Left Axis: Ratio of Varied Thrushs counted to number of
participants counting for that year. Right Axis: Percent of
total census circles reporting a Varied Thrush that year. The
Christmas Bird Count is the longest running bird census in the
world. From 1900 until the present, the cbc has been performed
and steadily expanded throughout North America and beyond. With
more than 2000 census circles in existance today, the data is
much more extensive in later counts than earlier counts.
Additionally, the data collected has been expanded to reduce
some of the intrinsic variability in the technique. By dividing
the number of birds counted by the number of participants doing
the counting a certain amount of varibility is reduced and a
more accurate analysis is possible. However, other biases exist
in the data and a good reference is available online from the
USGS Patuxent Wildlife Research Center.