Estimating Relative Densities of Breeding Birds by the Line Transect Method

In line transect censuses of breeding birds, all observations are registered. Observations made within 25 m of the transect, about 20 % of total, constitute the records, while records refer to all observations. This study shows how observations outside the main belt can be utilized to estimate densities of different species. Calculations are based on 1609 km of Finnish line transects, censused 1941 through 1974. Altogether 62905 individuals (pairs) were included. The effect of distance on the delectability of birds was approximated starting from three assumptions: delectability decreases according to a linear, a negative exponential, or a normal function of distance. The necessary parameters were estimated from main belt percentages and used to calculate relative density. Different models produce similar results for all species, except a few shortdistance species such as wildfowl. This correction method renders survey belt data suitable for estimating relative densities; a five-fold increase of data being available compared with main belt estimates alone. This is important in estimating densities of rarer species, in monitoring long-term fluctuations, and in analyzing species diversity.