary stood at about 150 words, but Alex didn't just imitate human speech as pet parrots often do. He comprehended what he said. For instance, after learning colours, Alex asked what colour he was. He learnt "grey" after being told the answer just six times.

Pepperberg attributed Alex's ability to reason and process complex information to her training methods. When starting out, Pepperberg adopted some of the techniques used by the previous generation of researchers, but she rejected others as flawed. Pepperberg believed, for instance, that the traditional training method of giving birds unrelated rewards (e.g. a food treat) when they learnt to do something correctly simply caused confusion. So she trained Alex using related rewards. This meant she would reward Alex with the object he correctly labelled rather than an unrelated object. When Alex correctly identified a cork, for example, he was given the cork as his reward, not his favourite food, a cashew. To 'sweeten' the task, however, she would then allow Alex to request a nut or a slice of banana.

Alex became an international celebrity during his lifetime; so much so that when he died unexpectedly in 2007, his obituary appeared in publications all over the globe - from the New York Times to the Economist. Alex was learning till the very end, getting his head around the number seven and enjoying teaching the two younger parrots in Pepperberg's lab - 12-year-old Griffin and 8-year-old Arthur-telling them to "talk better" when they mumbled their words. "Alex was so extraordinary in breaking the perceptions of birds as not being intelligent," says Pepperberg. "He had the emotional maturity of a 2-year-old child and the intellectual capabilities of a 5-year-old." Alex was, in short, no birdbrain.