The original Royle-Nichols model estimates abundance from a history of detection and non-detection of a species during repeated visits to a sampling site. In this case, we consider each pentad surveyed during the SABAP2 programme to be a site. The basic model builds on the fact that detection of a species at a site depends on the abundance of that species. More specifically if \(r\) is the probability of observing any one individual of the species of interest in site \(i\), then the probability \(p\) of observing at least one individual is,