The National Weather Service at Boulder posed a bit of a mystery Tuesday afternoon - they posted GIF images of something quickly engulfing the Denver metro area on radar.

At first, they thought it might be a bunch of birds heading south for the winter:

"Look what's flying into Denver!" they wrote. "Radar from last hour showing what we believe to be birds. Any bird experts know what kind? #ornithology"

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The weather service's Twitter question generated quite the response:

Jeff Wells, whose Twitter bio says he's a bird conservationist, pointed out to NWS that very few birds actually migrate in the daytime; most do so at night.

Someone tried to say it was insects, but NWS shot that down, saying insects are normally more dispersed than the radar image.

A 9NEWS producer even reached out - and NWS doubled-down on the bird thing, adding that no expert being able to figure out the breed.

Then someone pointed out that the radar image showed whatever it was moving north-northwest - birds would be going south.

Multiple people started arguing for the butterfly theory - saying that they'd seen tons of them around lately and it would make sense.

"Could it be butterflies?" asked Brett Perry. "Butterflies EVERYWHERE in #Denver !"

Just before noon on Thursday, NWS tweeted four more times, confirming that it was, indeed, butterflies showing up on radar.

Another Twitter user explained why, if the butterflies were heading south, were they going to the northwest; the headwinds on Tuesday were so strong it was pushing all the butterflies away - even though they wanted to go southwest.

So why was NWS so sure it was birds?

"Things with big wings need to fly together in the same direction with the wind to generate that signature in ZDR (see the image attached to this story)," NWS wrote on Twitter. "Insects rarely produce such a coherent radar signature. Migrating birds do all the time."

There you have it; NWS was going off examples they've seen probably hundreds of times.

As for the butterflies, NWS said they haven't created any similar image and are instead staying low to the ground.