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Thousands of Band-tailed Pigeons have along the Golden State’s coast. Since mid-December, caracasses of the migrartory species have been found in forests and mountains from San Diego to San Francisco (the birds winter in California). Here’s our guide to what’s going on.

Why is this happening?

The West Coast birds are being killed by a disease called avian trichomoniasis, caused by a single-celled protozoan called Trichomonas gallinae,(A similar but distinct parasite , called Trichomonas stableri, may be responsible for some of the bird deaths.)

The parasite sets up camp in the oral-nasal cavity and respiratory tract of the Band-taileds, which are slightly bigger game birds than typical park pigeons. The disease causes “cheese-like” lesions in the bird’s esophagus and mouth, making it difficult for the birds to eat the acorns and berries on which they rely. It also makes breathing difficult, and these lesions can lead to death from starvation or suffocation.

That sounds gruesome.

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How many Band-tailed Pigeons have died in the outbreak?

It’s hard to tell exactly. Krysta Rogers, an environmental scientist at the CDFW’s Wildlife Investigations Laboratory performs necropsies -- that’s an animal autopsy -- on pigeon carcasses to determine cause of death. In a single week in January, she examined a total of 200 dead birds. That’s more than usual.

She estimates the current total body count to be 5,000 to 10,000--and rising. Researchers rely on the general public to report dead birds, so it's possible that disease has felled more pigeons, but their carcasses haven't been discovered (possibly because they're in rural areas with fewer people).

Is this the biggest “mortality event”—more than five dead birds in the same geographical area in the same time frame—for Band-tailed Pigeons yet?  

Good vocabulary word. Again, its hard to tell—if past avian trichomoniasis outbreaks killed pigeons outside of urban centers, their carcasses could have gone unnoticed, and therefore uncounted. took the lives of 16,000 pigeons, and outbreaks have been occurring more frequently in the past decade. The current one “is certainly up there,” Rogers says.

How does the disease spread?

Quickly, and when birds congregate. The parasite can't survive for long outside its host. Infected birds deposit the parasite into watering holes, or birdseed at feeders, where uninfected birds pick it up when they consume the tainted water or food." Also, adults can pass it to their young through the “pigeon milk,” the.

How long has this parasite been wrecking havoc?

Millions of years. Scientists think that the protozoan might have been responsible for the death of the , distant ancient ancestor of the pigeon. In an investigation of 60 tyrannosaur jaws, nearly 15 percent featured the small, smooth holes characteristic of the parasite’s handy work. It might have been passed during dino-on-dino fights, or via cannibalism.  

In the modern era, it was ferried to North America by European settlers who brought along doves and pigeons. A centruy ago, the disease, the band-tailed’s cousin. Today, it affects pigeons and other birds, notably hawks, owls, and songbirds. (And other breeds of pigeons too, but they’re better at fighting it.)

What’s spurring the latest outbreak?

Drought conditions reduce the number of drinking spots available to the birds, so they tend to congregate together around the remaining resources,

Does the parasite hurt any other birds in the region?

Mourning Doves are susceptible, but not as much as the Band-tailed Pigeon. Rock Pigeons also carry the parasite, but it doesn’t usually kill them. Officials are worried that hawks and owls that prey on pigeons may come in contact with the parasite, but it doesn’t seem to be affecting these birds yet.

What can Californians do to help band-tails?

Removing bird baths and bird feeders can help quell transmission of the current outbreak, says the CDFW’s Rogers. These hot spots can bring birds into close contact and promote a quick transfer of the parasite, which doesn’t survive for long outside a host. “Let the birds move around more naturally,” says Rogers.  

Also, to help Rogers and other researchers get a handle on where and how transmission is occurring.