These figures are totals of the maximum counts recorded from each square in a given year. The maximum count is calculated for each species separately and is based on all birds seen and heard, including those in flight. These figures provide a useful indication of the relative abundance of different species although they do not take account of differences in detectability between species. They do not provide measure of absolute abundance because BBS surveys do not detect all of the birds present in a particular square. For comparative purposes the data can be used to calculate the mean count per square by dividing the species totals in the table by the total number of squares surveyed. This measure has the advantage that it can be used to compare results based on different numbers of squares. Particular care should be taken in interpreting such results where only a few squares could be surveyed (less than 30-50 squares). While these figures may provide a general indication of major changes in abundance over time they do not provide a statistically robust measure of such changes because they do not take account of the BBS sampling strategy or of temporal variation in the sample of squares surveyed. BBS trend analyses are available here