| Looking At Data |
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Abundance of Feeder Birds |
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Abundance
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Abundance of feeder birds FeederWatch abundance data indicate how many individuals of each species are seen. This information can be used to measure changes in species abundance over time and to track changes in the winter ranges of species. How are FeederWatch abundance data calculated? Data are summarized in two ways: percentage of feeders visited and mean group size. Percentage of
feeders visited is calculated as the number of FeederWatch sites that
reported a species at least once during the winter divided by the total number of
FeederWatch sites that sent in data. This value is often calculated for specific states or
FeederWatch regions and varies from 0 to 100%. 150/200 = 0.75 x 100 = 75% Click
here for more extensive sample calculations using
percentage of feeders visited. Mean group size is an average of all FeederWatch counts for a species over a specified geographic area. A "count" in this case is the highest number of birds seen at one time at a feeder. In these calculations, the average does not include zero counts. The mean group size tells you the average number of a species seen at those places where a species is present at feeders. This value can vary quite a bit depending on the species; those species that tend to roam in large flocks (e.g., Common Redpoll) have mean group sizes as high as 50 or more. Species that tend to feed alone or in pairs in winter (e.g., Downy Woodpecker) are usually seen in much lower numbers. For example: You can now calculate the mean group size: (NOTE: although you counted for 15 weeks, you saw juncos in only 10 weeks so divide by 10.) Click here for more extensive sample calculations using mean group size.
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