393 The Auk 117(2):393–408, 2000 A DOUBLE-OBSERVER APPROACH FOR ESTIMATING DETECTION PROBABILITY AND ABUNDANCE FROM POINT COUNTS JAMES D. NICHOLS, 1,4 JAMES E. HINES, 1 JOHN R. SAUER, 1 FREDERICK W. FALLON, 2 JANE E. FALLON, 1 AND PATRICIA J. HEGLUND 3 1 U.S. Geological Survey, Biological Resources Division, Patuxent Wildlife Research Center, Laurel, Maryland 20708 USA; 2 AScI Corporation, 1365 Beverly Road, McLean, Virginia 22101 USA; and 3 Department of Biological Sciences, University of Idaho, Moscow, Idaho 83844 USA ABSTRACT.—Although point counts are frequently used in ornithological studies, basic as- sumptions about detection probabilities often are untested. We apply a double-observer ap- proach developed to estimate detection probabilities for aerial surveys (Cook and Jacobson 1979) to avian point counts. At each point count, a designated ‘‘primary’’ observer indicates to another (‘‘secondary’’) observer all birds detected. The secondary observer records all detections of the primary observer as well as any birds not detected by the primary observer. Observers alternate primary and secondary roles during the course of the survey. The ap- proach permits estimation of observer-specific detection probabilities and bird abundance. We developed a set of models that incorporate different assumptions about sources of var- iation (e.g. observer, bird species) in detection probability. Seventeen field trials were con- ducted, and models were fit to the resulting data using program SURVIV. Single-observer point counts generally miss varying proportions of the birds actually present, and observer and bird species were found to be relevant sources of variation in detection probabilities. Overall detection probabilities (probability of being detected by at least one of the two ob- servers) estimated using the double-observer approach were very high (0.95), yielding pre- cise estimates of avian abundance. We consider problems with the approach and recommend possible solutions, including restriction of the approach to fixed-radius counts to reduce the effect of variation in the effective radius of detection among various observers and to provide a basis for using spatial sampling to estimate bird abundance on large areas of interest. We believe that most questions meriting the effort required to carry out point counts also merit serious attempts to estimate detection probabilities associated with the counts. The double- observer approach is a method that can be used for this purpose. Received 16 November 1998, accepted 1 October 1999. A BEWILDERING VARIETY of methods exists for assessing animal abundance (e.g. Ralph and Scott 1981, Seber 1982, Lancia et al. 1994). How- ever, all methods involve the collection of some sort of count statistic. Count statistics are as varied as the methods by which they are ob- tained and include number of birds seen and heard at a point-count location, number of un- gulates seen while walking a line transect, number of small mammals caught on a trap- ping grid, number of kangaroos seen from an airplane flying an aerial transect, and number of tigers photographed by camera traps. The relationship between a count statistic and the quantity of interest, abundance or population size, can be written as follows (see Barker and Sauer 1992, Nichols 1992, Lancia et al. 1994): 4 E-mail: jimnichols@usgs.gov E (C i ) = N i p i , (1) where C i denotes the count, N i the true abun- dance, and p i the detection probability, all as- sociated with time and location i. Two basic approaches use count statistics to draw inferences about animal abundance and changes in abundance over time (Lancia et al. 1994, Wilson et al. 1996). One approach is to collect the count data in a manner that permits estimation of the associated detection proba- bility. Such estimates then permit direct esti- mation of population size: C i ˆ N = , (2) i p ˆ i where the hats denote estimates. Resulting es- timates of population size can be used to draw inferences about changes in abundance over time and/or space. If the estimates of detection Downloaded from https://academic.oup.com/auk/article/117/2/393/5561729 by guest on 05 March 2022