One of the recurring dilemmas of conservation biology is whether, and how far, to intervene in predator-prey relationships: in particular, what action is appropriate when an endangered species is being eaten to extinction by a more common one. That question has come up with raven predation on desert tortoises in the Mojave, golden eagle predation on island foxes on Santa Cruz Island, California gull predation on western snowy plovers and least terns in the South Bay.
It’s trickiest in cases like those three where both predator and prey are native species. When the predator is an invasive exotic—as with red foxes versus California clapper rails, or feral pigs and goats versus any number of native plants—there are usually administrative resources at hand, although even the pigs and goats have their defenders (as in When the Killing Stops, T. C. Boyle’s new novel about the Channel Islands wildlife wars.)
What do you do, though, when the victim isn’t officially recognized as endangered and the non-native predator happens to be legally protected?
The victim in question is the tricolored blackbird, a California near-endemic (there are small populations in Baja California, Nevada, and Oregon.) This bird, a close relative of the widespread red-winged blackbird, was once incredibly abundant. John Neff, writing in the 1930s, estimated the statewide population as two to three million.
Tricolors, named from the male’s black, white, and crimson plumage, nested in huge colonies in freshwater wetlands throughout the Central Valley and coastal central and southern California. Most are itinerant breeders, raising one brood at a San Joaquin Valley location, then moving north en masse for second and sometimes third attempts in the Sacramento Valley.
That’s how it was before the development of rural California. More recent estimates have ranged from 150-260,000 between 2000-2005 and 400,000 in 2008. That apparent increase may be an artifact of methodological inconsistencies between the two studies. Biologists who study the tricolor, like Robert Meese of UC Davis, believe numbers are continuing to fall.
Meese presented the grim news in a recent Golden Gate Audubon Society program. He pointed to the usual suspects: the loss of wetlands to agriculture and urbanization, exacerbated by water diversions. The replacement of crops like alfalfa and sunflower with vineyards and orchards has eliminated additional suitable nesting and foraging habitat. Until 1989, it was legal to shoot tricolored blackbirds as agricultural pests. Some Sacramento Valley rice growers (Meese wasn’t naming names) still do so; enforcement is spotty.
Tricolors are also falling victim to a kind of attractive nuisance. They’ve been adaptable enough to switch from cattails and bulrushes to other nesting substrates: Himalayan blackberries, nettles, mustard, milk thistles, giant reeds, and—unfortunately for them—the triticale that’s grown to feed cattle. Growers have been harvesting the triticale before the birds complete their nesting cycle, killing thousands of fledglings. There used to be a federal program that, among other things, provided financial incentives to delay the harvest. It has been defunded, of course.
And now the beleaguered blackbirds, at least the San Joaquin Valley population, are facing a new threat: predation by cattle egrets. “In 2006, they went into a tricolored blackbird colony in Tulare County and basically wiped out the colony,” says Meese. “Three colonies were lost last year. No young fledged.” The worst-case scenario: young egrets are learning that blackbird nests are an easy food source. What happens when they strike out on their own? Meese’s answer: “We’ve got a huge, huge problem on our hands.”
Cattle egrets evolved in Africa and Asia in the company of large grazing animals, following them around and snatching the insects they stir up. They’re equally at home with elephants, rhinos, water buffalos, capybaras, tortoises, or tractors. Relatively new immigrants to the Americas, the birds appeared to have crossed the Atlantic from West Africa to South America, then worked their way through the Caribbean up to Florida, thence north and west, reaching California in 1964. Although they’re now established as far north as the Sacramento Valley, the bulk of the state’s cattle egret population is in Southern California’s Imperial Valley.
Like most herons, cattle egrets do not scruple at taking the occasional smaller bird. (I once saw a great egret grab and swallow a song sparrow at the Hayward Regional Shoreline.) Some have taken to waiting on Gulf Coast beaches to pick up exhausted northbound migrants. But the mass attacks on nesting colonies are new.
So why not cull the egrets, at least in Tulare and nearby counties, to give the blackbirds a respite? First, the tricolored blackbird is not recognized on either the state or federal levels as an endangered or threatened species. It’s listed as a California Species of Special Concern, which confers no meaningful protection. Second, the cattle egret is specifically protected under the federal Migratory Bird Treaty Act. OK, it’s kind of migratory. But it’s not a native bird! I’d love to know whose bright idea this was.
Meanwhile, Meese and other biologists, organized as the Tricolored Blackbird working group, have drafted a conservation plan in lieu of a formal endangered listing. To learn more, visit the Tricolored Blackbird Portal (tricolor.ice.davis.edu.)