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“Percentage of sites visited” is calculated as the number of FeederWatch sites that reported a species at least once over the winter divided by the total number of FeederWatch sites in the area of interest (see example, below). This value can be from 0 to 100% and is often calculated for specific states or FeederWatch regions.
Example: In this example, only 8 participants are signed up for Project FeederWatch, and they have submitted data about Species A from the following 8 locations.
Percentage of sites visited by Species A = 5 site reports/8 total sites = 63%
The FeederWatch “average group size” is calculated by computing the geometric mean of all the FeederWatch counts during the season for a species. This method of calculation takes the flock sizes reported by FeederWatch participants, without including any “zero” counts (counts from FeederWatchers who did not report the species at their site) and uses a formula to compute the mean that de-emphasizes rare high counts to provide a mean that more closely matches what a typical FeederWatcher is likely to see.
The average group size can vary quite a bit depending on the species; those that tend to roam in large flocks (e.g., Pine Siskin) have average group sizes from 1 to over 100, whereas species that tend to feed alone or in pairs in winter (e.g., Downy Woodpecker) are usually seen in much lower numbers.
The FeederWatch Abundance Index is a combination of the “percent of sites visited” and the “average group size,” but it also takes into account how frequently the bird was reported at each site within a count season. The FeederWatch Abundance Index tells you the average number of birds that you would see if you were to watch a series of randomly chosen feeder areas for single, randomly chosen count periods during the winter.
Each of these three summary measures is important. Some bird species (such as the irruptive species) vary more in the percentage of sites 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 observation and numbers at feeder areas, making the FeederWatch Abundance Index a useful overall measure when starting to track population changes from year to year.
The data collected by FeederWatchers provide an index of abundance for birds that visit feeders in winter. This index often moves in the same direction and magnitude as the true population size, and therefore can be used as an estimate. However, environmental factors can influence bird populations in complex ways, and scientists need to keep the big picture in mind. For example, a decline in a species abundance index sometimes means that birds are away from feeders but just feasting on natural foods in the wild. In short, scientists must blend data from a variety of sources (and mix it with some good, old-fashioned common sense) to determine how a population is actually changing.