Massive Bird Invasions from the North Linked to Climate Shifts

It sounds more like something out of Alfred Hitchcock's The Birds than something that's actually occurring: bird invasions. Scientists have found that vast numbers birds that are migrating hundreds of thousands of miles south from their usual winter range may be explained by climate shifts.

These unusual migration patterns are called irruptions, and many seed-eating boreal species are subject to these patterns. These species include the 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. In this latest study, the researchers focused on the pine siskin, a species that was featured prominently in earlier work on irruptive migrations.

In previous work, scientists found evidence that irruptions are triggered by food shortages caused by the large-scale collapse of seed production in northern pine, spruce and fir forests.

In order to see whether climate also had a role, the scientists examined data gathered by backyard birders as part of Project Feederwatch, which is a citizen science initiative run by the Cornell Lab of Ornithology. This project systematically records bird sightings from November through early April and gives scientists more than two million observations-just of pine siskins-to work with since 1989.

Pine siskins in eastern North America largely stay put in the northern coniferous forests of Canada through the winter. When seed production is poor, though, these birds travel as far south as the Appalachian mountains.

Over the years, the birdwatchers recorded dramatic shifts in siskin migrations. In 1990, there was a massive "superflight" south of the boreal forest. After comparing this data to climate data, the scientists found that extremely cold winters tend to drive birds south during the irruption year.

"The boreal forest is the world's largest terrestrial biome and is home to more than half of North America's bird species," said Benjamin Zuckerberg, one of the researchers, in a news release. "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 are published in the journal Proceedings of the National Academy of Sciences.

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