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Updated 11:00 AM March 8, 2004
 

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  Research
Chickadee populations show long-term decline


If you've noticed a decline in the number of black-capped chickadees visiting your backyard feeder, you're not alone.

"Beginning last winter, many people commented on the sharp decline of chickadees at their feeders," says Julie Craves, research associate at U-M-Dearborn's Environmental Interpretive Center and lead researcher in the campus's Rouge River Bird Observatory.

Many attributed the decline to West Nile virus, but Craves says data show a long-term decline in chickadee numbers on the U-M-Dearborn campus.

"The annual Winter Bird Population Survey (WBPS) is a good indicator of the chickadee population on campus," Craves says. In the WBPS, the campus's Natural Areas are monitored by experienced observers, and all birds are counted on an average of 12 days between Dec. 20 and Feb. 20 each year.

WBPS data show a 70 percent decrease in chickadees counted per average visit since 1992-93; each visit represents a rough estimate of the population on the site. "It appears that numbers have been waning on campus for a decade, long before West Nile virus was reported in North America," Craves says.

Data from U-M-Dearborn correspond closely with national figures compiled by ornithologists at Cornell University, which show a 15-year decline in chickadee numbers.
“It appears that numbers have been waning on campus for a decade, long before West Nile virus was reported in North America.”— Julie Craves

Although the declines have been going on for some years, the trend appears to be accelerating, Craves says. On campus, the number of chickadees counted last winter was at an 11-year low, and was more than 80 percent below the previous 10-year average. This year there was a 43 percent increase in the number of chickadees counted on campus. In the national data, last winter's number of chickadees reported was at a 15-year low, including significant drops in numbers in areas where West Nile virus was not present.

"This fact, coupled with long-term trends, suggests that something besides this virus is driving diminishing chickadee numbers," Craves says.

In addition to chickadees, woodpeckers will be closely monitored in the coming years on the WBPS. The emerald ash borer is killing ash trees across southeast Michigan, Craves says. Woodpeckers already are active on campus digging for the larva of this beetle.

As the trees decay, additional insect prey will become available, which may contribute to increased survivorship over the winter, and higher reproductive rates for woodpeckers and other species. Finally, the dead trees will provide a housing boom for cavity-nesting birds such as woodpeckers and chickadees. "In areas where dead ash trees are left standing, such as the campus Natural Areas, the ash borer may well indirectly aid in the recovery of chickadee populations," Craves says.

Craves says this kind of analysis demonstrates the importance of maintaining long-term records of observations of wild bird populations. "Without long-term data, we would not be able to discern population trends, or have baseline numbers to compare with data gathered after an important ecological event—such as the spread of West Nile virus or ash borer infestation."

"It also illustrates how the data compiled on the U-M-Dearborn campus accurately reflects trends obtained from much larger data sets," she says.

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