The recent expansion of Fox Sparrow (Passerella iliaca iliaca) breeding range into the northeastern United States.

John D Lloyd

doi:10.7717/peerj.6087

Abstract

The breeding range of the Eastern Fox Sparrow (Passerella iliaca iliaca) is generally recognized as comprising the boreal forest of Canada. However, recent observations suggest that the species is present during the summer months throughout much of the northeastern US, unexpected for a species characterized as a passage migrant in the region. To clarify, I conducted a literature review to document the historical status of the species in the northeastern US and then analyzed observations submitted to eBird to describe its recent and current status in the region. Historical accounts consistently identify Fox Sparrow as a passage migrant through the region during early spring and late fall. Beginning in the early 1980s, observers began noting regular extralimital records of Fox Sparrow in northern Maine. A single nest was discovered in the state in 1983, and another in northern New Hampshire in 1997. Despite the paucity of breeding records, observations submitted to eBird suggest that the southern limit of the breeding range of Fox Sparrow has expanded rapidly to the south and west in recent years. The proportion of complete checklists submitted to eBird that contained at least one observation of Fox Sparrow grew at an annual rate of 18% from 2003–2016 and was independent of observer effort. Fox Sparrow now occurs regularly on mountaintops and in young stands of spruce (Picea spp.) and balsam fir (Abies balsamea) during the breeding season throughout northern and western Maine and northern New Hampshire, with occasional records from the Green Mountains of Vermont and the Adirondack Mountains of New York. The cause of this rapid expansion of its breeding range is unknown, but may be related to an increase in the amount of young conifer forest in the northeastern US created by commercial timber harvest.

Highlights

Fox Sparrow (Passerella iliaca) is a polytypic species that breeds in montane and boreal forest across western and northern North America
Anecdotal reports from birders and observations submitted to eBird, a web platform that documents bird distributions using observations submitted by citizen-scientists (Sullivan et al, 2009), suggest that Fox Sparrow occurs regularly during the breeding season throughout northern and western Maine and as far south and west as the mountains of central and northern New Hampshire
In an effort to address this gap and to clarify the status of Fox Sparrow as a breeding species in the northeastern US, I review historical and current literature describing the distribution of Fox Sparrow in this region and describe temporal changes in occurrence using data submitted to eBird by citizen scientists

Summary

Introduction

Fox Sparrow (Passerella iliaca) is a polytypic species that breeds in montane and boreal forest across western and northern North America. Anecdotal reports from birders and observations submitted to eBird (http://www.ebird.org), a web platform that documents bird distributions using observations submitted by citizen-scientists (Sullivan et al, 2009), suggest that Fox Sparrow occurs regularly during the breeding season throughout northern and western Maine and as far south and west as the mountains of central and northern New Hampshire. This would constitute a fairly rapid southward expansion of the species’ breeding range, on the order of several hundred kilometers, yet this phenomenon has remained undescribed in the ornithological literature.

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