Home & Garden Columns

Wild Neighbors: Cowbird Extortion: Nice Little Nest You’ve Got There

By Joe Eaton
Tuesday April 03, 2007

A couple of years ago (have I really been doing this for a couple of years?) I wrote about the sneaky reproductive tactics of the brown-headed cowbird, one of a handful of bird species that are brood parasites. Instead of building their own nests and raising their own young, they dump their eggs in the nest of a host and go away. Apart from the New World cowbirds, avian brood parasites include Old World cuckoos, some African finches, African and Asian honeyguides, and the South American black-headed duck. Opportunistic egg-dumping occurs among swallows, waterfowl, and others, but these guys are pros.  

Well, it appears now that the female cowbirds don’t just go away. They stick around and monitor the fate of their egg. Some host birds will incubate the alien egg and feed the resulting chick like one of their own. Others toss it out of the nest, build over it (and any of their own eggs), or pick up and move. There’s variation within a host species, but general trends are known. Some species—robins, kingbirds, waxwings, orioles—reject cowbird eggs almost 90 percent of the time. Acceptor species have much lower rejection rates, 20 percent or below. Although they may succeed in rearing some chicks, they run the risk of the larger and faster-growing cowbirds starving out their own biological offspring. 

Recently published research by Jeff Hoover at the Florida Museum of Natural History at Gainesville shows that some cowbird populations will return and trash the nests of rejectors, destroying the host’s own eggs. It’s essentially a protection racket—Hoover calls it “mafia tactics.” 

Hoover and the museum’s natural history chair Scott Robinson did their fieldwork in the bottomland swamps along the Cache River in southern Illinois, using an acceptor species, the prothonotary warbler. The prothonotary, known to some as the golden swamp warbler, figured briefly in the Alger Hiss perjury trial—the prosecution, as I recall, using a birding recollection by Whittaker Chambers to impeach Hiss’s credibility. Stray prothonotaries sometimes reach California during migration, and they’re worth seeking out. 

Over four breeding seasons, Hoover and Robinson experimentally removed cowbird eggs from warbler nests and monitored what happened next. When cowbirds were allowed access, 56 percent of the host nests were ransacked. With access denied, none were. (Six percent of nests where the intruders’ eggs were not removed and cowbirds could get at the nest were trashed. Since all the study nests were supposedly predator-proofed, I’m not sure what to make of that datum. Maybe it reflects competition among cowbirds—a rival muscling in.) 

The cowbirds’ mafia tactics could work on two levels. In the short term, the owners of the ransacked nests may respond by leaving the cowbird eggs alone on their next attempt—either a same-year renest or the following year’s nest. But destroying the eggs of rejectors would also have the effect of reducing the frequency of rejector genes in the warbler population. In a sense, the cowbirds would be selectively breeding their hosts. 

Hoover also says he found other evidence for “farming” behavior: 20 percent of prothonotary warbler nests that had never been parasitized were still trashed, presumably by cowbirds. When these warblers renested, 85% were cowbird victims. The cowbirds appeared to be inducing the host to lay a new clutch of eggs so they could add their own. 

I’d like to see a California study along these lines. Cowbirds arrived here from the Great Plains around the end of the 19th century, finding a population of naïve hosts. They’ve wreaked havoc with species like the yellow warbler, least Bell’s vireo, and southwestern willow flycatcher. Retaliating against the minority of rejectors in those species would be a preemptive strike against the evolution of rejecting behavior. After all, the environment within which a species evolves isn’t just food and weather: it’s a bunch of other species—parasites, hosts, predators, prey, and symbiotes of all degrees—as well.  

 

Joe Eaton’s “Wild Neighbors” column appears every other Tuesday in the Berkeley Daily Planet, alternating with Ron Sullivan’s “Green Neighbors” column on East Bay trees. 

 

Photograph by Ron Storey 

A male brown-headed cowbird, guilty by association.