It was some time before Peter Warzybok believed he was looking at a northern gannet. The large birds are distinctive, with creamy plumage and black-tipped wings—but they aren’t supposed to be anywhere near the Farallon Islands. Their range is the northern Atlantic Ocean, separated from the Pacific by various continents and the inhospitable Arctic sea.
“At first I thought it was going to be a red-footed booby or something like that,” said Mr. Warzybok, a biologist who lives part-time on the cluster of islands that lies due west of San Francisco. He first spotted the bird during a routine morning seabird survey, and “once I got a good look at it, I got really excited,” he said.
The bird, which showed up last April and is still hanging around, is the first northern gannet ever documented in the Pacific Ocean. It created a huge stir within the bird-watching community, and is one in a small string of animal sightings worldwide that have been tentatively attributed to global climate change.
“For a bird that only is found in the Atlantic to show up on the Farallones is pretty extraordinary,” said Mr. Warzybok, who works for PRBO Conservation Science, the organization that conducts most biological research on the islands. “Of course the first thing we started talking about is how it got there.”
The northern gannet is a seabird that depends on chilly, open water to survive; it is common from Newfoundland to Ireland, and as far south as northern Mexico. Scientists agree it would be impossible for one of these birds to cross the continent. The most probable route Farallon gannet could have taken is north across the Arctic Sea, over the open water that increasingly has been replacing summer ice as temperatures rise.
“It seems pretty coincidental to have all these eastern north-American birds suddenly showing up in the Pacific,” said Peter Pyle, a biologist with the Institute for Bird Populations in Point Reyes Station.
Arctic ice reached record lows in 2012, and the Northwest Passage—historically impassible year-round—has been melting open during summers since 2007. As the ice recedes, a relatively direct path between the Atlantic and Pacific Oceans has opened to the aquatic creatures living in the northern parts of both seas.
And they seem to be taking advantage of it. A species of plankton that only lives in the Pacific showed up in the Atlantic after the passage melted open in 2007, according to the National Oceanic and Atmospheric Administration.
Other, larger organisms soon followed suit. In the last few years, a tufted puffin and a gray whale—both previously found only in the Pacific—have appeared half a world away, while an Atlantic eider showed up in Crescent City.
The whale was a particularly impressive voyager. This Pacific mammal was seen in the Mediterranean Sea near Israel in 2010; scientists speculated that its journey—if indeed the whale traversed the Northwest Passage—ended at a logical destination.
Gray whales typically feed off the shores of Canada before they head south and hang a left into their breeding grounds in the lagoons of Baja California. But if a whale overshot the mark, continuing west until it reached the coastline of Europe before heading south, the first major inlet on the left would lead it into the Mediterranean Sea.
The only direct documentation of migration through the Northwest Passage was of two male radio-tagged bowhead whales. These creatures occur in both the Atlantic and the Pacific, and in 2010 one whale from each ocean entered the Northwest Passage when it was largely free from ice.
Both spent the first two weeks in September in the Viscount Melville Sound before returning to their original territory. Because their radio tags showed they crossed paths before turning around, presumably they could have continued on to the opposite sea.
Though the Farallon gannet is the first to be documented in the Pacific Ocean, there were two unconfirmed sightings of a northern gannet in the Chukchi and Bering Seas last year. There is no proof that this was the same bird that eventually wound up here in California—but it would make sense, Mr. Bradley said.
“The only other way it could have come is if it actually rode a ship through the Panama Canal and came up that way,” he added. “Either way is pretty extraordinary.”
Now that it is here, the bird seems to have settled in. When Mr. Warzybok first saw it in late April he got on his radio to alert his colleagues.
“I said, ‘Come up to the lighthouse if you want to see a gannet,” he recalled. “But then it took off and flew around the island right by the house; people were able to just walk out the front door and see it fly by.”
Within hours, news of the sighting had ricocheted through the California birding community.
Part of the reason for the excitement is competitive. Serious bird watchers keep lists of the birds they’ve seen in their lifetimes, or in certain states or regions. For birders who have seen northern gannets in the east, it’s still a big deal to spot one in California, said Mr. Pyle—who added that he personally had been eager to see it himself, since he’s seen the largest number of birds on the Farallones and didn’t want to miss adding this rare visitor to his list.
Throughout the summer and fall, whale-watching boats and bird-watching cruises visited the remote islands, hoping for a glimpse of the gannet, which has been spending most of its time on a small spire of stone called Saddle Rock.
Though both Point Reyes and the Farallon Islands are renowned among birders for attracting off-course birds, or “vagrants,” the gannet was a particularly exceptional sight.
“Part of everyday life on the Farallones is keeping your eyes open because anything could show up ant any time—but even though we say that, we don’t really mean anything,” Mr. Warzybok said.
Most vagrants are blown off-course, or perhaps they turn left instead of right in a storm or fog bank. Substantially more vagrants show up in cloudy or foggy weather—but not ones that have never even been seen in the same ocean.
“This individual took an incredible journey to get to the Farallones and then it stayed there,” Mr. Bradley said. “It’s definitely just an incredibly rare occurrence.”
The gannet may have decided to stay on the rugged, rocky Farallones because they resemble the species’ nesting colonies back east—right down to the other birds, such as common murres, that nest nearby.
“I’ve seen [the gannet] land among the common murres and look like it wanted to nest—it was displaying and looking like it wanted to mate,” Mr. Warzybok said. “It probably was perplexed as to why there weren’t any other gannets around.”