this winter, chicago birders are preparing for an unusual influx of northern visitors as some organizations predict that winter 2025–26 may bring one of the largest migration surges in the past five years. these movements from canada’s boreal forests are more than a seasonal novelty, revealing clues to a changing climate.

unlike routine migrations, which follow relatively stable routes and schedules, migration surges, called irruptions, are erratic. some species irrupt every few years, while others, like snowy owls and other birds of prey, move only after major disruptions. the result is an unusual, often dramatic appearance of species that birders may see only a handful of times in a decade.

“there’s evidence that we’re going to get at least moderate movement,” said doug stotz, senior conservation ecologist for the field museum’s keller science action center. 

according to tyler hoar, consultant and winter finch forecaster for the finch research network (firn), the last irruptive “superflight” occurred in 2020–21. though 2025–26 is not currently considered a superflight year, birders are watching this season closely.

the firn winter finch forecast predicts moderate to strong irruptions for many finch species, meaning chicago feeders could see elevated numbers of evening grosbeaks, pine siskins, and possibly common redpolls, depending on how the boreal crop map shakes out. recent sightings of snowy owls at the montrose point bird sanctuary at montrose harbor in chicago also sparked conversations about an irruption for the raptor species, last seen in the area nearly a decade ago.

“it’s a fly-or-die kind of winter,” hoar said. “pretty much every tree species that provides food sources … is either not producing any cones or is very poor.” 

spikes in winter visitors don’t happen at random; they begin far from chicago, in the boom-and-bust rhythms of the boreal forest in canada. driven by widespread seed and berry failures across canada, irruptions offer a glimpse into how these forests’ residents are responding to a warming climate and unstable food supplies.

what are irruptions, and what do they say about canada’s seed crops?

“mast years” are defined as years where some trees produce unusually large, synchronized seed crops. instead of a steady, year-round production cycle, some trees experience several years of low production followed by a sudden surge.

with warming in high latitudes, altered precipitation patterns, and intensifying droughts, species composition and reproductive timing in the boreal are changing. these changes can alter how often and where mast years occur, increasing the unpredictability that fuels irruptions. 

some recent studies have suggested that even moderate warming — an additional 1.6 °c (around 3°f) —  impacts species growth and survival. global temperature rise has reached the 1.6 °c threshold.

“there are quite a number of weather cues that people have shown are really strong correlates of big production years,” said ian pearse, research ecologist for the u.s. geological survey. 

for example, white spruce trees in the boreal forests are primarily driven by temperature variability.

two major climate threads change the irruption story. first, climatic dipoles can create a push-pull dynamic that suppresses seed production in one boreal sector while boosting it in another. a climate dipole occurs when opposing weather conditions occur at the same time in different regions, such as warm and dry conditions in the eastern boreal forest and cold and wet conditions in the western boreal forest. for chicago’s winter birds, these dipoles matter because they can reshape weather across the midwest, in turn shaping food supplies and migrations.

second, a more extreme climate is increasing the frequency of very wet or dry years that cue mast events in complex ways, potentially reshuffling masting frequency. this may mean climate change makes irruptions less predictable and, in some cases, more dramatic. 

“what people have hypothesized is that with a warming climate … you’re going to be more and more resource-limited in the size of that seed crop. you’d get more mast events, but they would be smaller,” pearse said. 

with more frequent masting events, pearse said, prey species would experience more frequent population booms during mast years, followed by increases in predator populations the following year. that imbalance can intensify predation and competition, depleting food resources and ultimately driving large-scale migration in subsequent years. 

more frequent irruption years could be a clue to how these changes are manifesting, but much of the science behind masting events is still a mystery.

“our ability to predict things like what’s going to happen in 20 years is really quite poor. it’s basically nonexistent, being that [masting years] are driven by weather,” pearse said. “to extrapolate what’s happening in a changing world is something really important and pretty difficult to do.”

urban sanctuaries: species on the move

as climatic conditions shift, bird species will continue to irrupt southward, increasingly finding themselves navigating to urban environments like chicago. cities once seen as biological deserts now offer refuge through strategic plantings of fruit-bearing ornamental trees and feeders.

parks and lakefront preserves offer stopover habitats where finches, redpolls, and even some raptor species can rest and refuel when food supplies decrease farther north. for example, stotz works with other scientists and local communities to revitalize areas of the city like the 100-acre burnham wildlife corridor, which stretches from the lakefront museum campus to jackson park on the south side. 

“i think urban areas are actually hugely important positives,” stotz said. “so much of the rest of the state is what i call the corn and soybean desert, where we’ve lost habitats entirely … chicago is an important place because it’s the first spot [where] a lot of these birds can refuel.”

but these same urban environments introduce new risks. glass buildings, artificial lighting, and environmental toxins pose significant threats to birds arriving in a city for the first time. 

stotz recommends efforts to restore native habitats and plant native vegetation to assist irruptive bird species this winter. hoar and firn president matthew young say that planting native and ornamental shrubs and maintaining bird feeders stocked with seeds like black oil sunflower could also provide some comfort to avian visitors.

for researchers and citizen scientists, irruptive species in cities offer both warning and opportunity. urban sightings can serve as indicators of food failures and potential climate anomalies in the boreal forests. at the same time, densely populated areas generate vast amounts of citizen-science data through platforms like inaturalist and the cornell lab of ornithology’s ebird platform, which help fill gaps in formal monitoring networks. 

on these platforms, ordinary people can log bird and other wildlife sightings in real time. these platforms create open-source biodiversity records used in conservation research and planning. other citizen-science efforts include project feederwatch, the winter finch forecast, and the audubon christmas bird count, which allow users to choose data-collection locations or participate in an organized count event. 

for now, birders across chicago and the nation are scanning trees and backyard feeders for signs of new northern visitors. whether they arrive in the small bursts expected this season or sweeping waves, their movements could underscore how closely the city is tied to shifting boreal conditions up north. in tracking them, researchers and residents are also tracing the contours of a changing climate, revealed through decades of forest data and flight patterns seen each winter.