FeederWatch Abundance Index 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.