Every day on her way to work, Lijin Zeng would ride her bike along the rim of Kīlauea Caldera next to the world's largest active volcano, Mauna Loa, on Hawaii's Big Island. Steam rose from the ground as the warmth emanating from beneath the soil met the cool ocean breeze. Eventually, the biologist would reach her field site, Kipuka Puaulu, a 40-acre patch of undisturbed, native forest.

For millennia, lava flows tumbled down the side of the mountain and surrounded the forest below, burning a rock hard fence around it over and over again. As a result, the habitat encircled by the molten rock remains home to some of Hawaii's rarest endemic plants and animals, such as the i'iwi, or Hawaiian Honeycreeper, and the 'io, or Hawaiian Hawk. But Zeng didn't make her trek to study these birds or any of the other rarities. She went to observe an intruder called the Kalij Pheasant.

The species was first introduced to Hawaii in 1962 for hunting purposes at the Pu'u Wa'awa'a Ranch. Enjoying a habitat largely free of predation, the birds reproduced and eventually expanded beyond the borders of the ranch. The entire island population today is believed to have originated from the 67 individuals first imported from game farms in Michigan and Texas. Indeed, by 1984, Kalij Pheasants could be found in just about every forest type from sea level to 2,500 meters up, and a 2003 study suggested that their range was still expanding. They first reached Kipuka Puaulu—protected today within the borders of Hawaii Volcanoes National Park—by the late 1970s, and have been abundant there since at least the 1990s.

Eventually, some researchers with keen eyes began to notice that these birds seemed to behave differently than those in their native range. That's what led Zeng, a biologist at the University of California, Riverside, to combine data from nearly 11 years of observations. When she did, Zeng and her colleagues discovered a wholly different social system in place among the Kalij Pheasants of Hawaii: Unlike their South Asian counterparts, Hawaii's pheasants live in large social groups. The findings were published this week in The Auk: Ornithological Advances.

In their native range along the western slopes of the Himalayas, Kalij Pheasants were the focus of just one in-depth field study in the early 1990s. The researchers noted then that the birds occurred in pairs of a single male and a single female, and occasionally a male would be seen with multiple females. As a result of that study, it was assumed that the birds led a monogamous lifestyle, though polygyny was at least a possibility.

But in Hawaii, Kalij Pheasants live a more gregarious lifestyle. In all, the researchers followed 31 different groups, each comprised of just one female, between one and six males, and their offspring. While the females incubated the eggs alone, the males all helped with parental duties like defending the chicks and providing food. This social system is called "cooperative breeding."

The behavior is surprising not just because this is a social system undetected in their native range, but because when Kalij Pheasants first hatch, they don't really need much looking after. "We know that cooperative breeding is rare on its own, and it's more rare in precocial species," Zeng says. In an altricial species, where offspring are born especially helpless (like humans), this sort of system makes a lot more sense. Newly hatched chicks might need more care than their parents alone could provide.

So why would cooperative breeding have emerged in a precocial species like this one?

The researchers think it's encouraged by the unique Hawaiian habitat in which the Kalij Pheasants have found themselves. While they are at risk from feral cats, mongooses, domestic dogs, and raptors, the pheasants live a largely peaceful existence. But living on an island means their range is restricted. Combined, these two factors can result in overcrowding. If all the available territories and nest sites are taken, and all of the food resources are spoken for, then teenage pheasants that would otherwise leave their family group in search of new opportunities might be prevented from doing so. And since they're staying around, they might as well help with childcare.

It's not entirely a raw deal. Eventually the males work their way up the social hierarchy and become dominant breeders. Some even become successful parents long before that. While dominant males fathered more than two-thirds of the chicks assessed by Zeng's genetic analysis, nearly one in every five chicks was fathered by a subordinate. Those aren't bad odds. And the chicks that the subordinates help raise, even if not their own, are probably their relatives anyway.

To Zeng, this suggests that social behaviors are a lot more flexible than we typically assume. We usually think of species as either social or asocial, but these island-living pheasants suggest that habitat plays a fundamental role in determining social behavior. "Their behavior is shaped by the environment to a larger degree than we expected," she says. So what would happen if the Hawaiian pheasants were returned to their natural habitat in the Himalayas? Zeng thinks they would probably return to their original monogamous lifestyle.

Such social flexiblity is fascinating to consider and an important reminder that species aren't simply superimposed onto their environment. Rather, the Kalij Pheasants of Hawaii demonstrate that animals in the real world, including humans, are joint products of their genetic endowment and the circumstances in which they live. 

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