Thousands of Band-tailed Pigeons have dropped dead 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, autopsies at the California Department of Fish and Wildlife (CDFW) have shown. (A similar but distinct parasite discovered in July 2014, 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...