Boom and Bust in the Boreal Forest: Climate Signals Seen in Bird Populations

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Weaving concepts of ecology and climatology, recent interdisciplinary research by USGS and several university partners reveals how large-scale climate variability appears to connect boom-and-bust cycles in the seed production of the boreal (northern conifer) forests of Canada to massive, irregular movements of boreal birds.

A pine siskin stands on the branch of a northern conifer tree.
A pine siskin stands on the branch of a northern conifer tree. Photo, USFWS National Digital Library

Weaving concepts of ecology and climatology, recent interdisciplinary research by USGS and several university partners reveals how large-scale climate variability appears to connect boom-and-bust cycles in the seed production of the boreal (northern conifer) forests of Canada to massive, irregular movements of boreal birds.

These boreal bird “irruptions” — extended migrations of immense numbers of birds to areas far outside their normal range — have been recorded for decades by birders, but the ultimate causes of the irruptions have never been fully explained. 

“This study is a textbook example of interdisciplinary research, establishing an exciting new link between climate and bird migrations” said USGS acting Director Suzette Kimball. “A vital strength of our organization is our ability to pursue scientific issues across the boundaries of traditional academic disciplines.”

The investigation was based on statistical analysis of two million observations of the pine siskin (a finch, Spinus pinus) recorded since 1989 by Project FeederWatch, a citizen science program managed by the Cornell Lab of Ornithology. By methodically counting the birds they see at their feeders from November through early April, FeederWatchers help scientists track continent-wide movements of winter bird populations.

One of several nomadic birds that breed during summer in Canadian boreal forests, pine siskins feed on seed crops of conifers and other tree species. When seed is abundant locally, pine siskins also spend the autumn and winter there. In other years, they may irrupt, migrating unpredictably hundreds or even thousands of kilometers to the south and east in search of seed and favorable habitat. “Superflights” is the term applied to winters (e.g.1997-1998, 2012-2013) when boreal species have blanketed bird feeders across the U.S. 

The irruptions of pine siskins and other boreal species follow a lagging pattern of  intermittent, but broadly synchronous, accelerated seed production (“masting”) by trees in the boreal forest. Widespread masting in pines, spruces, and firs is driven primarily by favorable climate during the two or three consecutive years required to initiate and mature seed crops. Leading up to masting events, the green developing cones and the promise of abundant seed stimulate higher reproductive rates in birds.

However, seed production is expensive for trees and tends to be much reduced in the years following masting. Consequently, meager seed crops in the years following masting drive boreal birds to search elsewhere for food and overwintering habitat.

The key finding of the new research is that the two principal pine siskin irruption modes – North to South and West to East – correlate closely with spatial patterns of climate variability across North America that are well understood by climatologists. Not surprisingly, severely cold winters tend to drive birds south during the irruption year.

More subtly, the researchers found that favorable and unfavorable climatic conditions of regularly juxtaposed regions called “climate dipoles” two years prior to the irruption also appear to push and pull bird migrations across the continent.

USGS co-author Julio Betancourt commented, “Our study underscores the value of continent-wide biological monitoring. In this case, avid birders across the U.S. and Canada have contributed sustained observations of birds at the same broad geographic scale in which weather and climate have also been observed and understood.”

Other similar examples of biological monitoring within USGS include the Breeding Bird Survey and the National Phenology Network.

The research study, authored by Court Strong (University of Utah), Ben Zuckerberg (University of Wisconsin-Madison), Julio Betancourt (USGS-Reston), and Walt Koenig (Cornell University), was published May 11 online in the Proceedings of the National Academy of Sciences.