A swan in Europe was a white swan. So, naturally, swans were thought of as only white by Europeans until news came from travelers to Australia in the late 1600s where black swans are in fact relatively common. Black swan events can turn a worldview upside down. They are unprecedented, unexpected, rare, and with a significant impact.

The infrequency of black swan events means prior data about them is too little to reliably guess the likelihood of one, and you may not even think to anticipate the event anyway. Except after the fact, when all of a sudden many people will claim to have predicted it — black swan events are prone to be rationalised in hindsight, believing that all the signs were there beforehand even if they weren't.

The idea of black swan events is from Nassim Nicholas Taleb. He writes:

"One single observation can invalidate a general statement derived from millennia of confirmatory sightings of millions of white swans. All you need is one single black bird."

Examples Taleb gives include the September 11 terrorist attack, the 2008 financial crisis, World War I, or the rise of the Internet.

More Taleb sketches: on mistakes, the firehouse effect, and the Lucretius problem. Also, hindsight bias.

Black swan events

When we notice two things change with each other — sunny days and ice cream sales, or player height and goals scored — it's natural to think that one thing is the cause of the other. And it's only a small step to consider that if you do more of one, you'll get more of the other. But this instinct and our natural sense-making behaviours can easily lead us astray. Just because two things change together — are correlated — doesn't mean that one causes the other: correlation is not causation.

You might reasonably notice a correlation between the number of children in a household and the number of storks nests on the roof of houses in Denmark or Holland. Fortunately, while the two may be correlated it's not hard to see that it's most likely not the storks that are causing the larger families and instead another factor such as the size of the house that is the more likely cause. As such it's a handy reminder that correlation is not causation.

This is also the subject of my favourite xkcd on correlation, and another on cell phones to drive the point home.

For more see the excellent spurious correlations.

The stork nest example is from Darrel Huff's How to Lie with Statistics, as is the example on Sampling Bias.

Correlation is not causation

It all matters. Be it one person, one bird, or one tree. We are all connected. As John Muir wrote in My First Summer in the Sierra:

"As soon as we take one thing by itself, we find it hitched to everything else in the universe."

The sketch is Heart Lake in his beloved Sierra Nevada.

Hitched to everything else in the universe

Funny how it can take nearly 40 years to learn that something as commonplace as the stereotypical owl sound is made, not by one owl, but as a duet between the male and female tawny owls.

The classic twit-twoo is actually a duet

Owls are famous for being able to turn their heads seemingly full circle. It turns out some owls can turn their necks an amazing 270 degrees whereas our necks turn a measly 180 or so — see if you can turn your chin along your shoulder on both sides.

However, it’s likely the cause for this amazing feat is that, unlike our eyes, theirs are set facing forward and don’t rotate. So rotating their head becomes a key skill. Biomechanically they are able to turn so far by having twice the neck vertebrae as we do. If each vertebrae turns a little then having more helps them twist much further than us.

And yet the real trick is turning your neck while maintaining blood supply to your head through the arteries that also run up your neck. Rotate too far and the arteries can twist, stretch or be impeded. Owls have evolved a number of nifty strategies to help them turn their necks while maintaining blood flow. These include having the large carotid arteries towards the centre where they twist less than ours do, which run up the sides of our vertebrae. They also have loads of space around the arteries that run up inside the vertebrae and an area that may pool with blood at the base of the skull to provide a kind of reservoir if flow gets restricted. And more. All kind of amazing.

How owls’ necks turn so far round

…regulate temperature and (maybe) to keep feathers clean while eating. Keeping the feathers clean was my inspiration for this sketchplanation - there’s something about the cold practicality of not messing up your feathers while you’re tucking into some carrion. However, it looks like it is more likely to be for the more prosaic reason of regulating temperature and keeping cool. As Dominic McCafferty, at the University of Glasgow points out:

‘…other groups of scavengers, particularly giant petrels, have feathered heads. Large mammalian scavengers also don’t have bald heads, so why vultures?’

Should you be interested, read more at the University of Glasgow’s Why do vultures have bald heads?

Birds sketches