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Tucked out of sight, away from the showy exhibits at your nearest natural history museum, lie the institution’s real treasures: its collections. Stored inside drawers and cabinets are thousands or millions of individual specimens—dead, shelf-stable plants and animals gathered from the wild. For scientists, they are a window into past life on Earth, showing how species and their surroundings have changed over decades or centuries.

“How the environment changes is going to be reflected in the things that live in that environment, and the only insight we have into that, through time, is in museum collections,” says , the curator of birds at the Field Museum in Chicago, which holds 40 million artifacts and specimens.

But a new study, , finds that the female sex is underrepresented in bird collections from five of the world’s foremost natural history museums. This male bias must be addressed, curators say, so that current and future scientists can properly study and find solutions to leading challenges facing bird populations.

The news isn't a surprise to , assistant curator of ornithology at the Natural History Museums of Los Angeles County, who has had trouble finding female specimens of tanagers and hummingbirds, forcing her to adjust and sometimes limit her research. “There’s so much biology we’ve been missing because of male bias,” says Shultz, who was not involved in the study. “It’s great to see it documented in a rigorous framework.”

A group of researchers at the Natural History Museum in London compiled records from 1.3 million bird specimens collected between 1751 and 2018 from the American Museum of Natural History in New York, the Field Museum in Chicago, Muséum National d'Histoire Naturelle in Paris, Smithsonian National Museum of Natural History in Washington, D.C., and their own institution. Nearly half of those specimens weren’t labeled by sex. Of those that remained, 60 percent were male and 40 percent were female. That translated to over 143,000 more male birds than female birds in those five major museums.

The pattern holds across bird groups: In all six of the largest avian orders—songbirds, hummingbirds/swifts, woodpeckers and relatives, parrots, shorebirds, and doves—museums had more male specimens than females. And for "type" specimens, which officially define a given species, the male bias is even more pronounced: Only 25 percent are female. That means that crucial elements of species’ biology and behavior—including sex-specific behaviors, egg production, and often rearing young—are not captured in these biodiversity storehouses.

This collection bias can be partly explained by biology. In species where male and female birds differ physically, the male is often a colorful show-off, flashing brilliant feathers and sometimes performing absurdist choreography to attract a mate. He might perch out in the open, predators be damned, and sing extravagantly to claim territory.

“If you’re showy or bright, you’re that way for a reason: to get attention,” Hackett, who was not involved in the new research, says. The male birds aren’t trying to lure scientists seeking museum specimens, but humans may be drawn to the same bright colors and performance that attract female birds. “If the males with the brighter plumage are easier to find, you’re going to capture more males,” she says.

Female birds are often trying to avoid attention of a different sort: from predators eager to snatch defenseless eggs or chicks from the nest. As a result, female coloring is often—but not always—muted or cryptic. They’re trying to hide, and as a result they hide from scientists, too.

However, curators say that an element of human nature could also have led to this bias. When many of these older collections were created centuries ago, researchers liked to best each other: They wanted to capture the most colorful or biggest trophy, and in birds, that often means the male. “If we open up our case of birds-of-paradise, we often have more males than females because people want to collect them,” Shultz says. What’s more, historically, some ornithologists have argued that females are a mere subset of a given species, she says, playing a supporting role to the male’s true expression—an outdated concept that's become less prominent, but not extinct, yet.

No matter the reason for the male collecting bias, the result is that natural history museums likely don’t describe the full scope of bird ecology, Shultz says. As a bird moves through its environment, behavioral differences—for instance in migration, diet, or habitat—can be captured in the composition of its feathers and tissues and later studied. “If we don’t have female representatives of some of these species, that limits our ability to understand the female side of the ecology,” she says.

Crucially, male bias impacts genomic studies of bird populations, the authors write, which are increasingly important for crafting conservation strategies to recover threatened species or identifying genetically-unique species that need protection. Female specimens are necessary in these studies because female birds carry an entire chromosome that’s not found in males.

“When many of these specimens were collected, scientists didn’t know about the structure of DNA,” Hackett says. “Female birds are really important in a way that 20 or 50 years ago we wouldn’t have known.” Similar stories are playing out in other fields of ornithology, notably in the increasing recognition and study of female birdsong, which was sidelined in the past. 

That’s why, curators say, it’s important to reduce the male bias of collections. Whenever Hackett goes out into the field, she’s mindful of gathering females and males. Modern methods make that easier. Even 100 years ago, crews of researchers shot birds by sight. This continues today, but increasingly museums receive salvage collections, which are donations of birds which died by other means, such as window strikes. These aren’t typically sex-biased, Hackett and Shultz say. Meanwhile, mist-netting, a common technique, indiscriminately catches all birds that fly into a net—although, they note, researchers often attract species by playing male bird calls, a strategy likely to selectively pull in territorial males.

It might take some extra effort, Hackett says, but it’s worth ensuring biological collections represent the full breadth of bird diversity. Who knows—future technologies could unlock other secrets of bird specimens and inform conservation. “What else don’t we know?” she says. “I just know that, no matter what, the specimens are at the heart of whatever we’ll be studying.”