Least Flycatcher Range Expansion into Washington State

The Least Flycatcher (Empidonax minimus) has expanded its range west over the past 50 years at least.It has occurred annually in Washington since the 1970s, and its subsequent numbers there have increased steadily.Records extend from late April to late September, with spring migration likely peaking during late May and early June and fall migration peaking during early September.Many birds appear to be seeking territories and/or mates, but as of 2004 there were only two records of nesting.No obvious habitat changes coincided with this species' arrival in Washington, and appropriate habitat seems to have long been present.The explanation for the Least Flycatcher's range expansion may be related to population increases in its core range and/or microhabitat changes not yet described.The Least Flycatcher's expansion parallels that of several other woodland birds also currently expanding west from eastern North America.The fifth edition of the A.O.U.checklist (1957) described the Least Flycatcher (Empidonax minimus) as breeding west to southwestern Yukon, northeastern British Columbia, and western Montana.By the end of the 1950s, this species had moved into portions of British Columbia where it was previously absent: the Cariboo and Chilcotin areas of the interior and the Fort Nelson area of the province's extreme northeast (Johnson 1994, Campbell et al. 1997).It colonized southern British Columbia's Okanagan Valley in the mid-and late 1970s (Cannings et al. 1987).Since 1995, Least Flycatcher sightings in Idaho have also increased dramatically, perhaps partly the result of increased observer effort and skill.Idaho recorded its sixth through eighth records as recently as the summer of 1996 (Svingen 1996), but only two years later eight were recorded during one summer (Trochlell 1998).By the summer of 2002, reports from Idaho were too many to be enumerated in North American Birds, and only new nesting locations, in five counties, were mentioned (Trochlell 2002).South of Washington, Least Flycatchers were first noted in Oregon in 1977(Mewaldt 1977) and have been found annually there during migration since 1981 (Marshall et al. 2003).Territorial birds have been noted at a number of Oregon locations, and nesting was confirmed near Mount Vernon, Grant County, in 1985, 1995, and 1997(Marshall et al. 2003).The Least Flycatcher is also annual as a migrant in California, much more frequent in fall than in spring (Small 1994), in contrast to the situation in Oregon and Washington.California has two confirmed nesting records, from Modoc County in 1984 (Campbell and LeValley 1984) and Humboldt County in 2003(Rogers et al. 2004). METHODSThis study is based on records published in North American Birds and its predecessors through fall 2004.For seasons during which records were not individually listed, I obtained the details of specific records from the journal's editors.As the Least Flycatcher is not a species reviewed by the Washington Mlodinow: Least Flycatcher Range Expansion into Washington State

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