Natural Inquiries

Despised pests throughout North America, starlings and house sparrows are mysteriously declining in their native Europe

04-01-2004 // Scott Weidensaul

IT’S A CONSERVATION HEADLINE of the man-bites-dog variety: "Bird watchers are frantic about vanishing house sparrows and starlings."

The trouble is not in North America, where both species remain abundant (and reviled) alien pests, but in their native Europe, where urban sparrows are beloved as "cheeky chappies" and starlings are symbolic of the rural countryside—and where populations of both species have plummeted in recent decades.

In countries like the United States, concern about these birds may seem ludicrous. Indigenous to Europe, northern Asia and northern Africa (with the sparrow’s range extending east to India), the house sparrow and European starling have proved to be the avian world’s most successful carpetbaggers. Today both species are entrenched in North and South America, the Caribbean, southern Africa, Australia, New Zealand and Iceland. Though the birds have moved on their own to a few places (such as Iceland), most colonists arrived thanks to deliberate human introductions. European starlings, for instance, were released in New York City between 1890 and 1891 by a society dedicated to importing every bird mentioned in the works of Shakespeare. Beginning in 1851, house sparrows were introduced to the United States in an unsuccessful attempt to control agricultural pests.

Instead, the sparrows themselves became pests. By 1900 they were the most plentiful wild birds on the continent, crowding out native cavity nesters such as bluebirds. The disappearance of horse-drawn vehicles (and thus a ready supply of grain) reduced North American sparrow numbers, which declined further in the East due to competition from house finches moving in from the West. Yet sparrows remain abundant in many areas, particularly urban, though no current population estimate exists. Starling numbers, meanwhile, may still be on the rise. One estimate pegs their continental population at 200 million.

But both species have suffered flagging fortunes in their native lands. In Great Britain, where house sparrows have all but vanished from some traditional haunts such as London’s Kensington Gardens, a 2000 survey found just eight birds where hundreds once flew. The most comprehensive population estimates come from a consortium of scientists led by the British Trust for Ornithology (BTO). Its study, published in 2002, found that British populations of house sparrows have fallen by about half since the 1970s to approximately 13 million today.

Likewise, the study concluded that starling populations have decreased by 66 percent since 1965 to approximately 8.5 million today, with the greatest declines in rural areas. The scientists’ findings were worrisome enough that last year the house sparrow and starling were added to the trust’s "Red List" of species of conservation concern. Anecdotal data from other countries—summarized in the BTO report—suggest that both species are declining elsewhere on the continent as well.

The reason for the declines remains something of a mystery. Because sparrows and starlings had always been so common, they were largely ignored by ornithologists, who now find themselves scrambling for answers. Nor is the attention all academic: In 2000, a major British newspaper offered a £5,000 reward to anyone who could identify the cause of the house sparrow’s plight. The prize remains unclaimed, but not for lack of trying.

Now several major studies are beginning to shed light on the pressures affecting both species. The most promising clues come from the BTO report, which found that throughout Great Britain, house sparrow nesting success has actually been increasing over the past 40 years. The problem, the scientists discovered, is that rising nesting success has been eclipsed by an even higher mortality rate for first-year sparrows, especially in more urban and southerly areas. Yet so far they’ve uncovered no single smoking gun for what’s killing so many young birds.

One hypothesis is inadequate food in winter. "With sparrows, winter food supply seems critical, particularly in rural areas," says ornithologist Robert Robinson, a coauthor of the report. "More efficient harvesting and more hygienic storage of grain seem to be key culprits."

In towns, the cause of the declines is less clear. Because sparrows seem to be most abundant in areas with weedy corners and overgrown sites, a trend toward reusing abandoned industrial sites—otherwise laudable—may be a partial culprit, says Robinson. Another possibility is that the birds may find it more difficult to forage in cereal fields the way they once did. "Because sparrows have very limited dispersal," suggests Robinson, "urban spread may mean that parts of town are simply too far away from the fields now."

Another study, conducted in 2001–2002 by the city of Bristol, discovered a link between sparrow numbers and human poverty, with sparrows absent from wealthy suburbs but doing well around housing projects. Such poorer neighborhoods have more trash and food scraps, more weedy, overgrown lots, and older buildings with plenty of nooks and crannies for nesting. It appears that improving economic conditions, while good for people, may have been bad for the sparrows.

For starlings, the trouble seems to stem from intensified agriculture, particularly changes in the way livestock is raised. The actual problems spawned by modern methods are uncertain, "but they are likely to include loss of unfertilized grasslands that harbor the soil invertebrates that starlings feed on and the use of insecticides that reduce invertebrate numbers," says Robinson. He also suspects that better livestock management has decreased waste grain, which used to be readily available to starlings in winter when insect prey is scarcest.

If scientists remain unsure about why the birds are declining, one thing is already certain—these species are overlooked no longer. "I was out with a group of pretty hard-bitten birders last weekend, and we stopped to admire a flock of about a hundred [house] sparrows," Robinson recalls. "That caused some amusement when I pointed it out, but none of us could remember having seen such a flock lately."

Scott Weidensaul is the author of more than two dozen books on birds and nature, including the Pulitzer Prize-nominated Living on the Wind.

 

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More Than a Pest

Long reviled for ousting native birds from prime habitat, house sparrows may now be affecting North American birds in a new—also troubling—way.

Four years ago, the year after West Nile virus arrived on the continent for the first time, biologist Nicholas Komar of the Centers for Disease Control and Prevention (CDC) in Fort Collins, Colorado, analyzed the blood of several kinds of birds that had been bitten by mosquitoes carrying the virus. Of these species, which included house sparrows, pigeons and starlings, sparrows developed the highest level of virus in their blood. They also carried the infectious microbes for a longer period of time—up to five days—than any of the other species. And because house sparrows usually survive West Nile infection—possibly because the birds, like the virus, hail from Europe and may have developed immunity—Komar suggested that sparrows may serve as an important "reservoir" for the virus, allowing the microbes to multiply and build up large populations that can be picked up by mosquitoes and transferred to new, more susceptible, hosts.

More recently, scientists have begun to suspect that house sparrows may also play a role in spreading the virus to new parts of the country. In 1999, the CDC asked John Rappole, a National Zoo ornithologist based at the Smithsonian Conservation and Research Center in Front Royal, Virginia, to investigate how the virus was traveling so quickly from New York City, its likely point of origin. At first, Rappole and most other biologists considered migratory birds the primary culprits. But when he and colleague Zdenek Hubalek from the Institute of Vertebrate Biology in the Czech Republic analyzed their data a couple years later, they were surprised by their results.

"It turns out the virus did not move anywhere near as fast as it might have if migratory birds were involved," says Rappole. "If migrants had been moving it, West Nile should have reached Florida within a month and spread through the hemisphere within a few months." Instead, the virus did not show up in Florida until a year after it appeared in New York and was only detected in California in late 2002. Another problem with the migratory bird hypothesis, he says, is that migrants tend to move north or south, yet the virus has spread west as well.

Rappole and other researchers are now looking at other, more sedentary, candidate species. One strong possibility is the house sparrow. In addition to building up large amounts of virus for long periods, house sparrows are extremely abundant in many areas. And, while not migratory, the birds do move around. "Each year, about a quarter of the population travels at least 18 miles, which may be enough to account for West Nile’s spread," says Rappole. By the end of last year, the virus’s fifth on the continent, West Nile had spread to 46 U.S. states, 6 Mexican states and 7 Canadian provinces. At least 226 native bird species have died from the virus.—Laura Tangley

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