Effective area of acoustic lure surveys for Mexican spotted owls (Strix occidentalis lucida)

During acoustic lure surveys for birds, topography and ambient noise are likely to be important determinants of detectability. Examinations of propagation were conducted for acoustic lures (human‐made calls) and owl responses recorded during acoustic surveys for Mexican spotted owls in the Gila National Forest (2005). Lure surveys were designed based on formal agency protocols, which assumed a 0.43‐km detection range under typical conditions. A total of 558 points was called over a heavily forested, topographically complex 20×24‐km area. Real‐time measurements of owl calls and lures were made with a calibrated recording system. Ambient noise was collected using an array of 39 Larson‐Davis 820 and 824 sound‐level meters. The NMSIM (Wyle Laboratories) single‐event propagation simulator was used to model propagation of both owl and human calls. The resulting model of survey effort was compared with a simple two‐dimensional statistical model. Probability of detecting owls did not fit the expectations of the agency protocol, suggesting that acoustic propagation should be considered during owl surveys. [Work supported by U.S. Air Force ACC/CEVP; USFWS Permit No. TE024429]