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The Rise and Fall of the Varied
Thrush
Tracking the ups and downs of a popular West Coast Songbird Birders are captivated by the Varied Thrush, not only because of its ethereal song, which emanates from the cool, wet forests of the Pacific Northwest, but also for its vagrant habits. This species often wanders out of its normal winter range to show up at feeders throughout eastern North America and the southwestern United States. FeederWatchers find these birds fascinating for another reason: since 1988 Varied Thrushes have shown a remarkable biennial cycle, increasing in numbers one year, then decreasing the next. To confirm this FeederWatch finding, we compared our data to information from two other bird surveys, the Christmas Bird Count (CBC) and the Breeding Bird Survey (BBS). We were pleased to discover that all three surveys showed the same pattern of alternating high- and low-abundance years (Figure 1). The biennial cycle was not as obvious in CBC and BBS data before 1988-89, but the birds did tend to peak in abundance every two to three years. Given their vagrant habits, you might wonder whether Varied Thrushes expand their winter range in years when they are abundant, so that they occur over a larger geographic area than usual. According to FeederWatch data, the answer is no; the birds stay within their normal range. The percentage of FeederWatch participants hosting Varied Thrushes increases, however, from an average of 21 percent in years of low abundance to 37 percent in years of high abundance. We found that in years of high abundance, 31 percent of the FeederWatchers who hosted Varied Thrushes were visited by a single bird; 4 percent hosted groups of 10 or more thrushes. In years of low abundance, on the other hand, only 12 percent hosted a single bird and less than 1 percent hosted groups of 10 or more. In other words, Varied Thrush numbers vary more from site to site in years when the birds are most abundant. Other species also show a regular two- or three-year "boom-or-bust" cycle. Northern birds such as the Common Redpoll and Red-breasted Nuthatch regularly "irrupt," leaving their normal winter ranges in the far north to show up at feeders in the south. Irruptions are linked to changes in the food supply (birch and spruce seeds) in the far north. A good seed supply one year allows the bird population to increase. But a good seed crop one year is usually followed by a small crop the next year—so the now-abundant birds must leave the area to search for food. We suspect that the Varied Thrush population cycle is also related to changes in their food supply. According to the scientific literature, Varied Thrushes like acorns. Many species of oak produce acorns in regular two-year cycles. So the cyclic changes in Varied Thrush abundance may be coupled to changes in acorn production. Although Varied Thrushes irrupt within their normal wintering range, some thrushes do show up outside their normal range each year. For almost 50 years, Audubon Field Notes has published summaries of seasonal bird observations compiled by regional editors throughout North America. Using these data, we tallied the number of Varied Thrushes seen outside their normal range each year and compared them to FeederWatch Varied Thrush counts. The years when FeederWatchers
reported high numbers of thrushes were not the same as the years when many vagrant birds
were sighted. This result suggests that the vagrant birds are coming from a portion of
their range that is not well sampled by any existing bird survey—perhaps interior
Alaska and the Northwest Territories.
Jeffrey Wells
is a research associate and
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