Wild Neighbors: West Nile Virus Hits the Yellow-billed Magpie

By Joe Eaton
Thursday October 09, 2008 - 09:56:00 AM

            Yellow-billed magpie exploring plumbing in Dell Valle Regional Park.
Ron Sullivan
Yellow-billed magpie exploring plumbing in Dell Valle Regional Park.

No, this is not turning into the Magpie Column. But I just got wind of disturbing news about these birds that needs to reach a wider audience. The July issue of The Auk, published by the American Ornithologists’ Union, has an article entitled “Early Impact of West Nile Virus on the Yellow-billed Magpie (Pica nuttalli.)”  

I’ve spoken in the past to the lead author, Scott Crosbie of the Veterinary Genetics Laboratory at UC Davis, and several of his co-authors: Walter Koenig at UC’s Hastings Reservation, contracting biologist Ginger Bolen, East Bay Regional Parks District wildlife manager Doug Bell. These folks are not alarmists, and I’m inclined to take their findings seriously. Based on multiple indicators, they report steep declines in yellow-billed magpie populations since the advent of the disease.  

The magpie is a true California endemic, one of two bird species that occur only within the state; the other is the island scrub-jay of Santa Cruz Island.  

Its range is limited to the Central Valley and the Coast Ranges south of San Francisco Bay; there’s no overlap with the closely related black-billed magpie of the Great Basin. Since acorns are a major part of the yellow-bill’s diet, it’s usually found in association with oaks.  

The pre-West Nile population was estimated at 50,000 to 180,000 individuals; National Audubon and Partners in Flight go with the high end. 

Since corvids—members of the crow family, including magpies and jays—were known to be more susceptible to West Nile than other birds, there was early concern that California’s unique magpie might be at risk. The first infected magpie was detected in 2004, in Sacramento County.  

The year before that, 223 magpie carcasses had been reported to the state’s Dead Bird Surveillance Program. In 2004, that number increased to 2667; in 2005, to 8571. For both years, over 80 percent tested positive for the virus-the highest rate for any bird species with a meaningful sample size. (For comparison, 70 percent of western scrub-jay carcasses and 56 percent of American crow carcasses tested positive.) 

Breeding Bird Surveys and Audubon Christmas Bird Counts are widely used indicators of bird populations. The spring surveys involve point counts along fixed routes; for Christmas Counts, observers collect data on all the species they can see or hear within a defined count circle. Crosbie and his co-authors report a 22 percent decline in yellow-billed magpies on Breeding Bird Surveys from 2004 to 2005, and a 42 percent drop in Christmas Count numbers through 2006.  

Observations at three traditional communal magpie roosts in Sacramento County mirrored those trends. One roost with a peak occupancy of 913 birds in November 2003 had been completely abandoned by March 2006, as had a smaller roost. The third showed a precipitous decline. Magpies are set in their ways and may use the same roost sites for decades. There’s no indication that the missing birds went somewhere else. 

Another line of evidence suggests that West Nile is fatal to most magpies that contract it. Only one of 21 magpies trapped for blood samples in Davis had West Nile-specific antibodies, a sign that it had survived a bout of the disease. (In contrast, 25 percent of 48 scrub-jays had the antibodies.) 

So: fewer breeding birds, lower winter numbers, lots of carcasses, empty roosts. Crosbie and his co-authors extrapolate a possible 49 percent decline from carcass retrievals alone. They suggest several reasons why magpies would be harder hit than other birds: their habitat overlaps with the mosquitoes that spread the disease, the magpies gather in large night roosts and are highly social during the day, and they eat carrion, a reported transmission route for West Nile. If the birds don’t develop resistance to the disease, they could be headed for range contraction, a genetic bottleneck, and what population biologists call an “extinction vortex.” 

What difference would that make? No species exists in isolation. It’s all hitched together, as Muir said. One study suggests that Bullock’s orioles nest near yellow-billed magpies for protection from predators. Magpies cache acorns; those they don’t retrieve are well situated to become the next generation of native oaks. Oak woodland and savanna ecosystems are in enough trouble already without losing an important propagator.  

At a Magpie Working Group conference at UC Davis in 2005, scientists and wildlife managers acknowledged that the magpie may be on the brink and recommended range-wide monitoring, protection of its habitat, and consideration of endangered species status and a captive breeding program.  

Sadly, it’s hard to imagine any of these being acted on in the present political and economic climate. The California Department of Fish and Game is already strapped, and it’s anyone’s guess what will be left of federal programs. Bad timing for the yellow-billed magpie, and for all of us who value intact ecosystems.