Why Did the Siskin Cross the Continent?

If you spend as much time outside on British Columbia’s rural Galiano Island as professional gardener Ana Todorović does, it’s a sad inevitability that you will encounter dead birds. But when Todorović returned home from work on May 16, she noticed something distinctive about the goldfinch-size, streaky-brown bird corpse on her lawn: It had a metal ring on one leg.

The unique code on that band would later reveal that the bird had completed an amazing cross-continent journey. Scientists last week reported in a Facebook post that this particular Pine Siskin had traveled 2,403 miles west from where they banded it in Maryland in 2020—an impressive feat, but not unheard of for this far-ranging species.

Scientists had banded this siskin at Foreman’s Branch Bird Observatory, a long-term research site affiliated with Washington College on Maryland’s eastern shore. It’s one of many stations where ornithologists catch birds in delicate mist nets, attach leg bands, and send them on their way. The unique sequence of characters on each band allows researchers to track migratory movements and bird demographics. Simply capturing and measuring birds at these stations provides important data such as avian abundance and distribution, but the most exciting results often come when banded birds are caught again or reported by community scientists to the U.S. Geological Survey Bird Banding Laboratory.

Among the 2020 banding season’s most memorable days at Foreman’s Branch was Halloween; birds were flying into nets by the flock, and there was little time for banders to eat or rest. The crew ultimately banded 623 birds, including 124 mostly docile and cooperative Pine Siskins. One of those siskins would eventually end up on Todorović’s lawn, representing one of the observatory’s farthest-flung resightings—farther, even, than some reencounters in South America of Ospreys banded at Foreman’s Branch.

“When we got this siskin report, I thought, is it real? Did someone type the number wrong?” says Maren Gimpel, an ecologist at at Washington College and bander at the observatory. “But no, [Ana] had photographed the bird with the band on it. It was for sure real, and you obviously can’t go much further west.”

The vast majority of banded passerines are never seen again, but the data on those that are can reveal all sorts of insights, says Herb Wilson, professor emeritus at Colby College. One of his studies found that within winters, Pine Siskins are resighted an average of 75 miles from where they were banded. But between winters, the average reencounter was nearly 540 miles away.

Pine Siskins are one of North America’s winter finches, an informal group of boreal-breeding birds famous for their erratic, and occasionally long-distance movements. Unlike migrating birds that take annual north-south journeys, many siskins instead move east and west across the country. One USGS dataset prior to Todorović’s sighting included at least 33 siskins encountered more than 2,000 miles from where they were first banded. The record holder is a bird banded in Canada’s Yukon Territory in summer 2020 that was found in Mississippi the following spring, 2,869 miles away.

So, what explains these cross-continent treks? In a word, food. Like some other winter finches, Pine Siskins eat the seeds inside small conifer cones. Conifers are known for boom-and-bust cycles of seed production, which scientists believe is an adaptation to control populations of squirrels and other animals that eat those seeds. Finches have wings, however, and in years of few cones will undertake “irruptions,” traveling elsewhere in search of food.

The fall and winter of 2020 into 2021 brought an exceptional irruption of winter finches across North America, especially Pine Siskins. They showed up in flocks of dozens to hundreds, appearing as far south as the Gulf of Mexico and as far east as Bermuda. The siskin that Todorović found was one of 896 banded at Foreman’s Branch in fall of 2020, 662 of them in their first year of life. The crew there banded zero Pine Siskins in fall of 2021, demonstrating that these movements aren’t simply annual migrations.

While some winter finches like Common Redpolls and Evening Grosbeaks will return north to appropriate habitats after irruptions, Pine Siskins are habitat generalists that may travel and breed wherever they find a good conifer crop. It’s possible that after the 2020 irruption Todorović’s siskin decided to head west, following food sources until it ended up at the Pacific Ocean.

“It’s not uncommon for siskins to undertake these east-west movements because they do fine in both boreal and montane habitats,” says Matt Young, president and founder of the Finch Research Network, which now publishes the annual forecast predicting the movements of these finches. Pine Grosbeaks, for example, have a boreal subspecies and multiple montane subspecies in North America, with little mixing between them. Though there are isolated Pine Siskin populations in Central America, those north of Mexico represent a single subspecies with birds moving across different habitats and breeding where food supplies can support them.

What makes some birds travel so much farther than the rest of the population, and where they go between resightings, remains a mystery. But the more observant people like Todorović take note of banded birds, the more we’ll learn. “The scientists do their part, but if someone didn’t take the time to write down and report the band number, we wouldn’t know about it,” says Gimpel. “Regular people are part of the loop.”