eff Hoover reaches into a birdhouse
This young brown-headed cowbird—deposited as an egg in the warbler nest about two weeks ago by its mother—has strategically hatched before its warbler nest mates.
By the time the two warbler eggs hatch, the young, begging cowbird—its belly already full and round from being fed by the adult birds—will be considerably larger and more aggressive than the warbler hatchlings. The cowbird will be able to muscle past the warbler host’s own chicks for food. “The warblers may be able to feed them all,” Hoover says, “but when food is limited, the warbler nestlings may not survive.”
Cowbirds are known for their parasitic practice of laying their eggs in the nests of other species, giving the birds a reputation for being lazy parents.
During the course of his research in the Cache River watershed, Hoover discovered a nasty wrinkle in the parasitic cowbird-songbird dynamic: Cowbird moms, it turns out, have a lot in common with Livia Soprano and the Corleone family. The cowbirds aren’t just lazy parents abandoning their hatchlings. Instead, they stick around and use tactics of intimidation and retaliation—like avian mobsters—to make sure that warblers and other bird species raise their young.
And what’s worse, Hoover found, is that cowbirds around the Cache River were able to expand their range and their tactics because of the destruction of warbler habitat by people. It’s like a tale of urban blight: When the natural neighborhood goes into decline, the riffraff start moving in. The newly cleared farm fields, logged-over forests and drained swamplands help cowbirds proliferate and provide them an edge over the local songbirds.
The Cache River watershed is home to the nation’s northernmost cypress-tupelo swamp. The flared trunks and pointy knees of the bald cypress trees and the mud flats studded by shin-high crayfish chimneys say “Louisiana.” But the map says this is Illinois.
Along the Cache River, the wetland habitat of the Gulf Coast improbably intersects with the plains of the Midwest. The biological diversity created by this juxtaposition has been recognized by the Ramsar international wetlands treaty, which puts the Cache River wetlands in a class with the Everglades and the Okefenokee Swamp.
The diversity found here includes threatened copperbelly water snakes, huge swamp rabbits and nesting anhingas—water birds with snakelike necks that are more closely associated with Brazil than Illinois. All told, these wetlands are home to more than 100 species listed as threatened or endangered in Illinois. Along with its ecological richness, the region’s agricultural potential has not gone unnoticed. Starting more than 100 years ago, parts of the Cache River watershed were converted to farmland. The cypress and the tupelo trees were cut down, the river was diverted, and fields of corn, wheat and soy were planted. In the 1950s, the pace of agricultural conversion exploded.
By 1993, things had become grim for migratory songbirds in the region. A survey that year of a broad range of species parasitized by cowbirds—including indigo buntings, prothonotary warblers, Acadian flycatchers, Kentucky warblers, northern cardinals and white-eyed vireos—found nearly half of all nests contained a cowbird egg. But the effect was likely much larger, as cowbirds target the nests of many more species than were included in the survey (more than more than 120 species in total). And one cowbird egg in a prothonotary warbler nest, for example, lowers by 40 percent the chance that the host’s own young will survive to reproduce, says Matt McKim-Louder, a doctoral student at the University of Illinois at Urbana-Champaign, who works with Hoover.
In 1995, Hoover and his collaborators began a series of studies exploring whether and why prothonotary warblers return to the same nest site every breeding season. As part of the experiment, they needed a group of warblers to be particularly successful at raising their young.
Arranging that success seemed easy enough. Cowbirds had parasitized more than half of the prothonotary warbler nests in the area. The researchers decided they would try to undo the damage and boost the warblers’ nesting success by plucking the cowbird eggs from the host nests.
When the research team returned to the nests from which they had removed cowbird eggs, they found only broken warbler eggs and torn-up nests.
To figure out what was going on, Hoover in 2003 conducted another experiment. In some of his milk carton birdhouses, he blocked the cowbirds from re-entering after they had laid their eggs by using a cardboard insert that made the entry hole too small for cowbirds but large enough for the adult songbirds. He left other nests alone.
Hoover discovered that the cowbirds were keeping an eye on the nests where they had laid their eggs and would retaliate after their eggs were removed. These cowbirds were not just freeloaders but Mafioso-like birds that ruthlessly enforced vendettas against birds that thwarted their schemes.
That’s why the Cache River Joint Venture Partnership—a collaboration between conservation groups and federal and state agencies—began working to make some big changes here. The partnership, which is composed of the Illinois Department of Natural Resources, the U.S. Fish and Wildlife Service, the Natural Resources Conservation Service, Ducks Unlimited, and The Nature Conservancy, worked with Hoover to identify areas where migratory birds were still holding their own against the cowbirds. Hoover quantified how these areas were different from those overrun by cowbirds and developed a restoration strategy.
“If you buy a 10-acre parcel in the middle of a sea of agriculture, it’s really going to do nothing to help the bird community locally,” says Hoover. The key, he discovered, is to restore the gaps in the middle of relatively healthy forests. “By acquiring a 10-acre clearing and making it forest again, you can undo a lot of harm for the songbirds locally.”
Those findings helped the Conservancy target a 2,775-acre tomato and bell pepper farm in the Cache River watershed that had become a haven for cowbirds. This property—now called Grassy Slough—was the ecological equivalent of a graffiti-tagged back alley.
The farm was wedged between the two halves of the Cache River State Natural Area, giving local cowbirds ample access to both insects in the farm fields and songbird nests in the forests. State officials were interested in purchasing and reforesting the property, but the government just didn’t have the cash on hand. So in 1999 the Conservancy made the purchase and kicked off reforestation efforts.
“The opportunity to acquire Grassy Slough was an excellent one for The Nature Conservancy to use its expertise in land acquisition to support the restoration of the Cache River watershed,” says Jeff Walk, the Conservancy’s director of conservation science in Illinois.
Volunteer efforts helped accelerate the old farm’s return to nature. Local Boy Scout troops organized annual acorn roundups to help grow swamp oak seedlings for plantings. And volunteer paddlers used canoes and kayaks to gather tupelo seeds in nearby swamps, which they tossed into the backwaters at Grassy Slough to promote regrowth.
After a decade of restoration work, the Grassy Slough Preserve’s 1,600 acres of young forest and 700 acres of wetlands now help connect the nearly 35,000 acres that have been protected in the Cache River watershed.
“With the success restoring the forests on the property, the restoration phase is largely complete,” says Walk.
In 1993, nearly 80 percent of songbird nests in some areas here were parasitized by cowbirds. But in the past two years only 21 percent of nests were hit. “I was back to the region recently,” says Robinson. “Forest bird populations are rebounding. Some incredible progress is being made.”
These days, when Hoover’s research team needs to collect a cowbird egg for a genetic study, they slip a fake one in its place to keep the host nest safe. Cowbirds may use Mafia tactics, says Hoover, but they are not villains. “The cowbirds are doing what cowbirds do. It’s because of what humans have done to the landscape that there isn’t a balance.”
Certainly, the nearly naked bird balanced on Hoover’s hand near the beaver swamp looks as vulnerable and innocent as any baby bird. With the balance between songbird hosts and cowbirds nearly restored here, a cowbird is no longer a threat, but just another part of the ecosystem.
Still, Hoover’s team hasn’t stopped studying cowbird tactics. McKim-Louder, the doctoral student in Hoover’s lab, is studying how fledgling cowbirds figure out that they are not members of the host species. He’ll need every cowbird fledgling he can monitor to untangle that mystery, so Hoover reaches back into the soy milk container and gently replaces the tiny mobster in its nest.