Recently, we did the squiggle birds exercise where we drew random squiggles and then passed the paper along in the group. Each member of the group was then supposed to add other features to make the squiggle look like a bird--hence the name squiggle bird.
This exercise is based off the fact that humans are pattern recognizing machines and we are basically always looking for patterns even when there really isn't any, which leads to some pretty interesting news articles (i.e. toast Jesus). I think it's probably a leftover survival trait from our cavemen days when a good visual wasn't always guaranteed and so our brains fill in the missing gaps. For example, if night already fell and you see what might be a face peeking through the bushes, you would pick up a nearby stick to defend yourself. It could just be the shadows playing off the leaves but it's better to be safe than sorry.
Not a super fun theory about how this phenomonal came to be but at least we got some cute and some wonky birds out of it right?