Climatic dipoles drive two principal modes of North American boreal bird irruption

• Published in: Proceedings of the National Academy of Sciences - PNAS 2015-05, Vol.112 (21), p.E2795-E2802
• Main Authors: Strong, Courtenay, Zuckerberg, Benjamin, Betancourt, Julio L., Koenig, Walter D.
• Format: Article
• Language: English
• Subjects: Animal Migration - physiology; Animal Nutritional Physiological Phenomena; Animals; Biological Sciences; Birds; Climate; Correlation analysis; Diet; Ecosystem; Finches - physiology; Habitats; North America; Physical Sciences; PNAS Plus; Precipitation; Seasons; Seeds
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1418414112

Pine Siskins exemplify normally boreal seed-eating birds that can be sparse or absent across entire regions of North America in one year and then appear in large numbers the next. These dramatic avian “irruptions” are thought to stem from intermittent but broadly synchronous seed production (masting) in one year and meager seed crops in the next. A prevalent hypothesis is that widespread masting in the boreal forest at high latitudes is driven primarily by favorable climate during the two to three consecutive years required to initiate and mature seed crops in most conifers. Seed production is expensive for trees and is much reduced in the years following masting, driving boreal birds to search elsewhere for food and overwintering habitat. Despite this plausible logic, prior efforts to discover climate-irruption relationships have been inconclusive. Here, analysis of more than 2 million Pine Siskin observations from Project FeederWatch, a citizen science program, reveals two principal irruption modes (North-South and West-East), both of which are correlated with climate variability. The North-South irruption mode is, in part, influenced by winter harshness, but the predominant climate drivers of both modes manifest in the warm season as continental-scale pairs of oppositely signed precipitation and temperature anomalies (i.e., dipoles). The climate dipoles juxtapose favorable and unfavorable conditions for seed production and wintering habitat, motivating a push-pull paradigm to explain irruptions of Pine Siskins and possibly other boreal bird populations in North America. Significance This study is the first, to our knowledge, to reveal how climate variability drives irruptions of North American boreal seed-eating birds. Patterns of Pine Siskin irruption and associated climate drivers manifest as two modes (North-South and West-East) in which dipoles of temperature and precipitation anomalies push and pull irruptive movements across the continent at biennial to decadal periodicities. Our study accentuates the value of sustained and synoptic biological observations, contributed by citizen scientists in this case, that match the spatial and temporal scales at which climatic phenomena are observed and understood. Such observations can help probe new questions, such as the role of climatic dipoles in other large-scale ecological processes.