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DATE: Feb. 24, 2004

Chickadee populations show long-term decline, according to UM-Dearborn ornithologist

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

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

Many birders attributed the decline to West Nile virus, but Craves said banding and survey data show a long-term decline in chickadee numbers on the UM-Dearborn campus.

In fact, the number of chickadees banded on campus has declined approximately 80 percent since 1992, but that number might not tell the whole story because of the relatively small numbers of chickadees banded each year.

"The annual Winter Bird Population Survey (WBPS) is perhaps a better indicator of the chickadee population on campus," Craves said. 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.

The data collected in that survey show a 60 percent decrease in the chickadee numbers since 1992-1993. "While this also represents only a few less chickadees counted per year, it appears that the number has been waning on campus for a decade, long before West Nile virus was reported in North America," Craves said.
Data from UM-Dearborn correspond very closely with national figures compiled by ornithologists at Cornell University, which show a 15-year decline in chickadee numbers.

Although the declines have been going on for some years, the trend appears to be accelerating, Craves said. On campus, the number of chickadees counted per hour was at an 11-year low, and was 80 percent below the previous 10-year average. 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 said.

Both the UM-Dearborn and national surveys show a peak in chickadee populations in the mid-1990s. "These peaks and valleys may be normal population fluctuations," Craves said. "Due to crowding and competition, chickadee mortality is high in winters following big population increases, a phenomenon known as 'density-dependent survival.' The eastern United States had a huge increase in chickadee numbers in 2001, which probably led to lower numbers in the last couple of years."

Craves said that this kind of analysis demonstrates the importance of maintaining long-term records of observations of wild bird populations. "It also illustrates how the data compiled on the UM-Dearborn campus accurately reflects trends obtained from much larger data sets," she said. "Continued monitoring will provide valuable clues to help us unravel the causes of chickadee declines."

[Click on graph to enlarge]





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