Bird Irruption: A Sudden Surge of Birds

Redpolls in Texas, white-winged crossbills in the Great Plains, and snowy owls scattered like confetti across the continent. When you hear of these unexpected sightings, it means one thing—it’s an irruption year.

What is a Bird Irruption?

An irruptive migrant is a species that usually migrates short distances at the most, but occasionally moves far south in very large numbers. The reason for these unique migrations is not straightforward, and researchers have found that the causes vary with the species.

Winter Bird Irruption

Steve and Dave Maslowski

In the winter of 2012, an abundance of boreal songbirds headed south out of their normal range. Bohemian waxwings, pine siskins, red-breasted nuthatches, crossbills, redpolls, pine and evening grosbeaks all appeared in unusual places. Check out the top 10 plants that attract waxwings.

This sudden boom began in the spring, which was unusually cool and rainy. With those weather conditions, pine and spruce trees in the boreal forest produced fewer cones, the main food for northern songbirds. As winter approached, the fare became scarce, and the desperate birds headed south in search of more productive foraging.

Steven Matsuoka, a research wildlife biologist for the U.S. Geological Survey in Anchorage, Alaska, described the activity of these birds as resource tracking. “The birds are essentially going where there is food,” he said. “It’s part of what makes these species hard to study—they are constantly moving.”

While each species has its own threshold for an irruptive migration, most years at least one makes the journey. Although it happened in 2012, it’s rare for conditions to push many types of birds south at once.

Why Owl Irruptions Happen

Richard Day/Daybreak Imagery

Northern owls like snowies, great grays, boreals, saw-whets and northern hawk owls have different reasons for heading south.

“Some owls have irruptive migrations due to a lack of food, while others move because abundant food has led to high productivity,” said Scott Weidensaul, one of the founders of Project SNOWstorm, a research effort to study snowy owls. “Boreal forest owls tend to be the first, while snowy owls tend to be the second.”

Northern hawk owls, boreal owls and great gray owls fly south when food, such as voles and lemmings, is scarce. Like the cone-dependent songbirds, they move out of their northern homes to find food. But snowies and saw-whets appear in largest numbers when the populations of small mammals are up.

According to Scott, when lots of small mammals are around, snowy owls are very productive nesters. Broods of chicks are large and easily fed, and due to the abundant food, most of the youngsters survive the normally dangerous first weeks away from the nest. When snow arrives, draping the hunting grounds in a deep layer of fluff, all of those tasty lemmings and voles are suddenly much less accessible and there is a lot of competition for good hunting grounds. The young birds, unequipped to cope with these hardships, irrupt south, in search of something better.

How Scientists Predict a Bird Irruption

Steve and Dave Maslowski

As biologists hash out the different causes of irruptive migrations, they are also learning how to predict them.

Project FeederWatch, administered in the U.S. by the Cornell Lab of Ornithology, describes itself as “a winter-long survey of birds that visit feeders at backyards, nature centers, community areas, and other locales in North America.” Emma Greig, project leader there, said, “By looking at feeder count data in past years, we can clearly see when irruptive migrations have occurred. From there we can look at the climate and weather conditions and see what those years may have had in common.”

By analyzing that data across the north over time, it’s possible to determine which species, if any, are likely to appear further south the following winter. The trouble, however, is that the arctic and boreal forests are vast regions with a lot of variability from one part to another. What is happening in Alaska may not reflect what’s going on in eastern Canada. Those who study this phenomenon admit that irruptive migrations are still not fully understood. (Read more: Ron Pittaway’s Finch Irruption Forecast)

There’s good news for birders in the Lower 48, though. The snowy owls of Canada’s eastern Arctic had a good summer, Scott said. “Birders along the East Coast should keep their eyes open for snowy owls this winter.” That’s a forecast every birder loves to hear.