Better Know a Bird: The Massive, Fickle Breeding Colonies of Tricolored Blackbirds

At first glance, the species might be hard to distinguish from Red-winged Blackbirds, but wait until they gather by the tens of thousands to nest.

In California’s winter months, hordes of Tricolored Blackbirds haunt dairy farms. Males steal grain from feedlots, and females trail plows as the farm equipment churns up insect larvae buried in the soil. The birds mingle in mixed flocks with Red-Winged Blackbirds, European Starlings, and Brown-headed Cowbirds, where they are easily mistaken for their Red-Winged relatives. To an inexperienced eye, the two species appear nearly identical, but Tricolored males have red and white shoulder adornments, not red and yellow, and sport a glossy blue sheen. 

Tricolored and Red-winged Blackbirds may look alike, but that’s where their similarities end. “They couldn’t be much more different,” says Robert Meese, an ornithologist at University of California, Davis, who has banded more than 90,000 Tricolored Blackbirds in the last 15 years, by his count. While Red-winged Blackbirds range across virtually the entire continent, Tricoloreds are much more selective. Nearly the entire population lives within the state of California, where the species hangs out in wetlands and grain fields. But where the birds do reside, they show up in force.

Each spring, Tricoloreds flock by the tens of thousands to breed. They’re the only remaining land birds in North America that nest in such immense crowds, reaching upward of 20,000 individuals at one site. A century ago, those throngs could be more than 10 times as large and number up to 300,000 birds in one colony. That was before urban development and expansion of nut orchards and vineyards in California encroached on what had been one of the world’s largest wetlands, reducing the birds’ preferred habitat to less than 10 percent of its original expanse.

Nowadays, the species often nest in farmland—the closest thing to wetland they can find. But they’re a picky bunch. Tricoloreds raise their chicks only in brambly plants, like cattails or bulrushes in the remaining wetlands, or fields of blackberry, grain, or thistle. The nesting sites need to be close to a water source: a stream, wetland, or even a watering trough. And the birds require a bountiful source of bugs within a few miles of their colony, at any grassland or open field without insect-killing pesticides.

To choose where to nest, thousands of males scour the landscape, shifting breeding locations from year to year. When the birds finally select a nest site, they descend all at once. An empty field transforms into a dense bird city in a matter of minutes.

The newcomers aren’t shy. Males make a high-pitched screech like a meow on first arrival to a nest site. "They literally sound like cats,” Meese says. They shriek at each other or scuffle to stake out their initial claims to a small territory, though they remain nestled up close, within a few feet of one other. The din can carry for more than half a mile, Meese says. But for the remainder of the season, the birds remain quiet to avoid alerting potential predators of their whereabouts. 

Females typically show up a day or so after the males to choose a partner. Often two or more females will mate with each male. While this set up sounds cushy for the males, females hold the final say in who they’ll mate with. Once a female decides she likes a male and his territory, she solicits copulations and sets to work building her nest. She dips sturdy grasses into nearby water to soften the fibers, before weaving the material onto hardy stems to sculpt a cup-shaped nest. Soon, she’ll lay three or four eggs inside.

Sometimes, days after a colony has settled at a site, the birds will get up and leave without laying a single egg. Meese suspects these great desertions happens when females can’t find enough food to produce eggs. Then, the birds fly off all at once—and settle down at a new nesting site somewhere else. "Sometimes they’ll hop to a field across the street. Sometimes, they might move to a field a mile away," says Nicole Michel, a quantitative analyst with the National Audubon Society who's modeled the species' population. "Sometimes they leave for no reason that we can tell—they just up and move."

Tricoloreds don’t go through this fussy nesting process only once each year. After breeding early in the spring, they pick up shop and do the whole thing over again at a site farther north. "To make things more confusing, they breed twice," Michel says.

These idiosyncrasies make Tricoloreds one of the most unique birds in North America, particularly in comparison to the ubiquitous Red-winged Blackbird. But there’s one other major difference between the two species: Tricolored Blackbirds are at risk of extinction, particularly as nut farms continue to expand in California, diminishing the birds’ already reduced foraging habitat. “[The species is] struggling to accommodate change,” Meese says.

Concern over the birds' population declines has spurred immense volunteer survey efforts led by Audubon California and its partners to better monitor the species. It was partially due to these efforts in conjunction with other data, including an eBird analysis performed by Cornell's Lab of Ornithology, that led the state of California to list Tricoloreds as threatened in April. Now, the federal government is currently reviewing the species for listing under the Endangered Species Act, and will likely announce their decision in the coming weeks. 

Despite the protections gained from endangered species listing, Meese continues to worry about the birds’ future. Whenever he gives presentations about the species, he urges the people to see Tricolored Blackbird colonies while they can. “Please try to get out next spring,” he says, “because they’re disappearing.”