Science

Is It Okay to Remove Cowbird Eggs From Host Nests?

It can be tempting to interfere with these brood parasites. But there are many reasons—legal and behavioral—to leave them alone.

No parent appreciates another meddling in their child-rearing efforts. Unfortunately for many songbirds in North America, meddling is the Brown-headed Cowbird's modus operandi.

The species is our best-known brood parasite—organisms (like some birds, insects, and fish) that rely on others to raise their young. In birds, this typically describes a species that lays its eggs in a host’s nest and lets that other parent do the chick-raising, often to the detriment of the host’s own offspring. Cowbird chicks don’t directly harm their nestmates (by pushing them out of the nest, for instance, like some cuckoo species), but tend to grow faster and outcompete them for resources.

Due to a perceived sense of injustice, cowbirds are often vilified by humans, who occasionally take matters into their own hands by removing cowbird eggs from nests. These are usually well-intentioned attempts to “save” the chicks of other species—but is it a good idea to allow people’s drive to nurture interfere with nature?

The short answer: no. “The best solution is to leave cowbirds eggs alone," says Steve Rothstein, Emeritus Professor of Zoology at the University of California, Santa Barbara, who has researched the effects of cowbird parasitism on endangered species. "It’s a natural process and we shouldn’t attach human values about killing or being sneaky to the natural world." 

The reasons are multifold, and encompass both the law and unintended consequences.

U.S. law already says that people should not interfere with cowbird eggs. As a native species, the Brown-headed Cowbird is protected under the Migratory Bird Treaty Act, and taking eggs is illegal without a permit. While permits for cowbird control have been granted, it’s only done when they’re considered a threat to endangered birds. For example, in Michigan, the U.S. Fish and Wildlife Service traps an average of 4,000 cowbirds every year to protect endangered Kirtland’s Warblers.

The law is only one reason to leave eggs untouched, Rothstein says; interference can have unintended effects. That’s because “most birds in North America don’t have egg-recognition abilities”—often not even for their own eggs. However, parents do keep track of the total mass of eggs in their nest. “Many seem to assume the cowbird egg is part of their clutch and will have a nest-desertion response if we remove a certain proportion of eggs,” Rothstein, who has examined this behavior in his research, says. They may even desert the entire area and find a new nesting spot.

 “This response is universal among birds, as they have the option to re-nest,” he continues. “However, if it’s late in the breeding season, the bird might not have time to.”

Worse, egg removal can result in retribution by cowbird parents. A 2007 study, led by avian ecologist Jeffrey Hoover with the Illinois Natural History Survey, was the first to document what’s known as “mafia behavior.” In the experiment, scientists observed the effects of removing Brown-headed Cowbird eggs from parasitized warbler nests. In 56 percent of cases, cowbird mothers returned and ransacked the nest, destroying most or all eggs. Comparatively, six percent of nests were destroyed when humans didn’t interfere.

While no additional research has confirmed this behavior in Brown-headed Cowbirds, it highlights “a potential further complication to removing cowbird eggs,” says Matthew Louder, who studies cowbird brood parasitism at the University of Illinois at Urbana-Champaign. For Louder, the only instances where cowbird egg removal is justifiable is when it’s legally conducted to help one of three endangered species: Golden-cheeked Warblers, Black-capped Vireos, and Kirtland’s Warblers.

“Cowbirds aren’t the main reason for their decline, and probably not even a great contributing factor. The only factor in the decline of these species is humans,” he says. “Now numbers are so low that any other problems [such as brood parasitism] would push them over the edge.” If cowbird control is permitted, Louder says it must be done alongside habitat restoration measures.

Rothstein agrees that cowbird control and habitat improvement must go together. “Cowbirds have been in North America for a million years,” he says. “If a species needs help, it’s because we have damaged their habitat. Any species that would go extinct just because of cowbirds and not human interference would have gone extinct long ago.”

All this evidence points to a single conclusion: Cowbird eggs should be left alone. It can be disturbing to observe what looks like “cheating” at the expense of less common species—but it's just nature's way, even if it's ugly. Sarah Winnicki, an avian ecologist at Kansas State University, has found a way to adjust her own perspective on the species to avoid moralizing their behavior.

“I try to convince people about how amazing [cowbirds’] evolutionary story is,” she says. “How did they learn to find nests, to lay eggs, and to re-find them later? How do they learn to time their reproduction to their hosts? As an ornithologist, this is incredible to me.”

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