Birds are one of the most well studied groups of animals in the world, and yet, we're still finding new species, some of which we've never seen before and others that have been hiding in plain sight for hundreds of years.

The gentoo penguin (Pygoscelis papua) was first discovered in the cold Southern Ocean in 1781. In the years since, scientists have split populations into two subspecies, one that lives in the Falkland Islands (P. p.papua) and one that lives in the South Shetland Islands and Western Antarctic Peninsula (P. p. ellsworthi).

Now it seems that one species has been split into four. Despite sharing roughly the same appearance, in habitats not so far apart, the gentoo penguin appears to have divided itself into four populations that have little to do with one another, according to new research.

"For the first time we've shown that these penguins are not only genetically distinct, but that they are also physically different too," says Jane Younger who studies population genomics at the University of Bath.

"Gentoos tend to stick close to their home colonies, and over hundreds of thousands of years have become geographically isolated from each other to the point where they don't interbreed with each other, even though they could easily swim the distance that separates them."

And that's essentially the concept of a species: an interbreeding group reproductively isolated from other such groups.

Using genome data and measurements from museum samples, researchers found clear differences in the genes and morphology of gentoo penguins.

The differences are great enough that the authors think both recognised 'subspecies' should be elevated to their own species, while two new species should also be added.

While these penguins once held a common ancestor, their breeding habitats are quite different today, spanning flat beaches, gravelly beaches, and tussock grass nests, while their diets are made up of various ratios of crustacean, fish, krill, and squid.

"They look very similar to the untrained eye," says evolutionary biologist at the University of Bath and the study's lead author Josh Tyler, "but when we measured their skeletons we found statistical differences in the lengths of their bones and the sizes and shape of their beaks."

Their DNA alone is a dead giveaway, confirming these groups are not breeding with each other. This basic difference suggests all four species are now evolving independently of one another.

One of the new distinct species was identified on Iles Kerguelen, and in 1927 it was briefly classified as a subspecies (P. p. taeniata). The fourth species lives on South Georgia Island and has never before been described.

The team now calls it Pygoscelis poncetii – named for Sally Poncet, an Australian scientist and explorer who spent much time on South Georgia Island surveying wildlife and writing conservation reports. She's even given birth on the island after one particularly cold winter when their boat reportedly became frozen in ice.

"Our results clearly support the division of gentoo penguins into at least four species," the authors conclude.

If it wasn't for genomic research, we might never have known. This diversity of hidden species is something we have only just started to untangle, and in the avian tree of life, we've probably overlooked a lot of little branches.

A study published in 2016, for instance, estimates there are about 18,000 bird species in the world, which is nearly double what we thought, and that's because many birds that look similar to one another are actually different species.

Like unpacking a Russian doll, scientists have discovered species within species of giraffe, dolphin, bird, and orangutan, all in the past ten years.

Researchers have even found the reverse in two distinct penguins species that appear to be melding into one.

Practically, of course, such discoveries don't change anything for the animal, but it does force us to reassess each species' habit, behaviour, and conservation status. This will allow us a greater understanding of each species and the threats they might face in the coming future.

"Currently gentoo penguins are fairly stable in numbers," says Younger, "however there is some evidence of the northern populations moving further south as the climate gets warmer, so we need to watch them closely."

The two new additions, P. taeniata and P. poncetii, both show evidence of decline, right as we began to notice their existence.

The study was published in Ecology and Evolution.