Betty the crow bends some wire into a hook to retrieve a treat-laden bucket from a well (footage: behavioural ecology group, Oxford University)
"In the past, people thought birds were stupid," laments the aptly named scientist Christopher Bird.
But in fact, some of our feathered friends are far cleverer than we might think.
And one group in particular - the corvids - has astonished scientists with extraordinary feats of memory, an ability to employ complex social reasoning and, perhaps most strikingly, a remarkable aptitude for crafting and using tools.
Some corvids, such as rooks, live in large groups
Mr Bird, who is based at the department of zoology at Cambridge University and is supervised by Dr Nathan Emery, says: "I would rate corvids as being as intelligent as primates in many ways."
The corvids - a group that includes crows, ravens, rooks, jackdaws, jays and magpies - contain some of the most social species of birds.
And some of their intelligence is played out against the backdrop of living with others, where being intelligent enough to recognize individuals, to form alliances and foster relationships is key.
However, group living can also lead to deceptive behaviour - and western scrub jays (Aphelocoma californica) can be the sneakiest of the bird-bunch.
Many corvids will hide stores of food for later consumption, especially during the cold winter months when resources are scarce, but western scrub jays take this one step further.
Sneaky scrub jays move their caches around when in the presence of other birds (footage: Christopher Bird/Cambridge University)
Mr Bird says: "If they are being watched, they will hide their food, but they will do some 'fake hides' as well - so they'll put their beak in the ground, but not place the food. It's a bit like a confusion strategy.
"Sometimes, if they are being watched, then they'll even go back and hide the food again."
You looking at me?
Corvids' cognisance of other birds has led scientists to ponder whether they are also aware of themselves.
And to test this, scientists use the Gallup mark test, where an animal is marked on a part of its body that it cannot normally see and is then shown its reflection in a mirror.
Can a magpie pass the mirror test? (footage: Plos Biology)
If it notices this mark and tries to remove it, then it suggests that the animal knows it is looking at itself and could possess some kind of self-awareness.
So far, only some species of primates have consistently passed this self-recognition test, although more recent studies suggest elephants and dolphins may also respond.
But last year, a German team revealed that magpies, marked with a coloured sticker under their beaks, tried to remove it when presented with a mirror - the first time a bird had been seen to pass this test.
Professor Onur Gunturkun, from Ruhr-University Bochum, one of the authors of the Plos paper, says: "It throws out the assumption that only higher mammals were capable of self-recognition."
While the birds' social intelligence has continued to impress, it is perhaps their physical intelligence, and in particular their tool use, that has stirred the most interest.
Recent studies reveal that corvids' tool-use may at least rival, and even surpass, that of primates, such as chimpanzees.
And one species in particular possesses an extraordinary ability - the New Caledonian crow (Corvus moneduloides), which is found on the Pacific island of New Caledonia.
Russell Gray and his colleagues from the department of psychology at the University of Auckland have studied this species extensively, and were the first to discover that the birds were crafting tools in the wild.
New Caledonian crows craft tools from branches to fish for grubs (footage: Russell Gray/University of Auckland)
Professor Gray tells BBC News: "They do some really complex looking things.
"We have seen that they take a whole branch, chop off the side branches and hone away at the end to create a hook, which they use to get grubs."
Other experiments carried out at field stations have even shown that the birds will use a number of different tools to reach a tasty snack.
Inside the laboratory, captive New Caledonian crows are also helping scientists to better understand tool use and corvid intelligence.
What has led to just this one species in this one little island in the Pacific being able to make these complex tools?
Professor Russell Gray
And one bird in particular seemed to posses a remarkable ability when it came to solving problems using tools - Betty.
Alex Kacelnik, who leads the behavioural ecology group at Oxford University, said: "Betty was captured as a juvenile from the field, and she must have been one-and-a-half years old when she came to us. And we didn't have any reason to suspect that she was an unusual animal."
Betty's tool use surprised the researchers
However the team discovered, by chance, that Betty was able to perform some remarkable feats that had never been seen before in any other animals.
The researchers were testing how New Caledonian crows selected tools by presenting them with a small bucket filled with some food, which was placed in a well, and pieces of wire, some straight and some with a hook at the end.
The aim was to see whether the crows would select the bent wire to retrieve the treat-laden bucket.
But Betty astonished researchers when she selected a straight piece of wire and then used her beak to bend it into a hook so she could pull up the bucket of food.
When she was later tested with just the straight wire, Betty repeatedly bent it into hooks - and other experiments with aluminium strips revealed how she would bend, shorten and lengthen the material to get to her food.
This was the first time that any animal had been seen to make a new tool for a specific task, without an extended period of trial-and-error learning.
'The million dollar question'
As scientists discover ever-more intelligent behaviour in corvids, they are now trying to understand why this group has developed these special abilities. And New Caledonian crows' tool-use is a key focus.
How do you define intelligence? How do you define what does it means to understand something?
Dr Christian Rutz
Professor Gray explains: "What has led to just this one species in this one little island in the Pacific being able to make these complex tools? It's an ongoing mystery."
Professor Kacelnik agrees: "This really is the million dollar question.
"We know that this is heritable - we have demonstrated that if you raise New Caledonian crows, without exposure to any social input, they still would want to use tools to solve problems."
Researchers are also looking at the cognitive processes that underpin this behaviour.
Mr Bird says: "The interesting thing is that they can do so many of these clever things that primates can do - sometimes they can do them even better. But their brain is completely different from the mammalian brain.
"They don't have the area of the mammalian brain that is thought to be the area of intelligent cognition - the neocortex.
"Interestingly, they have another area, the nidopallium, that might do the same job."
Corvid intelligence research is still in its early days
As scientists try to understand this, the research is also driving forward some more fundamental questions about intelligence.
Christian Rutz, who also works for Oxford's behavioural ecology group, says: "There are such enormous semantic issues. How do you define intelligence? How do you define what it means to understand something?"
We have to be careful with ascribing intelligence to seemingly impressive behaviours, he says.
He explains: "Not everything that looks smart to the human observer is actually smart.
"For example, take orb web spiders. These animals build sophisticated structures for foraging, but would we call this behaviour 'intelligent'? Probably not."
He says to understand what the birds are doing and whether this sets them apart in any way, the same experiments need to be carried out, multiple times, on many different species, to properly compare results.
Dr Rutz adds: "People tend to think corvid cognition research is now incredibly advanced and we've answered most of the questions - I don't think so, I think it is at the very beginning."
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