An adorable family of great horned owls (Bubo virginianus) has become an internet sensation after they were discovered nesting on the window of an office building in Reno, Nevada.

"I heard this racket outside my window," recalled Jim Thomas, a hydrologist at the Desert Research Institute.

Looking out onto the window ledge, Thomas saw a pair of great horned owls fighting off some ravens for the prime territory.

Soon enough, the male and female owl couple became famous around the office. But then, something odd happened: another female owl showed up.

The office watched in fascination as the two female owls began to lay eggs along the rocky window ledge. 

While the females were nesting, the male owl would bring back tasty treats from his hunting expeditions, like mice and an old rabbit.

The behavior from these owls isn't just unusual for an office environment, it's also completely unheard of in nature - mainly because great horned owls are monogamous.

Christian Artuso, an ornithologist with Bird Studies Canada, told National Geographic that this is the first time polygyny - which is when one male mates with more than one female - has ever been observed in the species.

The behaviour is even more confusing when considering that most great horned owls are solitary creatures. 

The species, which is quite territorial, doesn't usually flock together and they certainly don't nest near each other.

Still, the behaviour isn't completely out of the question for other owl species. For instance, scientists have noted instances of polygyny in barn owls and Eurasian eagle owls.

But overall, the behaviour is still very rare among raptors. Because unless there is bountiful food, Artuso explained, a male will generally not be able to provide two females with enough sustenance.

David Catalano, an ornithologist with the Nevada Department of Wildlife, agreed. 

"Very, very odd," he told National Geographic.

Realising how important the discovery was, Thomas' office set up a webcam to broadcast live footage of the owls. The owl family has since become an internet sensation.

"It's been quite frankly amazing," said Thomas.

It gets even more amazing. The second female owl did not look after her eggs well enough, and they failed to hatch.

So, when the first female's eggs hatched, the second female began caring for the two owlets, offering them shelter and food – despite the fact that she was not their biological mother.

Catalano said this is likely a classic case of misdirected parenting. In other words, the second owl mistook the owlets for her own.

Although, there is another explanation. The two female birds could also be related, maybe as sisters or as mother-daughter.

This could explain why the two birds generally get along well together. Like most family members, the two birds do get into "some pretty good battles" – although, according to Catalono, in general "they've co-parented quite well."

Still, without a genetic test, all of this is guesswork.

Never mind the odd family set up, the owlets appear to be doing just fine.

One of them has already flown out of the nest, landing safely below the ledge. The other owlet will soon follow.

"It could be any day now," Catalano said.

And with that, the three adult owls will be empty nesters.