Late November in the Solano County farmlands: Up ahead on the gravel road there are two vehicles on the shoulder and a huddle of birders with spotting scopes. We pull over, get out, set up, ask what they have. “Mountain plovers,” one guy says, offering a view in his scope. Far out in the bare field, a dirt clod gets up and runs a few steps. It’s a tan-and-white bird, like a killdeer without the black markings. It stops and vanishes. Then another moves, and another — about 15 altogether. The Prairie Ghosts are back for another winter.
There’s nothing montane about the mountain plover except its name, and although it’s a shorebird, you won’t see it chasing the waves at the beach. It’s a creature of the Great Plains, a shortgrass prairie specialist, nesting in a broad swath east of the Rockies from Montana south into New Mexico and west Texas. In winter, most of its population shifts to California’s interior valleys — the Sacramento, the San Joaquin, the Imperial — with a minority in Texas and northern Mexico.
Like many grassland birds, it’s in trouble. Ten years ago, biologist Fritz Knopf analyzed a quarter-century of breeding bird survey data and reported that 16 of the 25 bird species associated with North American grasslands were declining, some rapidly. Mountain plover numbers had fallen 3% or more per year, adding up to a 50% reduction in numbers since 1966. The species was once abundant enough to be hunted for the market (plovers of all species are reputed to be tasty; Mrs Beeton has recipes). Now it’s on the National Audubon Society’s Watch List and a candidate for Federal Endangered status.
What happened to the plover? The market hunters took their toll, as did the conversion of prairie to farmland, but the story is more complicated than that. It has to do with interactions between birds, mammals, insects, and people on the Plains, and the well-known Law of Unintended Consequences.
Mountain plovers like their shortgrass habitat seriously short. Their ideal nesting site contains at least 30% bare ground. Short grass and bare patches allow them to locate the grasshoppers and beetles that compose most of their diet and spot potential predators like swift foxes, coyotes, and ground squirrels.
Historically, it was the bison that kept the shortgrass short: 30 million hooved mowing machines, grazing, trampling, wallowing. (And it was other grazers — tule elk, pronghorn, smaller herbivores like kangaroo rats — that created a homelike environment in the California valleys where the plovers wintered.) The canonical figure for the bison is 60 million, but UC-Davis professor emeritus Dale Lott, allowing for the carrying capacity of the grasslands, annual losses to predators, and other factors, figures the population was half that size, maybe lower. Even so, that would have been a lot of mouths and hooves.
We all know what happened to the bison, or we think we do; for some of the complexities, see Lott’s splendid new book, American Bison: A Natural History. But bison weren’t the only grazers on the Plains. Mountain plovers also took advantage of the lawnlike shortgrass habitat in prairie dog towns. Range maps show a striking overlap between the historic distribution of the plover and that of the most widespread prairie dog species, the black-tailed. Today, many of the plover’s breeding sites are among the burrows of these sociable rodents — in some areas, like Montana’s Charles M. Russell National Wildlife Refuge, over 90 percent.
Prairie dog towns once covered a mind-boggling hundred million acres. They now occupy only 1% of that area, and most colonies are small (less than 10,000 acres) and isolated. Ranchers, who believed — without much supporting evidence — that prairie dogs competed with cattle for forage, wanted the dogs eradicated. The government responded with wholesale poisoning campaigns, and the introduced flea-borne disease called sylvatic plague (in humans, it’s bubonic plague) almost finished the job. The remaining prairie dogs are still popular targets for recreational varmint hunters, although the black-tailed is now itself being considered for endangered status. The fewer prairie dogs that remain, the less suitable nesting habitat there is for the plover.
There’s another connection that seems plausible enough, although I’ve never seen it mentioned anywhere. Mountain plovers, remember, eat grasshoppers. In some parts of the plover’s range, like Montana, these are or were the primary food item. And the Great Plains used to have grasshoppers in almost unimaginable numbers.
In 1875, a Dr. A. L. Child watched a swarm of Rocky Mountain locusts passing over Plattsmouth, Nebraska. He did some back-of-the-envelope calculations and estimated the swarm was 1,800 miles long and at least 110 miles wide, covering an area equivalent to New England plus Pennsylvania, New York, and New Jersey. Jeffrey Lockwood, an entomologist at the University of Wyoming who is the leading modern authority on the locust, has figured that Dr. Child’s swarm contained 10 billion individual insects, an aggregate of 6,000 tons. Think of that in terms of potential plover chow.
Lockwood’s recent article in Wild Earth includes an 1891 Department of Agriculture map of the locust’s breeding grounds. What’s shown as the “permanent region,” the core of the insect’s range, forms another overlap with the range of the mountain plover.
Rocky Mountain locusts erupted over the Plains several times in the nineteenth century, devouring all the crops in their path. They were a plague of Biblical proportions. But for better or worse, they’re all gone: the last living specimen was collected just over a hundred years ago. This was long before the advent of modern pesticides, of course. Lockwood believes the species perished not by the sword but by the plowshare.
The locusts laid their eggs in the fertile valleys of the rivers that laced the Plains: “precisely the lands that the early pioneers sought to convert to agricultural production.” The transformation of the river valleys — not just by plowing, but by diverting the streams for irrigation and bringing in sheep and cattle — doomed the insects.
It’s pure speculation on my part, but you’d figure taking thousands of tons of biomass out of any ecosystem is bound to have some major effects. If the mountain plover suffered from the loss of that food source, so did other insectivorous birds. But other species may have been buffered by broader ranges or more diverse food preferences and not hit as hard as the plover.
So now what, though? We can’t — even if the farmers would stand for it — bring back the Rocky Mountain locust. But the bison and prairie dogs are still around, albeit much reduced in numbers. And it seems domestic livestock can act as bison surrogates in keeping the grass at the proper height for plovers. On the Pawnee National Grassland in Colorado, the plover population declined after cattle were taken off the public range. It’s ironic to admit it, but those hooved locusts may have a positive ecological role to play after all.
“The prescription for mountain plover [at the Pawnee] is more prairie dogs and heavier grazing,” Fritz Knopf says.
At last count, California hosted around 3,000 mountain plovers, about a third of the global population. Their winter habitat is a long way from pristine, and plovers in California have been found to carry high levels of pesticide residue. But the birds’ decline appears to have been set in motion over a century ago and half a continent away, when the bison were slaughtered and the first plows broke the Plains.
