How to interpret the trend graphs:

These graphs show how the abundance of birds has varied over the course of Project FeederWatch. We have used two different measures of abundance, each of which conveys different information about the birds visiting feeders. The red square symbols show the percentage of feeders visited by a bird species.  Larger percentages mean that a species of bird was more widespread, being seen by a greater number of FeederWatchers.

The blue, diamond-shaped symbols indicate the mean group size (or average number seen). This value calculates the average maximum number of each species seen in a feeder area during a FeederWatch count. Zeros are not included in the calculation. This tells us how abundant a species is, on average, when that species is present at a count site. Larger means indicate that birds were more abundant if they occurred at all, but tell nothing about how many FeederWatchers saw this species.

If the percentage of feeders visited is small, the fluctuations in group size are probably biologically meaningless (i.e. a record of 3 South Polar Skuas at a feeder in South Dakota, though exciting, would not tell anything about that species' overall population size).

How the graphs were made:

The percentage of feeders was calculated as the percentage of FeederWatchers that reported a species at least once over the entire winter. Mean group sizes were calculated by calculating the mean group size seen for each FeederWatcher. An average of these individual means was than calculated for each region. For the technically inclined, note that all means were geometric, not arithmetic. No data were plotted for a species in a given year if no individuals were reported in that year.