The software and further information to undertake these analyses are freely available at: www.ruwpa.st-and.ac.uk/distance

Distance sampling takes account of the fact that the number of birds seen or heard declines with distance from the observer. The shape of this decline, the distance function, differs among species, among observers and, importantly, among habitats. Distance sampling models the ‘distance function’ and estimates density taking into account both the birds that were observed, plus those that were present but not detected.