Detection radius is the distance from the observer to the bird from which a bird can be perceived and thus counted. This distance has a strong influence on the calculation of bird density and population estimates because it affects the estimate of the total area sampled around a point count station (i.e. the denominator in density calculation).
The ability to detect a bird is affected by a number of factors that can be categorized by the ‘availability’ of cues to detect a species, and ‘perceptibility’ of the cues by the observer. Availability (e.g. whether a bird sings or not) can be influenced by factors including time of day, year, survey length, weather, mating status, and species. Perceptability (e.g. the likelihood that an observer hears a singing bird) can be influenced by observer ability, weather, habitat, song quality or quantity, and the distance of the bird from the observer. All of these factors affect the overall detectability of a species.
BAM uses “effective detection radius” (EDR) to correct for detectability in calculating estimates of abundance and density. EDR is the distance from the sampling point at which as many birds are detected beyond EDR as remained undetected within EDR. BAM selected this approach because it does not assume that all birds are detected perfectly within a set distance (e.g. maximum detection radius). EDR can be calculated for different habitat types or other strata, to test whether the assumption of equal detectability is met in different areas.