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鈥檚 nest and lets that other parent do the chick-raising, often to the detriment of the host鈥檚 own offspring. Cowbird chicks don鈥檛 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. And some people occasionally take matters into their own hands by removing cowbird eggs from nests. These are usually well-intentioned attempts by bird-loving folks trying "save鈥 the chicks of other species鈥攂ut is it a good idea to allow people鈥檚 drive to nurture interfere with nature?
The short answer: no.
鈥淭he best solution is to leave cowbirds eggs alone," says , Emeritus Professor of Zoology at the University of California, Santa Barbara, who has researched the effects of cowbird parasitism on endangered species. "It鈥檚 a natural process and we shouldn鈥檛 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 are granted, it鈥檚 only done when they鈥檙e considered a threat to endangered birds. For example, in Michigan, the U.S. Fish and Wildlife Service to protect endangered Kirtland鈥檚 Warblers. And in California, a cowbird trapping program by has led to a resurgence of Least Bell's Vireos at the Hedrick Ranch Nature Area.
The law is only one reason to leave eggs untouched, Rothstein says; interference can have unintended effects. That鈥檚 because 鈥渕ost birds in North America don鈥檛 have egg-recognition abilities鈥濃攐ften not even for their own eggs. However, parents do keep track of the total mass of eggs in their nest. 鈥淢any 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,鈥 Rothstein, who has examined this behavior in his , says. They may even desert the entire area and find a new nesting spot.
鈥淭his response is universal among birds, as they have the option to re-nest,鈥 he continues. 鈥淗owever, if it鈥檚 late in the breeding season, the bird might not have time to.鈥
Worse, egg removal can result in retribution by cowbird parents. A 2007 , led by avian ecologist Jeffrey Hoover with the Illinois Natural History Survey, was the first to document what鈥檚 known as 鈥渕afia behavior.鈥 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鈥檛 interfere.
While no additional research has confirmed this behavior in Brown-headed Cowbirds, it highlights 鈥渁 potential further complication to removing cowbird eggs,鈥 says , 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鈥檚 legally conducted to help certain endangered species.
鈥淐owbirds aren鈥檛 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,鈥 he says. 鈥淣ow numbers are so low that any other problems [such as brood parasitism] would push them over the edge.鈥 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. 鈥淐owbirds have been in North America for a million years,鈥 he says. 鈥淚f a species needs help, it鈥檚 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.鈥
All this evidence points to a single conclusion: Cowbird eggs should be left alone. It can be disturbing to observe what looks like 鈥渃heating鈥 at the expense of less common species鈥攂ut 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.
鈥淚 try to convince people about how amazing [cowbirds鈥橾 evolutionary story is,鈥 she says. 鈥淗ow 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.鈥
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