Pine siskins other bird species from the boreal forests of Canada are consistently unpredictable. One winter, they could be one of the most common birds in the southern U.S. and the next they might be nearly absent.
Such sporadic seasonal habits are highly unusual, and until recently ornithologists were unsure what was leading these birds to behave this way. A study published in the journal Proceedings of the National Academy of Sciences May 11 provides evidence for the prevalent hypothesis that these fluctuations are linked with climate shifts that result in changes in seed production in these species' boreal forest habitat in northern Canada.
"It was clear that climate plays a role, but it's never been clear whether its effects are direct or indirect," senior study author Walt Koenig of Cornell University said in an interview.
The study looked specifically at pine siskins, one of several seed-eating boreal bird species that is known to participate in these massive periodic southern invasions. Irregular migratory events during which many birds of the same species travel outside of their typical range are known as irruptions. Those who live along the East Coast of the U.S. may remember seeing snowy owls last winter, an unusual sight in that region. This unusual influx of snowy owls is another example of an irruption.
While other bird species are known to irrupt as well, "boreal seed-eating birds are the most dramatic" when it comes to this behavior, according to Koenig. Other boreal seed-eating birds include cedar waxwings, pine and evening grosbeaks, red-breasted nuthatches, and purple finches.
In their analysis, the researchers used more than 2 million pine siskin observations gathered through the citizen science program Project FeederWatch, which allows members of the general public to contribute to scientific research by submitted records of their bird sightings online.
"The beauty of citizen science data is that there are a lot of citizen scientists across the country, so we're able to get really good data on the birds' relative abundance going back many years," Koenig says.
Comparing the patterns seen in the citizen science data with climate records revealed that changes in rainfall and temperature matched up with the birds' curious migratory behavior. It was known from previous research that seed availability was a key factor involved in these irruptions, but researchers found that in this study that seed availability has a two-pronged effect that is governed by climate conditions. When the climate up north produces fails to produce a favorable seed crop, climate conditions in the southern U.S. tend to produce a very favorable seed crop, and vice versa. The resulting complementary boom-and-bust seed production cycles drive the extreme migratory patterns of pine siskins and other boreal birds.
While co-author Benjamin Zuckerberg noted in a press release that "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," Koenig points out that the mechanism relating weather to seed production among boreal trees is still unknown.
"We won't know the effects of climate change on these birds until we determine this mechanism," Koenig says.
Photo: Thomas Quine | Flickr
