AUTUMN 2000/VOLUME 14, NUMBER 4
Project FeederWatch data document changes in birds' feeder usage. Here's an explanation of some of the summary statistics calculated from the data.
Percentage of feeders visited tells what percentage of FeederWatchers reported seeing a species at least once during the winter, thus describing how widely a species is distributed.
Mean group size indicates the average number of individuals of a species seen at one time at a feeder. This tells us how abundant a species is when it is present at a feeder.
The FeederWatch Abundance Index is a combination of the first two summary measures but also takes into account how frequently the bird was reported at each feeder within a count season. The FeederWatch Abundance Index tells you the average number of birds that you might see if you were to watch a series of randomly chosen feeders for single, randomly chosen two-day periods during the winter.
Each of these three summary measures is important. Some bird species (such as irruptives) vary more in the percentage of feeders visited from year to year. Other species vary in group size at feeders but are typically seen by the same percentage of FeederWatchers. Variation in the FeederWatch Abundance Index captures changes in both frequency of sightings and numbers at feeders, making the FeederWatch Abundance Index a useful overall measure when tracking population changes from year to year.