Data obtained from acoustic recording units and from field observer point counts of Tasmanian forest birds are similar but not the same

Acoustic recording units (ARUs) have recently been used in place of field observers to survey birds. To test whether data obtained from ARUs and from point counts by a field observer are directly comparable, we surveyed Tasmanian forest birds using these two methods at exactly the same times and places. Data derived from ARUs were similar to those obtained by the field observer without distance constraints for (a) the total numbers of species detected at each of three points, and (b) both the number and composition of species within each of the 13 5-minute surveys at each point. However, data sets obtained by each method comprised more records of certain species, largely because of calls being detected by one method but not by the other. Additional differences between methods arose because ARUs could not detect birds seen by the field observer that were not calling, and ARUs sometimes led to false positive identifications when the person listening to recordings was unable to distinguish mimetic calls from those of the species being mimicked. Because ARUs appeared to detect many calls from beyond 100 m, their use to survey a particular patch of vegetation requires placement (within the vegetation patch) at distances greater than 100 m from the edge of the patch to prevent the inclusion of calls from non-target vegetation.