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If the winds are right, a morning on the berm overlooking the Cape May Canal in New Jersey could yield thousands of migrating birds flying past from a hundred or more different species. But in late October of 2018, dozens of individuals from an utterly unexpected species whizzed by morning-flight counter Andrew Dreelin: White-breasted Nuthatches.

Familiar backyard birds across the United States and Canada, White-breasted Nuthatches have long been assumed to be fairly sedentary residents, inhabiting cavities in deciduous or mixed deciduous-coniferous forests year-round. But in 2018, Dreelin and other birders in his Morning Flight Working Group on Facebook, which studies the phenomenon of birds reorienting themselves following a night of migration, began noting an incredible number of White-breasted Nuthatches on the move. 

The reports piqued the interest of fellow group member Joe Gyekis, an associate teaching professor of biobehavioral health at Penn State. Noticing that there was a dearth of information on the phenomenon, Gyekis wrangled Dreelin, Paul Heveran, a volunteer at Hawk Mountain in Pennsylvania, Andra Florea, a bird bander at Tadoussac Bird Observatory in Quebec, and Erica Dunn, a veteran scientist with access to nuthatch data from Long Point Bird Observatory in Toronto, to gather and compile nuthatch data from major migration hotspots across the eastern United States and Canada. 

Together, the team of birders and others, working in various Facebook groups and on email chains, are demystifying the seasonal movements of this often-overlooked bird. So far, according to from the group last month on Cornell’s BirdCast blog, a pattern has emerged in the data they’ve collected: White-breasted Nuthatches seem to move cyclically, in approximately two-year increments, just like Red-breasted Nuthatches. And, as with their smaller cousins, White-breasted Nuthatch movements correlate with years of poor tree seed and fruit abundance in Canada's boreal forest. 

These findings suggest that White-breasted Nuthatches might also undergo food-driven “irruptions” out of their northern habitats. In fact, two years after the 2018 irruption, White-breasted Nuthatches are once again on the move in 2020, with record numbers flying past Cape May . The flight has presented the group with yet another opportunity to study this phenomenon. (It’s important to note that this research deals only with birds in the eastern U.S. The complex cycles of tree crops in the Mountain West makes studying irruptive movements far more difficult.) 

“Me as a birder, and I think a lot of people, categorize birds as those that migrate and those that don’t,” Gyekis says. “I always thought that White-breasted Nuthatches don’t migrate, so I didn’t expect thousands of them to be streaming out of Canada in some years and not in other years.” 

The earliest record of White-breasted Nuthatches moving in significant numbers the team has found came in 1968, when ornithologists Donald Heintzelman and Robert MacClay noticed flying past Bake Oven Knob in Pennsylvania, after observing only 53 total in the preceding seven years. Since then, there have been other accounts of similar, though smaller, flights. But otherwise, White-breasted Nuthatch movements have remained largely a mystery. 

The main reason for this is that White-breasted Nuthatches are such a widespread and common resident species in the eastern U.S. that a sudden influx of birds is hard to detect. When a Red-breasted Nuthatch shows up in a backyard in Arkansas or Virginia, for example, it's clearly a migrant from elsewhere because the species doesn't breed anywhere nearby. But the White-breasted Nuthatch's year-round status essentially cloaks any widespread migratory movement of the species. 

Vladimir Pravosudov, a scientist not involved in the research and a foundation professor in the department of biology at the University of Nevada, Reno, says he also finds the species' movements surprising, but he points out that it’s unclear whether these reports show irruptions or simply cycles of juveniles dispersing from their birth habitats. “To really document irruption, we need data on banded birds—that the birds are adults and that they all took off and went somewhere,” he says. To determine what exactly is going on, Pravosudov would want to see data about the age of the birds observed at migration hotspots, as well as population studies of the northern birds and even physiological data of moving birds, such as hormone levels. 

Florea, who bands birds at Quebec's Tadoussac Bird Observatory, a major migration hotspot, agrees that more work is required to determine what kind of movement she and the rest of the team has observed. Still, the research has changed the way she thinks about nuthatches. “The work has unravelled a mysterious pattern that awaits more studies, and I realize (as always, with every new question) how much more there is to know about birds and how creative we can be as ornithologists if we don’t take our bird knowledge for granted,” she said in an email. 

Dreelin hopes that eventually White-breasted Nuthatches will appear in the popular Winter Finch Forecast that tracks the movements of irruptive species like boreal finches, Red-breasted Nuthatches, and Blue Jays. Ultimately, though, the team is happy to be shedding new light on such a common species. “Now, whenever I see or hear a White-breasted Nuthatch, I squint and ask whether it’s a migrant or not," he says.