Crows and parrots have sophisticated thinking skills on a par with those of apes such as chimpanzees, researchers claim.
The birds' brains are about a tenth the size of the mammals' and their structures are completely different, but scientists believe they developed equal cognitive abilities through facing the same challenges in the wild over 300million years of evolution.
The new assessment of research results gathered in recent decades says bird cognition includes abilities such as delaying gratification - for example in hoarding food - and reasoning.
Corvids - the bird family that includes crows - are also known to use tools and think logically. Previous research, for example, has confirmed that a crow will drop stones into a beaker of water to raise its surface level so it can drink - a form of behaviour central to the Aesop's Fable The Crow and the Pitcher.
'The mental abilities of corvids and parrots are as sophisticated and diverse as those of apes,' the latest study says.