Northern birds invade the south thanks to a shifting climate

“Facultative migrants” are animals whose annual travels aren’t locked in. Their migration pathways are uncertain, as is whether they migrate at all during any given year. As weather and food fluctuate over time, so too do the movements of facultative migrants. Perhaps the most well known facultative migrants are birds. While some birds fly south for the winter and north in the spring at intervals you can predict with astonishing accuracy, others are more flexible in their movements. They can breed or overwinter over consecutive years in the same places, before suddenly switching their attention elsewhere. With food and weather as their guides, the birds seek out the most favorable habitats.

One important avian facultative migrant is the pine siskin (Spinus pinus), an inhabitant of North America’s vast boreal forests. The seed eaters are widespread and easy to spot, spending the entire autumn and winter in the northern forests in some years, but flying up to several thousands of kilometers south in others. The prevailing hypothesis has been that dwindling seed supplies send the birds packing in search of more to eat. Seed production follows a cycle of boom-and-bust, which means the birds can find enough to eat without leaving during good years, but must travel farther to eat well at other times. These occasional migrations are called “irruptions.”

But explanations for the broader climate- and environment-scale trends that drive avian irruptions have thus far eluded ornithologists. For one thing, they’re difficult to monitor. Effective observations require lots of data at a wide variety of locations, spread across a vast landscape. Second, the phenomenon could be driven by trends that play out not just over large landscapes but also over long time periods.

Recent years have seen innovations that now allow researchers better access to data at those spatial and temporal scales – such as citizen science efforts like Project FeederWatch – finally allowing scientists to better understand avian irruptions.

To see whether he could predict avian irruptions, University of Utah atmospheric scientist and assistant professor Courtenay Strong compared climate data with observations collected by birders through Cornell University’s Project FeederWatch. The project goes back to 1989, includes observations from the entire winter, from November through April, and includes more than 10,000 backyard birders each year. That gave Strong and his colleagues more than 2 million pine siskin records to analyze.

They found that weather patterns in one year could predict the pine siskins’ movements for as much as the next two years. When one winter was extremely cold, seed production the following year – or even two – was reduced. That’s why the siskins were forced to abandon their boreal forests and fly as far as the Appalachian Mountains the following years. Striking irruptions occurred in 1990 and 2009, with thousands of birds turning up in the US. But the birds were all but absent from the US during the winter of 2004.

The data revealed that the avian trends followed both north-south and east-west climate seesaws called “climate dipoles.”

When weather in the north was bad for seeds (wet and cold), the weather in the south was usually better for seed production (warm and dry). After two or three years, the balance shifted, and the north became warmer and drier, while the south became wetter and colder. The dipole pushes and pulls the birds across the continent as the shifting weather patterns affect seed generation. A similar pattern flows on a different time scale along the east-west axis.

Having identified these patterns, Strong and other researchers are now in a position to predict avian irruptions, at least two or three years in advance. And it’s not just for pine siskins. Other seed eaters, like 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 all likely behave similarly.

Their findings raise questions about the ways in which climate change might unbalance the two climate dipoles described in this study. “Nearly half of all North American birds breed in the boreal forest,” writes Strong, “and as such, the periodicity of avian irruptions and their connection to climate likely represents a critical bellwether of how climate variability influences North American biota.” Changes in climate are likely to lead to changes to local weather patterns, which in turn could alter the strength, periodicity, or orientation of the two North American climate seesaws.

“It’s a chain reaction from climate to seeds to birds,” says Strong. Could climate change hijack that chain reaction, leading to unfavorable seed conditions in both the north and south during the same year, leaving the birds with nothing to eat? Nobody knows. – Jason G. Goldman | 13 May 2015

Source: Courtenay Strong, Benjamin Zuckerberg, Julio L. Betancourt, and Walter D. Koenig. (2015). Climatic dipoles drive two principal modes of North American boreal bird irruption. Proceedings of the National Academy of Sciences USA. DOI: 10.1073/pnas.1418414112.

Header image: Pine siskins at a feeder via Flickr/George Thomas CC BY-NC-ND 2.0.

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