The maximum detection distance is the greatest distance between a bird and an observer from which the observer can perceive the bird. 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 such as 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.
Partners in Flight uses maximum detection distance (MDD) to correct for errors in bird detectability, so detection distance has a particularly strong effect on PIF population estimates for songbirds in North America . In this PIF method, each species was placed into one of five detection distance categories (80, 125, 200, 400 or 800 m), based on reports in the literature (e.g. Rosenberg and Blancher 2005 ) and a consideration of habitat, song loudness, and bird behaviour (amount of time spent in flight, and secretiveness). The distance classes used tended to be larger than those derived empirically, because PIF also adjusted density for pairs and time of day, and because movement of birds during counts means that a larger area has been sampled than is indicated by the distance to the detected bird.