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.