Around winter 2003-2004, many people commented on the sharp decline of Black-capped Chickadees at their feeders. Some automatically attributed these declines to West Nile virus. However, RRBO banding and survey data reveals a long-term decline in chickadee numbers on the UM-D campus.
There was a drop in the number of chickadees banded in both spring and fall seasons from 1992 to 2003, shown in figures 1 and 2, above. The overall decline was 81.7% in spring and 78.5% in fall. Because RRBO does not large numbers of chickadees, this translates to a decline of fewer than three birds per year, and there are swings in the numbers that make the trend less accurate.
The annual Winter Bird Population Survey (WBPS), however, is perhaps a better indicator of chickadee populations on campus, as it surveys birds throughout the entire campus Natural Area multiple times over the entire winter. In the WBPS, the campus Natural Area is surveyed and all birds are counted on an average of 12 days between 20 Dec and 20 Feb each year.
There was a 60.3% decrease in chickadees in the winter between 1992 to 2003 on the WBPS (Figure 3). This also represents only a few less chickadees counted per year, but agrees with our banding findings. It appears that chickadee numbers were waning at UM-D for a decade, long before West Nile virus was reported in North America.
How does our data compare with that of other researchers? Cornell Lab of Ornithology’s Project FeederWatch (PFW) tracks bird numbers at feeders continent wide each winter from November through early April. Their results were similar to RRBO’s, showing a 15-year decline.
Although the declines have ongoing, the dip in 2002 chickadee numbers was quite noticeable. On the WBPS that year, chickadees per hour were at their eleven-year low, and were 80.5% below the previous ten-year average. According to PFW, chickadees were at a 15-year low over the same period, with chickadees in the Midwest 32% below the previous 14-year average. PFW data, however, found drops in numbers in areas where West Nile was not present. This fact, coupled with the long-term trends, suggests that something besides this virus is driving diminishing chickadee numbers.
RRBO and PFW show a peak in chickadees in the mid-1990s. These peaks and valleys may be normal population fluctuations. Due to crowding and competition, chickadee mortality is high in winters following big population increases, a phenomena known as ‘density-dependent survival.’ The eastern United States had a huge increase in chickadee numbers in 2001, which probably led to the lower numbers in the last couple of years.
This analysis demonstrates the importance of having long-term monitoring methods in place, and also illustrates how RRBO data accurately reflects trends obtained from much larger data sets such as PFW. Continued monitoring will provide valuable clues to unravel the causes of chickadee declines.