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.
'Among other things, they are capable of thinking logically, of recognising themselves in the mirror and of empathy.'
The study highlights the fact that birds and apes use different brain structures to think.
Mammals' cognitive skills are controlled by a part of the brain called the neocortex, while crows and parrots manage complex mental tasks with a structure called the pallium.
Professor Onur Gunturkun, of Germany's Ruhr University, and Professor Thomas Bugnyar, of the University of Vienna in Austria, speculate that a common ancestor between apes and birds may have passed on a brain module that is similar in both.
However, they believe a more likely explanation is that they evolved independently of each other by facing the same challenges of survival.
The scientists, whose study is published in the journal Trends in Cognitive Sciences, think this would mean certain brain 'wiring' patterns are necessary to boost cognitive skills.
Professor Gunturkun said: 'What is clear is that the multi-layered mammalian cortex is not required for complex cognition. The absolute brain weight is not relevant for mental abilities, either.'
While ape brains weigh 10oz to just over a pound on average, those of birds weight 0.2-0.7oz.