Birders from across the country traveled to Marin County this month after a local resident recorded what is believed to be the first sighting of a Eurasian songbird on the North American mainland.
The willow warbler, native to northern Europe and Siberia, was first spotted by longtime bird-watcher William Legge of Mill Valley on Oct. 18 on the north side of Rodeo Lagoon in the Marin Headlands.
“I’ve found quite a lot of rare birds at Rodeo Lagoon over the years but nothing close to this,” Legge said.
Word spread quickly across social media about the sighting, attracting hundreds of birders from throughout the country hoping to catch a glimpse of the small yellow songbird.
“Some people flew in from Texas and southern California. Some of the most prominent birders in the country came and found it,” said Novato resident Daniel Edelstein, a consulting avian biologist, birding guide and author of the WarblerWatch.com blog. “It’s a life list bird for a lot of people, including for me, who have never seen it.”
The last confirmed sighting of the warbler in Marin was on Oct. 23, Legge said.
The sighting came about a week after another songbird, the closely related wood warbler, was sighted for the first time on the mainland of North America in southern California.
“It’s very interesting to have two extremely rare Siberian vagrants show up in the same year and a similar timing,” said Mark Dettling, an avian ecologist with the Point Blue Conservation Science nonprofit research group. “It’s also been just a very interesting year in general. Bird-watchers have been loving it.”
But how does a 5-inch-tall, half-ounce songbird like the willow warbler end up thousands of miles away from its native range?
Dettling said the reason is not certain, but there are several leading possibilities. One is that they have a genetic mutation that has thrown off their internal compasses. In the case of the willow warbler, it resides in northern Europe and Siberia but overwinters in sub-Saharan Africa.
“The bird in Siberia needs to go southwest,” Dettling said. “But if that internal compass is off by 90 degrees, it goes southeast. That brings it to the Siberian coast and to islands off of Alaska and just keeps going and starts following the coast further south to end up in this case in California.”
Another possible explanation is that a severe storm can cause birds to get disoriented. Birds have also been known to hitch a ride by ship, though Dettling said that this would be challenging for a willow warbler given the lack of insects it needs to feed on.
Willow warblers have been observed on the islands off Alaska but no recorded sighting has been made on the North American mainland.
More and more cases of birds astray have been recorded in recent years, according to Jack Dumbacher, ornithology curator at the California Academy of Sciences. Climate change could be causing birds to be responding differently compared to their predecessors from decades or hundreds of years ago, he said. But another factor for these increased sightings is that more people are bird-watching and are able to share their observations more rapidly than ever before, he said.
The chances are slim that the willow warbler found in Marin will survive and make its way back to its native territory, Dumbacher said. In most cases, birds with genes that alter their internal compasses often get weeded out. However, there are those fortunate few that play a pivotal role in the evolution of their species.
“There is a big risk and a big downside,” Dumbacher said. “But if you get lucky you end up in the Galapagos Islands and you end up founding a new population and becoming a new species. But that’s probably the rare exception.”
The last time Legge saw the willow warbler, it was flying around the coyote scrub and feeding on the abundant flies. While that might be the last time he sees the warbler, Legge said another rare bird is likely to follow soon enough.
“Ultimately you never quite know what you’re going to see out there,” Legge said. “October and September and the fall tend to be a time of the year where you get more chances of encountering a rare bird that has gone off the tracks a little bit.”
