Why did so many species vacate their higher-elevation winter homes? Like most large-scale winter dispersals, this one was probably related to food. Because these irruptive movements differed from the regular biennial movements we often write about, the cause was probably also an unusual event. Forest fires in Montana and low river levels in Washington state last year suggest that a lack of precipitation was a likely cause of widespread food shortages, prompting some species to travel in search of areas with more abundant food.

We examined the irruptive patterns of five species that Feeder-Watchers had observed frequently. Surprisingly, although all five species came from the same area and were found in higher abundance outside their normal range last winter, Cassin’s Finches dispersed to somewhat different regions than the other four species.

Mountain Chickadees and other irruptives

FeederWatchers recorded abnormally low numbers of Mountain Chickadees through much of the mountainous West from British Columbia through Wyoming. Many parts of the western coastal mountains south through northern California also reported lower numbers than usual. In contrast, Colorado, Arizona, and Texas reported unusually high numbers of Mountain Chickadees. Counts were also higher than normal in Southern California. Steller’s Jays, Red-breasted Nuthatches, and Dark-eyed Juncos showed similar patterns (see map 1, below).

Map 1. Abundance of Mountain Chickadees (left; similar pattern for Steller’s Jays, Red-breasted Nuthatches, and Dark-eyed Juncos) and Cassin’s Finches (right) in winter 2000–2001, compared with previous five winters. Each point represents the observations of a single FeederWatcher at a single site. Maximum numbers of Mountain Chickadees or Cassin’s Finches reported during any observation period during 2000–2001 were compared with the maximum reported by the same observer at any time during the previous five winters. Note: Cassin’s Finches were more abundant at many sites in the Great Basin (southernmost British Columbia through Utah and Nevada).

Cassin’s Finches go their own way

Like the other montane species that dispersed from the mountains, Cassin’s Finches were more abundant than usual in some parts of Arizona last winter. In other regions, however, the trends were the opposite of those shown by Mountain Chickadees and other montane species. For example, Cassin’s Finches were less abundant than usual in Colorado, and the majority of FeederWatchers in the Great Basin reported more Cassin’s Finches than in previous years (see map 1, above). What makes Cassin’s Finches different? We’re not certain, but we know from past irruptions that these finches rarely wander far from their nesting areas in the northern Great Basin mountains. The birds’ appearance at lower elevations in the region may indicate that they were simply moving the shortest distance possible to find food during the winter.

Illustration by John Schmitt

Last winter’s irruption brought five forms of Dark-eyed Junco (four of which are illustrated here) to the backyard feeders of Feeder Watcher Inga Brennan in Golden, Colorado.

Dark-eyed Juncos

Dark-eyed Juncos were especially abundant in the northeastern regions last winter. The junco has five distinct plumage forms (four are illustrated at left). While the “slate-colored” form is the most widespread (in the eastern and northern regions), the others are much more limited to several western regions. Remarkably, as part of the general movement of western montane species, several individuals of the “Oregon” form were observed in locations far outside their range, and a “Pink-sided” Junco was photographed at a feeder in Science Hill, Kentucky. Just as notable, an invasion of “White-winged” Juncos into the Southwest was the “birding event of the season” according to North American Birds journal (http://americanbirding.org/publications/nabgen.htm vol. 55, no. 2).

Please see "Flying Rumors?," "Top-10 Lists, Rare Birds, and Regional Highlights," and our web site http://www.birds.cornell.edu/pfw for additional results from last year’s count season.

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