For many types of birds, survey detectability is an issue because any comparison of the raw ´unadjusted´ counts between sites and through time must assume that the probability of detecting birds is the same. However, some birds present in a study area will always go undetected, regardless of the survey method, how well the survey is carried out, and the observers’ competence.
A comparison of unadjusted counts will only be valid if the numbers represent a constant proportion of the actual population present across space and time. Detectability is an important concept in wildlife surveys and has been a matter of much debate (Buckland et al., 2001; Rosenstock et al., 2002; Thompson, 2002) and recent statistical developments.
A solution is to ´adjust´ counts to take account of detectability, and some different methods have been proposed (Thompson, 2002). The ´double observer approach uses counts from primary and secondary observers, who alternate roles, model detection probabilities and adjust the counts (Nichols et al., 2000). The ´double sampling approach uses the findings from an intensive census at a sub-sample of sites to correct the unadjusted counts from a larger sample of sites (Bart & Earnst, 2002). The ´removal model´ assesses the detection probabilities of different species during the period of a point count and adjusts the counts accordingly (Farnsworth et al., 2002). ´Distance sampling´ models the decline in species’ detectability with increasing distance from an observer and corrects the counts appropriately (Buckland et al., 2001). The ´binomial mixture´ model uses counts from repeated visits within a period of closed population sizes (Royle & Dorazio, 2008).
Distance sampling is a way of estimating bird densities from the line or point count transect data and assessing the degree to which our ability to detect birds differs in different habitats and at different times (Buckland et al., 2001; Rosenstock et al., 2002). The software to undertake these analyses is freely available on the Distance project website. This method is often recommended because distance sampling in the field, e.g. recording a distance to each bird, or more often recording birds in distance bands (e.g. 0-25 m, 20-50 m, 100 m and over for line transects, 0-30 m and 30 m and over for point transects) is often practical when alternatives are not. While we flag the issue of bird detectability, most breeding bird surveys do not routinely adjust counts when assessing trends. Distance sampling and other methods are useful to provide improved estimates of population sizes, but so far, there is little evidence that detection probability adds significant bias to bird trends (Johnson, 2008).