Massive bird invasions taking place across the United States and Canada, a phenomenon that has long puzzled scientists and birdwatchers alike, is reportedly linked to climate shifts, new research indicates.

Over the years, vast numbers of birds from Canada's boreal forests have migrated hundreds-of-thousands of miles south from their usual winter range. While previous studies have blamed these so-called irruptions on food shortages, it seems that climate also plays a role. Specifically, persistent shifts in rainfall and temperature drive boom-and-bust cycles in forest seed production, which in turn drive the mass migrations of pine siskins.

"It's a chain reaction from climate to seeds to birds," atmospheric scientist and lead author Court Strong, from the University of Utah, said in a news release.

"We've known for a long time that weather was probably important, but prior analyses by ecologists have been unable to identify exactly what role weather was playing in this phenomenon," says ecologist Walt Koenig, a senior scientist at the Cornell Lab of Ornithology and co-author of the new study incorporating climate science. "It's a good example of the value of interdisciplinary work," added study co-author Walt Koenig.

Many seed-eating boreal species are subject to irruptions, including Bohemian and cedar waxwings, boreal chickadees, red and white-winged crossbills, purple finches, pine and evening grosbeaks, red-breasted nuthatches, and common and hoary redpolls. But during this study researchers chose to focus on the pine siskin - the most widespread and visible of these bird migrants. (Scroll to read on...)

Pine siskins breed during summer in Canadian boreal forests, where they rely heavily on tree seeds for food. When seeds are abundant, pine siskins in eastern North America largely stay put through the winter. But when seed production is poor, pine siskins and other boreal birds move elsewhere to find overwintering habitat with adequate food. During these irruptive years, the eastern populations of pine siskins forage as far south as the Appalachian Mountains, unlike western populations.

In fact, amateur birdwatchers have seen dramatic shifts in migration patterns over the years, For example, winter 1990 saw a massive "superflight" south of the boreal forest, while during the winter ending in 2004 there was a near absence of boreal pine siskins in the United States.

To get to the bottom of this phenomenon, researchers combined FeederWatch observations - a project that involved more than two million bird sightings - with climate data in a statistical analysis. This allowed them to link bird population movements with established patterns of climate variability across North America. As expected, they found that extremely cold winters tend to drive birds south during the irruption year.

More surprisingly, it seems that there is a teeter-tottering pattern between the north and south that influences bird migrations two to three years later. When it is wet and cold in one region, hindering seed production, the weather is warmer and more favorable in the other region.

By pinpointing this unique climate pattern that sets the stage for irruptions, the scientists can, in theory, predict the events more than a year in advance.

What's more, this study also raises important questions about how global climate change is affecting bird populations across the country, by leaving birds like the pine siskin with poor seed supplies.

"The boreal forest is the world's largest terrestrial biome and is home to more than half of North America's bird species," co-author Benjamin Zuckerberg concluded. "It is likely that these irruptions, driven by climate, are a critical indicator of how climate change will affect northern forests and their dependent species."

The findings were published in the journal PNAS.

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