How an epic bird migration hinges on just one tiny insect

Every year, millions of birds make epic flights, traveling thousands of miles from as far north as the Arctic to as far south as the tip of South America. Fueling these journeys is an obscure creature — the brine fly — whose presence on the ground creates a critical rest stop.

This under-appreciated insect swarms around the waters of salt lakes, consuming algae and bacteria. Billions of brine flies can congregate on the shores of a single lake and serve as an important reason for birds to stop — a fly buffet station of sorts — making completing the entire journey possible, says scientist Ryan Carle, who researches waterbird conservation, including at salt lakes.

Brine flies do an incredible amount of work, but they get little credit and are sometimes scorned. When birds are traveling south, “The heart of the whole migration is the source of alkali” — or brine — “flies,” said Carle, the science director for conservation group Oikonos Ecosystem Knowledge. “That’s why [birds] care about saline lakes, because saline lakes have alkali flies.”

The high densities of brine flies — so dense shorelines can look like they are matted with tar — are an indication of a healthy lake, not a pestilent one. The more flies, the more birds.

But these bugs, adapted to spend much of their lives underwater and hardened to harsh salty conditions, are in trouble.

By volume, saline lakes make up 44 percent of all lakes on Earth and serve as ecological refuges in some of the planet’s driest regions. Over the last century, they have shrunk across the globe, a trend largely driven by farms and cities that consume the water before it reaches the lakes. In some parts of the world, including the Western United States, climate change has made the problem worse.

And less water means higher concentrations of salts, which in turn strains the ability of flies to reproduce, adapt, and thrive. When lakes dry up, the flies can all but disappear. The shores become eerily vacant.

That’s what happened in 2022 at North America’s largest saline lake, the Great Salt Lake in Utah. For Bonnie Baxter, the director of the Great Salt Lake Institute, it was a wake-up call: “There were dead fly pupa and no adult flies,” she said. “It was so devastating.”

When the populations of flies collapse, the consequences ripple across half the world.

Birds need flies for food while migrating thousands of miles. When the flies are not there, their survival hangs in the balance. While the shorebirds are often the star of epic journeys across the hemisphere, brine flies play a central, if obscured, role in giving energy to these flights.

If the lakes form a network of rest areas along a vast migration route, the undistinguished flies stock the shelves, refueling birds on their journeys halfway across the world.

When the brine flies disappeared at the Great Salt Lake, it highlighted how quickly a linchpin species in the ecosystem could vanish. Invertebrates, animals with no spine and unfairly viewed as pests, help form the backbone for the entire path of migration.

“A lot of these species are eating alkali flies at saline lakes and going to the Gulf of Mexico where they are eating kelp flies,” Carle said. The “flies unite us.”

Brine flies thrive in strange, liminal habitats — not in freshwater, on land, or in the briny waters of the sea.

Using a special adaptation that filters saline compounds from their small bodies, brine flies have learned to live in some of the harshest landlocked waters imaginable. Scientists have found brine fly species at hot springs in Yellowstone and breeding near crude petroleum. “They can live in all kinds of bizarre places,” said David Herbst, a biologist at UC Santa Barbara’s Sierra Nevada Aquatic Research Laboratory.

The flies spend the larval and pupal stages of their life cycle underwater, only emerging at the surface as adults. As they clear salt out of their system, they excrete a mineral that latches onto nearby rock, imprinting their biology onto the geology of the lakes they inhabit.

Although birds also consume brine shrimp, the flies are the highly prized prey, packed with more nutrition than other invertebrates. These flies are so loaded with protein they help birds like the Wilson’s phalarope, a threatened shorebird with a rust-colored neck, double in size as they await their flight from North America to lakes in Argentina.

But even creatures adapted to the fringes of life on Earth — whether they be bacteria or worms — have their limits. In a recent paper, Herbst wrote salt-loving “halophilic organisms often do not so much as love salt as they have evolved means to tolerate it.”

And to tolerate it, they need water.

When lakes dry and the flies are faced with hypersaline conditions, they expend more energy trying to keep the concentrations of salt in their blood at a constant level. That can exhaust them and prevent them from growing and reproducing the next generation of baby brine flies. Lower lake levels risk exposing submerged “microbialite” or “tufa” rock that are important for fly life cycles.

“As lakes become more and more salty, the populations are going to be diminished,” Herbst said.

From the Aral Sea in Central Asia to Lake Urmia in Iran, saline lakes are struggling globally. They are especially sensitive to change because of their geography. They mostly exist in dry areas and in what scientists call endorheic basins, closed systems that do not reach the sea. Freshwater rivers instead end at inland or “terminal” lakes that are often saline. Over the past century, cities and farms have overtapped freshwater rivers and aquifers feeding these lakes. When less water makes it to them, salt concentrations rise beyond what the flies can handle.

In the Western US, the situation is especially bleak as scientists warn that the same overused rivers and aquifers face pressure from increasingly arid conditions, fueled by climate change. The Great Salt Lake faces ecological collapse, while in Oregon, ranchers have pulled out water before it can reach Lake Abert, which has dried up four times in the past decade. Over the last century, Los Angeles has diverted water from Mono Lake and Owens Lake, leaving them a fraction of what they once were.

As the landscapes home to these bodies of water become hotter and more arid, there will be even less to go around.

That’s a problem for the flies — and the birds. The West’s expansive Great Basin region — touching Oregon, Idaho, California, Nevada, and Utah — is like an airline hub for migration.

Birds congregate at Great Basin lakes to feast on flies before flying south in different paths to the California coast, the Gulf of Mexico, or South America. The combined water in the Great Basin, one of the driest parts of the US, is what makes it possible to refuel on brine flies.

The absence of water — and its effect on invertebrates like brine flies — is one of the reasons conservation groups, led by the Center for Biological Diversity, petitioned the US government to list the Wilson’s phalarope as a threatened species earlier this year.

Separated by mountain ranges, the Great Basin’s salt lakes often rise and fall together as they face similar threats from climate change and ever-increasing demand for water. When flies at one lake pour more energy into surviving in saltier water, it’s a good bet the fameless flies — on whose back a global bird migration rests — are fighting to keep going at another lake.

As the flies work harder to navigate the saltier environment, they have not been able to adapt as fast as humans are changing their habitats. “The expense of getting rid of those salts is at the cost of development and survival,” Herbst said.

A self-described “salt lake junkie,” Herbst has been studying saline lakes for five decades since he was a student at UC Davis. He’s spent much of that time studying alkali flies, a species of Ephydridae, at California’s Mono Lake, tucked beneath the steep foothills of the Sierra Nevada mountain range.

Starting in the 1940s, the city of Los Angeles began diverting water from the streams that feed Mono Lake, causing steep declines and rising salinity. In the early 1980s, the courts stepped in and said the state must balance diversions against how they affect “public trust” values like the environment and recreation. It meant there was a fighting chance for the ecosystem.

In recent years, he’s brought his knowledge to the Great Salt Lake to help monitor the flies and work toward lake recovery. He’s learned Ephydridae are sensitive to change but also resilient. Flies patiently wait for water. When water comes back, the flies will come roaring back too.

This is good news not only for the brine flies but for the bird migration that depends on them.

In the dry years, brine flies find hideouts in whatever water is available. Many persist by relying on nearby groundwater-fed springs and seeps on the edge of salt playas. When water comes back to saline lakes, the brine flies can repopulate quickly. He points to Lake Abert in Oregon. After completely drying several times, flies returned from their groundwater-fed shelters when the water returned.

As an even more extreme example, he points to Owens Lake. Similarly fed by streams running off the Sierra mountains, Owens Lake became a target of Los Angeles’s ambitions to grow.

In one of the most notorious water plays in US history, the city acquired massive landholdings in the area, capturing streams for urban development. Instead of going to Owens Lake, water was rerouted through the city’s aqueduct. This left the lake so dry it was considered a total loss, known only for the dust pollution generated off its desiccated shores.

But in the early 2000s, Los Angeles began bringing water back to the lake for dust control. In the process, the city created new aquatic habitat on a lake. It was just the right salinity for algae to grow, and brine flies moved from their hiding spots in shoreline springs and seeps. “Once conditions were permissive, they recolonized in a hurry,” Herbst said.

The birds returned quickly too. By 2013, an April survey counted about 115,000 birds at a lake once considered dead.

“These places can really recover — and recover quickly,” he said. “So we really should not be writing them off.”