Purple Finch / USFWS Photo
Ron Pittaway, a respected Canadian ornithologist, recently released his winter finch forecast for the upcoming winter. It has already bounced around a few birding listserves; that link points to the version on eBird. The forecast will eventually be posted on the Ontario Field Ornithologists website, where you can see an archive of forecasts from past years. Pittaway's forecasts are written primarily for birders in Ontario, but they are useful for birders in the northeastern and mid-Atlantic U.S. as well.
Most migratory songbird species, such as wood warblers and thrushes, migrate south each winter because their primary food sources (invertebrates) are not available during northern winters. Other species, such as cardinals and chickadees, tend to stay where they are since their food sources are usually not disrupted. A small group of boreal species, mostly finches, migrates south in some years but not in others. These migrations, known as irruptions, are triggered by year-to-year changes in the seed crops of conifers, birches, aspens, and mountain-ashes in the boreal forest.
Which bird species will irrupt in a given year can be predicted on the basis of which seed crops are abundant and which are scarce. Here are a few species that may migrate south this winter:
Purple Finch: This finch winters in the north when the majority of deciduous and coniferous seed crops are abundant, which is not the case this year. Most Purple Finches will migrate south of Ontario this fall. A few may frequent feeders in southern Ontario. Purple Finch numbers have declined significantly in recent decades due in part to a decrease of spruce budworm outbreaks since the 1980s (Leckie and Cadman in Atlas of Breeding Birds of Ontario 2007).Most other species are unlikely to come south because they have abundant food sources somewhere in the boreal forest. See the full forecast for comments on the rest of the irruptive species. The species comments are informative, even for birds that will not come south. Past forecasts usually included some notes on boreal raptors as well, but those seem to be left out this year.
Common Redpoll: Redpolls should irrupt into southern Canada and the northern United States this winter. The Common Redpoll's breeding range in Ontario is mainly in the Hudson Bay Lowlands from the Manitoba border southeast to southern James Bay (Leckie and Pittaway in Atlas of Breeding Birds of Ontario 2007). Redpolls in winter are a birch seed specialist and movements are linked in part to the size of the birch crop. The white birch crop is poor across much of northern Canada. Another indicator of an upcoming irruption was a good redpoll breeding season in 2010 with double and possibly triple broods reported in Quebec. High breeding success also was reported in Yukon. Samuel Denault of McGill University has shown that redpoll movements at Tadoussac, Quebec, are more related to reproductive success than to tree seed crops in the boreal forest. Redpolls will be attracted to the good birch seed crops on native white birch and European white birch in southern Ontario and to weedy fields. They should be frequent this winter at feeders offering nyger and black oil sunflower seeds. Watch for the larger, darker and browner "Greater" Common Redpolls (rostrata subspecies) in the flocks. It is reliably identified by its larger size and proportionally longer thicker bill and longer tail in direct comparison with "Southern" Common Redpolls (nominate flammea subspecies).
Hoary Redpoll: The breeding population in northern Ontario is the most southerly in the world (Leckie and Pittaway in Atlas of Breeding Birds of Ontario 2007). Careful checking of redpoll flocks should produce a few Hoary Redpolls. There are two subspecies. Most Hoaries seen in southern Canada and northern United States are "Southern" Hoary Redpolls (exilipes subspecies). During the last large redpoll irruption in 2007/2008, several "Hornemann's" Hoary Redpolls (nominate hornemanni subspecies) were found and supported by photographs. Hornemann's Redpoll was previously regarded as a great rarity south of the Arctic, but it may be more frequent than formerly believed. Hornemann's is most reliably identified by its much larger size in direct comparison with flammea Common Redpolls or exilipes Hoary Redpoll. Note that white birds loom larger than life among darker birds and size illusions are possible.
Red-breasted Nuthatch: This nuthatch is a conifer seed specialist when it winters in the north, thus its movements are triggered by the same crops as the boreal winter finches. The southward movement, which began in the summer, signaled the generally poor cone crops on spruces, balsam fir and white pine in the mixed coniferous/deciduous forest region across Ontario and in Atlantic Canada, New York and New England States. Red-breasted Nuthatches will be very scarce this winter in central Ontario such as Algonquin Park. White spruce crops are excellent in the northern half of the boreal forest, but it is uncertain how many Red-breasted Nuthatches will winter that far north.