The Island Scrub-jay of Channel Islands National Park. NPS photo.

That rare bird you spotted on your last trip to Channel Islands National Park may be a good bit rarer than you thought. The Island Scrub-jay (Aphelocome insularis) is now estimated to be one-fifth of what had been previously believed, according to a study by the Smithsonian Institution’s Migratory Bird Center.

The paper published in the journal Ecological Applications lists the jay’s population at about 2,500 individuals, all residents of Santa Cruz Island in the Channel Islands. This makes the species one of the rarest in the United States and considerably less numerous than some other celebrated endangered birds like Kirtland’s Warbler (about 5,000 individuals), Piping Plover (about 6,500), and the closely-related Florida Scrub-jay (about 8,000).

The Island Scrub-jay, however, carries no federal or state protection beyond that afforded to all migratory birds by the Migratory Bird Act. The International Union for the Conservation of Nature has automatically raised the level of the scrub-jay from “near threatened” to “vulnerable to extinction.”

With a range restricted to one island off the southern California coast, the species is particularly vulnerable to sudden impacts to its population from habitat disturbance, introduced species like pigs and sheep, and climate change.

Still, Santa Cruz Island is wholly within the protection of Channel Islands National Park (on its eastern quarter) and the Nature Conservancy (which co-manages the island with the National Park Service), making conservation efforts somewhat easier.

“The bad news is we only have about 2,500 of these birds left, a very small number for any species,” says lead author Dr. Scott Sillett of the Smithsonian’s Migratory Bird Center. “The good news is that we are seeing an increasing population trend. It appears that there has been about a 20-30 percent population increase in the last 25 years owing to a series of conservation actions on the island.”

The Island Scrub-jay, formerly a subspecies of the Western Scrub-jay, was split from that species based upon studies of genetic makeup. The insular species also has distinctly brighter plumage than the more common and widespread Western Scrub-jay. They are known for their especially large bills, an adaption for feeding on the large and thick-shelled acorns of the Channel Islands Oak (Quercus tomentella), itself endemic to only the Channel Islands and Guadalupe Island.

While the authors of the study take a cautiously optimistic stance toward the future of the Island Scrub-jay, Mike Parr of the American Bird Conservancy emphasizes the need for continued conservation efforts.

“Whenever a species exists only in small numbers in a singular location, it is cause for concern,” says Mr. Parr.

He added that he hopes this published study and downgrade by the IUCN will lead to new protections for the bird from the federal and California governments “in short order.”