Eight things a crow works out for itself

Faced with a treat floating out of reach in a tall tube of water, New Caledonian crows do exactly what the thirsty crow does in Aesop's old fable: they drop in stones to raise the water until the food rises to the beak. Tested carefully, they chose sinking stones over floating ones, and water-filled tubes over sand. Their grasp of the trick rivals that of a five-to-seven-year-old child.

Put a small coloured sticker on a magpie's throat, where it can only be seen in a mirror, and the bird scratches at its own throat to be rid of it, proof that it knows the reflection is itself. The 2008 experiment made the magpie the first non-mammal ever shown to pass the mirror test, settling that this kind of self-awareness can grow in a brain with no neocortex at all.

Ravens will pick out a tool and hold on to it for a job that won't come until much later, even turning down a smaller treat offered right now to keep the tool for a bigger reward to come. In one study they held the right object for up to seventeen hours before the chance to use it arrived. That kind of forward planning was long thought to belong to the great apes and us alone.

Given pieces too short on their own to reach a treat, New Caledonian crows slot them together into one longer tool, a hollow shaft and a plunger, with no help and no training. One bird went further, joining three and even four parts into a single reaching stick. Building a working tool from separate components had never been seen in any animal outside the great apes.

Shown a cue, crows can deliberately produce a set number of caws, one, two, three or four, then peck a key to signal they are done. The very first caw already carries a hint of how many will follow, a sign the bird plans the whole sequence before it begins. It is the clearest case yet of an animal counting out loud, much as a young child does on its fingers.

A Clark's nutcracker, a high-mountain cousin of the crow, hides pine seeds for winter, tens of thousands of them, in thousands of separate buried caches scattered across miles of forest. Months later, even under deep snow, it returns and digs them up from memory, using rocks and trees as landmarks. Birds have relocated their stores more than nine months after burying them.

When crows find one of their own lying dead, they gather and mob the spot in a noisy crowd, and they are learning, not mourning. They memorise whatever was near the body, a person or a predator, and afterwards scold that face on sight. The lesson spreads to crows that never saw the death, and the grudge can hold for weeks, passed bird to bird without anyone risking a thing.