Beware the mute swan

Species of birds never before found in the Central Valley are staging a broad-scale invasion. Though some have arrived as a direct result of human intervention, all have likely been abetted by humans in some way.

Most of the new arrivals adapt to the environment with little trouble. However, some species, such as the mute swan, are so destructive, even bird-watchers want to keep them out of the state.

Perhaps the most dramatic avian invasion of any native North American bird is that of the great-tailed grackle. This bird has historically lived in Latin America, from the Central American isthmus to as far north as the very southern tip of Texas.

But midway through last century, it began to move north. In 1964, a bird was seen in Imperial County in the extreme southeast corner of California. In 1978, a grackle appeared in San Francisco. In the following two years, the species began to colonize the Central Valley. Initially, these birds appeared on temporary breeding visits, but today they reside year-round in many parts of the state, have been sighted in every county in California and are considered relatively common throughout much of the western continental United States. The great-tailed grackle’s sudden invasion is one of the most abrupt and widespread range expansions ever documented in a wild animal.

Ed Pandolfino, a board member of Sierra Foothills Audubon Society, says the species’ range change, which appears at a glance like one spurred by climate change, probably has little or nothing to do with warming weather each season.

“The grackle has expanded way farther north than climate change would predict,” he explains. “They’re living now in Iowa and Oregon, and the climate there is certainly not like it is in Mexico and Texas.”

Pandolfino, who doubts that the grackle’s presence will be problematic for most native birds, says the grackles have moved into the Golden State at least partly to feed on our luxuriant garbage resources, often at public landfills and in parking-lot Dumpsters. While the birds primarily use marshlands for nesting purposes, islands of such habitat are frequently preserved within housing developments as part of wetland-preservation policies, making ideal suburban habitat for a bird that thrives in a human-altered landscape.

Numerous other species have entered the Central Valley both very recently and very abruptly. President of the Central Valley Bird Club John Sterling notes that six species of gulls, previously very rare in the Central Valley, have become commonplace around Davis and Sacramento in the last 10 years. Ravens, too, which historically have nested on cliff faces in mountainous areas, are moving into the area.

“Ravens have seen an incredibly rapid range expansion,” says Sterling. “A decade ago they didn’t occur in the Valley. Now they’re common.”

Like the gulls and grackles, ravens tend toward garbage and can be viewed at the Yolo County dump in Davis and the Sacramento County dump, among other large trash heaps. Meanwhile, expansion of orchards and ornamental gardens has provided a food source for a colorful array of other invasive species. Many of these birds, says Sterling, will probably find the space to coexist with pre-existing resident species.

On the non-native front, the Eurasian collared dove, a non-native species that was introduced back East in the early 1980s, has spread on its own all across the United States and has gone from nonexistent to common in much of the Central Valley in just a few years. Its impact on species such as the mourning dove are not known at this time.

But the beautiful yet aggressive mute swan is another story. It could become a serious problem if allowed to proliferate uncontrolled. This Eurasian native was introduced to New York state in the 1800s as a popular decorative bird for zoos and private gardens—but the birds got away. They have since established themselves across the Eastern Seaboard as wild, successfully breeding birds and in some cases edging native waterfowl from local environments. Scattered reports, say Pandolfino and Sterling, are now arriving of wild pairs residing in the San Francisco Bay Area. Pandolfino would like to see officials take action to control and eradicate the mute swan while they can.

“It’s way too late to do anything about them back East, but here, they’re just beginning to show up,” he says. “There might still be time to get rid of them.”

Though the Department of Fish and Game’s waterfowl coordinator Dan Yparraguirre recognizes the swan as an invasive species and as a serious threat to native wildlife, state officials have yet to take firm action. Pandolfino wishes they would—and it wouldn’t even require killing any adults. Anti-swan activists, he says, have had success in Eastern states by merely bullying into the nest clad in body armor, smearing the eggs with oil and leaving them in place. The slick coating suffocates the developing chicks inside while fooling the adult birds into wasting the entire nesting season roosting on a batch of rotten eggs.

But Sterling thinks shooting the swans would be more efficient than birth control, adding that “it’s just a matter of time” before the powerful pests get out of hand.

“If we act now,” says Sterling, “we could nip this one in the bud.”