Birds were long considered stupid, however laboratory research has shown that corvids – the group of birds including crows and jays – are actually highly intelligent. Their ability to make and use tools rivals that of chimpanzees. When hiding stores of food, corvids demonstrate their episodic memory and future planning ability; cognitive abilities previously thought unique to humans. Not only do corvids remember where they have caught food, they remember when they stored it. If seen catching food, corvids will return and re-hide it, unobserved by competitors. This anticipation of pilfering shows that corvids acknowledge the mental state of other individuals. Being the most social group of birds, corvids raise their young cooperatively and form long-term relationships. The social function of intellect theory, which hypothesised that social living was the impetus for the development of primate intelligence, is now applied to other species, such as corvids. Although capable of doing many of the same things as primates, corvids have smaller brains and lack the neocortex that is responsible for mammalian cognition. Instead, corvids have a nidopallium, which scientists believe fulfils a similar function more efficiently. Primate and corvid intelligence is sometimes used as an example of convergent evolution, whereby two unrelated species independently develop the same adaptations to similar environmental conditions. But animal intelligence is a controversial subject, with no consensus on its definition. Some scientists argue that corvid behaviour can be explained by adaptive specialization and is not equivalent to primate intelligence, and thus convergent evolution does not apply.