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As people have remade the American landscape, they’ve also shaped the fortunes of Common Grackles. The iridescent blackbirds flourished in the grain fields and pastures that European settlers cultivated after cutting down forests in the 18th and 19th centuries. By the 1970s, an estimated 150 million grackles inhabited a vast stretch from the East Coast to the Rocky Mountains, and today they are regulars across much of the continent. But the birds are disappearing—and no one knows why. A new tracking project aims to reveal what’s driving the mysterious decline.

Birders were among the first to gather evidence of the species’ troubling trajectory. In winter, grackles join Red-winged Blackbirds, European Starlings, and other birds in giant swirling congregations. Լ����Ƶ’s annual Christmas Bird Count (CBC) long tallied roosts of up to tens of millions of birds. But in the past couple of decades, participants have rarely found flocks of more than a few million, says former CBC director Geoff LeBaron. Other blackbirds are in decline, too, but grackles have become noticeably absent from winter roosts.

According to CBC data, Common Grackles have been dropping at a rate of 3 percent per year, which amounts to a roughly 78 percent decline since 1970, says Tim Meehan, a quantitative scientist at Լ����Ƶ. Meanwhile, federal Breeding Bird Surveys, which take place each summer, have found at least a 50 percent decline over the past half century. These data clearly indicate that there are fewer grackles across the board, Meehan says: “It’s a slam dunk.”

Michael Ward, a University of Illinois biologist, is one of the lead scientists delving into the conundrum. In 2021 he and his colleagues a hunch that grackles were failing to produce young: 60 percent of chicks in nearly 200 nests that he monitored fledged successfully, a higher rate than most songbirds achieve. Whatever is harming grackles is likely affecting adults, Ward says.

It’s possible the resourceful foragers, which eat everything from grubs to grains to garbage, are exposed to something dangerous in their diet. The researchers suspect insects and corn could be hazardous meals: Both may contain high levels of neonicotinoids—insecticides commonly applied to grains that have been linked to a decline in avian biodiversity in North America and beyond. Ward’s group plans to study what grackles eat on their breeding grounds to help determine how great a risk their food poses.

Meanwhile, the scientists want to better understand the challenges grackles face after leaving their breeding grounds. In addition to stringing up mist nets to snag birds in residential neighborhoods and at roost sites, Ward and U.S. Fish and Wildlife Service biologist Kelly VanBeek set live traps baited with seed and mealworms at Goose Pond Sanctuary. They outfitted 14 Common Grackles with satellite tags throughout the summer and early fall of 2023 as the birds prepared to depart for their wintering grounds in the Southeast and southern Midwest.

On those journeys, VanBeek says, there are plenty of opportunities for the migrants to encounter other possible chemical culprits like fungicides, which may disrupt birds’ hormones and metabolism and are typically applied in the fall when grackles are on the move. Blackbirds’ penchant for foraging on farmland in large flocks makes them a target for culling as well. Between 1974 and 1992, the federal government killed up to 18 million Common Grackles in Kentucky, Tennessee, and Alabama to reduce damage to agricultural crops. The pace has slowed, but the practice continues: Wildlife Services, a USDA division, killed more than 630,000 Common Grackles . That’s on top of birds taken by farmers under FWS permits.

The pilot project VanBeek and Ward launched last year aims to tease out the extent to which these various threats are driving losses. The scientists’ approach sounds straightforward: Follow the tagged birds through migration and beyond, and collect any carcasses to determine the cause of death.

Yet executing the plan has proven challenging. The team recovered three dead grackles, but not quickly enough to detect pesticides. “You’ve pretty much got to find the bird within a few days,” Ward says. It’s not an easy task: The satellite tags relay a location only once every five days, and gaining access to private property can be time-consuming. Researchers also never tracked down several birds whose signals blinked out long before the batteries’ expected expiration date last spring—leaving them uncertain whether the carcasses were scavenged or the tags failed. “It’s frustrating,” Ward says.

While they’re just beginning their investigation, scientists stress the importance of drawing attention to this decline. When common birds are struggling, it indicates that something in the environment isn’t right, Ward says. Understanding and addressing the threats that grackles face could help the many species, including humans, that share these landscapes.

This piece originally ran in the Fall 2024 issue as “Disappearing Act.” To receive our print magazine, become a member by .