Is This A Rabbit or A Bird?

Never mind what color that dress is -- the internet has a new optical illusion to puzzle over.

A viral video of a man petting an animal has Twitter in a tizzy -- because they're not sure if it's a bird or a bunny.

The 30 second clip was uploaded by Daniel Quintana, a senior researcher in biological psychiatry at the University of Oslo in Norway, on Sunday. It has since been viewed 4.3million times, retweeted 11.5k times, and has 45,000 likes.

At first, the cute vid appears obvious: a raven with his head cocked back, beak agape, absolutely loving a head scratch.

But after reading the caption — "Rabbits love getting stroked on their nose" — the beak suddenly morphs into ears, the scalp suddenly morphs into a snout, and the bird suddenly morphs into a sleek black bunny facing the opposite direction.

However Quintana (who does not know who owns the original vid) knows the correct answer — and the clue is in the blink.

"I thought it was fairly clear that the video was of a bird... as you can see the translucent nictitating membrane sweep across the eye horizontally (rabbits don't have membranes like this) and the positioning of the 'ears' are a little strange," he told CNET in an email.

"I made the rabbit comment to prime readers into thinking it was actually a rabbit, to give it a fighting chance at least," he said. "When you only see the beak in your peripheral vision, it really seems like they're ears. Without this misleading cue, I thought most people would have seen a bird."

He added: "I'm pretty confident this is a white-necked raven, but I'm not a corvid expert so I can't say this with 100 percent certainty."

Quintana pointed out that the "static bird/rabbit illusion" is well-known within psychology and philosophy, which is why he first decided to share it.

Indeed as the tweet went viral, users shared some of the many existing examples — but not many had ever seen a live version before.

The debate wasn't quite as decisive as the infamous "Dress" incident of 2015, when the internet went to war over whether a gown in a photograph was blue and black, or white and gold. It was later proven to be the former, with some people's color perception fooled by their brain making assumptions about the amount of light in the background.

Nor was it as divisive as the "Yanny or Laurel" auditory illusion, which found the web split almost down the middle over an audio recording of a voice repeating either the word "Yanny" or "Laurel", depending on who you asked. It was later confirmed both sets of sounds were present but at different frequencies, and that the brain can only focus on one or the other.