Illustration by John Schmitt

Counts at feeders this winter showed that numbers of Black-capped Chickadees (above) and Carolina Chickadees were down compared with the previous winter. In some regions, West Nile virus is a suspect, but apparent declines have also occurred elsewhere, in the absence of West Nile virus outbreaks.

As West Nile virus (WNV) spreads across North America, so does concern over the impact of the virus on wild bird populations. Limited testing has shown that thousands of birds have been infected by the virus, and many have died as a result of these infections. But has WNV-related mortality had a significant impact on bird populations? Anecdotal reports suggest that noticeably fewer birds are present in some backyards and neighborhoods, particularly in the Midwest, after the WNV outbreaks in 2002. We analyzed bird counts from Project FeederWatch to investigate whether impressions like these were corroborated by this year's data.

Our preliminary analyses confirm notable drops in counts of several bird species in the upper Midwest compared with the previous winter, as well as a continentwide decrease in sightings of Black-capped and Carolina chickadees. In contrast, counts of several other species have held steady or increased since last winter.

A look at this winter's data
In 2002, West Nile virus, a mosquito-borne pathogen, was more prevalent than ever in the United States and Canada. Historically, WNV has been widespread in Africa, the Middle East, and southern Eurasia. It first gained a foothold in the Western He misphere in 1999, when it caused an outbreak among birds and humans in and around New York City. By the end of 2002, WNV had been detected in 44 states and 5 Canadian provinces. A survey of American Crows and Black-capped Chickadees by Audubon–Chicago Region and the Bird Conservation Network found striking declines of both species in the Chicago region following an outbreak of WNV there in 2002 (see sidebar).

To see whether the declines in feeder birds in Chicago were part of a wider decline in the upper Midwest and other regions during the past year, we compared FeederWatch counts from this winter with data submitted by the same FeederWatchers the previous winter. These early results from November through mid-February were made possible by online data submissions.

Pinpointing declines
FeederWatchers have witnessed notable declines in the numbers of Black-capped and Carolina chickadees visiting their feeders this year. Chickadees have declined at 74 percent of 302 FeederWatch sites with greater than expected changes in counts (Figure 1). A high proportion of sites with declines was notable in the upper Midwest (Ohio, Illinois, Indiana, Michigan, and Wisconsin). The average number of chickadees counted was down by 26 percent in Illinois, 19 percent in the upper Midwest, and 10 percent elsewhere.

Similarly, American Crow counts have declined at 70 percent of 173 FeederWatch sites where changes were identified between winters. Declines were recorded in most regions, although there were some increases concentrated along the coast of New England (Figure 2). Crow counts decreased by 61 percent in the upper Midwest compared with last winter, and Illinois reported a staggering 91 percent decrease. Across the rest of the continent, crow counts were up by 6 percent.

Declines were also detected in the number of Tufted Titmice visiting feeders (63 percent of 234 sites with changes). The average number of titmice counted was down by 29 percent in the upper Midwest, but increased by 4 percent across the rest of their range.

Not all bad news
Reassuringly, the results show that, as a whole, FeederWatchers are continuing to see Blue Jays, House Sparrows, and Northern Cardinals in numbers comparable with those of the previous winter. Although the number of sites with declines in Blue Jay counts far exceeded those with increases in parts of the Midwest, counts of jays remained stable or increased across much of the species' range (Figure 3). Overall, reports of Blue Jays increased at 52 percent of 301 sites where changes were detected this winter. The average number of jays counted declined by 26 percent in Illinois and 6 percent in the upper Midwest, but increased by 15 percent across the remainder of their range. More FeederWatchers reported increases than declines in the number of House Sparrows at their feeders this winter (56 percent of 253 sites with changes). Similarly, 64 percent of 327 FeederWatchers reported increases in the number of Northern Cardinals.

Is WNV the culprit?
Is West Nile virus to blame for the observed declines in chickadee, crow, and titmouse populations? Without direct evidence, the jury is still out. We do know that the virus has infected at least 135 species of birds, ranging in size from Ruby-throated Hummingbirds to Bald Eagles. However, responses to infection vary greatly. American Crows are particularly susceptible—laboratory tests show that they suffer 100 percent mortality when exposed to WNV. In New York state, 44 percent of dead crows tested over the past four years had been exposed to WNV. Relatives of crows, such as ravens and jays, also test positive for WNV more frequently than many other species, according to data from the New York State Department of Environmental Conservation's Wildlife Pathology Unit.

WNV has not yet been directly linked to high mortality rates of small birds such as chickadees and titmice. These birds are more difficult to find after they die, and even when found, most are not tested for WNV. Circumstantial evidence suggests that WNV could be linked to the chickadee declines in the Chicago area (see sidebar, above). But the rangewide declines are more puzzling. Lower counts of chickadees this winter have been reported from regions such as the Pacific Northwest, where there were no WNV outbreaks (see Figure 1).

Numerous factors in addition to WNV may contribute to declines in the number of birds seen at feeders this winter. Perhaps the birds experienced a poor reproductive season last summer, leading to lower counts at feeders this winter. Maybe natural food supplies were low, forcing birds to move elsewhere. Or perhaps natural food resources were plentiful, and the birds were not enticed by the bountiful supply provided at feeders. We are uncertain what has caused the apparent declines in the Midwest, although some link with WNV is possible.

A FeederWatch Perspective
The data from Project FeederWatch have provided an important preliminary glimpse at striking decreases in counts of some birds at Midwestern feeders since last winter. At the same time, the broad view gained from examining continentwide patterns demonstrates that a common, widespread species such as the American Crow can undergo dramatic local or regional declines but remain relatively stable across its range.

However, the declines we detected also raise concerns about the vulnerability of species with more restricted ranges and numbers. Recently, birds infected with WNV were found in the Dominican Republic, the first cases confirmed in the Caribbean. An outbreak there could have much greater consequences for the already-diminishing White-necked Crow and Hispaniolan Palm Crow than for the more widespread and abundant American Crow.

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