Mad River Union
ARCATA – The Redwood Coast population of snowy plovers has been growing since 2010, but it remains less than optimal because of constant predation, mainly by ravens.
The local population in the tri-county region – Del Norte, Humboldt and Mendocino – reached 72 breeding adults this past summer from a low of 19 in 2010, a nearly four-fold increase of about 20-23 percent a year.
However, about two-thirds of the current 72 are immigrant plovers from Oregon.
Humboldt State University Wildlife Professor Mark Colwell and his students have been compiling snowy plover data for 17 years, in concert with state and federal agencies. An award-winning faculty member and author, Colwell explains why the tri-county populace is so heavily augmented by Oregon immigrants.
The state to the immediate north undertakes active management of human, predator and habitat impacts, he says, producing a population that “exports” plovers to the North Coast, where management is limited.
Specifically, the tri-county cohort is growing because Oregon carries out lethal control of common ravens and other predators in locations where plovers concentrate to breed. Ravens are the main predators of eggs and chicks.
Lethal predator control – traps, shooting, short-lived chemical poison without secondary ecosystem impacts – raises the survival rates of young plovers in Oregon and Washington State.
“While in Northern California there is limited management of plovers, the population has grown owing to active predator management elsewhere,” he points out.
On two occasions over the past 10 years or so, experiments to curb predation used exclosures, box-like cages designed to foil ravens and crows. While the devices protected eggs and chicks, they made adult plovers easier prey because they were slow at exiting the exclosures when alarmed.
Ravens sensed this vulnerability and exploited it, killing precious breeder adults.
Two cage sizes were tested; one design, short of 10 by 10 feet, was used in Humboldt County and took hours to set up. The other was smaller.
Neither size was effective and in any event, as Colwell summed up, “There’s no question that ravens and crows are really abundant now and they are incredibly smart.”
Having researched shorebirds since 1981 and snowy plovers since 2001, Colwell concludes that the most important negative impact on plover reproduction is from predators of eggs and chicks. Little or no active management occurs in the tri-counties to address that fact.
The local plover population declined from 2001-2010.
Habitat restoration produces mixed results. The South Spit (south end of Humboldt Bay opposite King Salmon) has very high reproductive success associated with a restoration area.
Little River State Beach restoration, in contrast, has failed to augment breeding success because ravens are so abundant there, Colwell reports.
Clam Beach is particularly dangerous territory for plovers and the reasons are not hard to find. The wildlife professor has guided 20 graduate students to their degrees over the years and one of them mapped the habitat associations of ravens, Clam Beach included.
That heavily used seashore features surrounding agricultural landscapes in a suburban setting with a lot of food. Ravens are opportunistic foragers. They are shrewd at pinpointing the few nests and chicks out there.
“But with everything else that washes up on the beach, garbage that’s left in parking lots; people feeding them purposefully; dead cows in pastures; all of this makes for a cornucopia for the ravens.
“The collateral damage is that plovers and probably other species in the area aren’t doing as well. We analyzed the data on how frequently we encounter ravens at different sites up and down the coast in relation to how plovers breed and there’s a negative correlation.
“Where there are more ravens, the plovers do less well at producing chicks. When there are a lot of ravens, in particular at places like Clam Beach, the nesting success is poor.”
Most adult plovers have a lifespan of only two to three years, owing to predation, but Colwell has monitored one locally that is 16 years old-plus and still going strong. He was hatched at Table Bluff and has set a longevity record.
Typically, the professor says, young plovers start growing up in Oregon and when they migrate in search of their first winter home, they head south and bivouac at places like Clam Beach, the South Spit and Centerville Beach, off Ferndale, in search of other plovers.
While ravens and crows feed on plover eggs and young, adult plovers contend with raptors, especially peregrine falcons.
“When the plovers are not breeding, they gather in flocks for vigilance and safety in numbers,” from a few to perhaps a hundred, Colwell observes. Locally, reproduction does not offset predation.
“I’m not in any way villainizing ravens, crows and peregrines,” he adds, “but peregrines are more abundant now than they were when they were first put on the endangered species list; they’re now delisted.”
Author of the 2010 book Shorebird Ecology, Conservation and Management, with a new book due early next year on the world’s plovers, Colwell doesn’t expect the birds to be taken off the endangered list on the West Coast anytime soon.
The data for the entire Pacific coastline population are tabulated every year in a coordinated effort to survey plovers in their breeding habitats. Researchers survey coastal habitats at the end of May or in early June and count them in situ.
Currently the total population is estimated at 2,200, which Colwell says is probably within 10 percent of the actual total.
The overall Pacific Coast trends are good. Numbers have increased by 500-700 since about 2004 or 2005 when surveys began up and down the Pacific coast.
But reproduction rates on the Redwood Coast are not large enough to increase the total on a steady basis indefinitely.
In Colwell’s words, “The average plover in the tri-county region doesn’t produce enough young in any year or over its lifetime to grow the population.”
The U.S. Fish and Wildlife Service endangered species recovery plan stipulates that if and when the total population reaches 3,000 and remains stable for 10 years, the snowy plover will be taken off the endangered roster.