Visibility bias in aerial surveys a review of estimation procedures

Aerial surveys, often used to estimate the density of wildlife populations, commonly underestimate population density because of animals being missed. This visibility bias can be serious. Methods of estimating visibility bias based on comparison with ground counts, use of a subpopulation of marked animals, mapping with multiple observers, line transect sampling, and multiple counts on the same area are reviewed. All of the methods are model based and the various model assumptions are discussed and compared. J. WILDL. MANAGE. 51(2):502-510 Aerial surveys are used to estimate the population size of many large birds (Caughley and Grice 1982), large terrestrial mammals (Rice and Harder 1977), and marine mammals (Burnham et al. 1980:76). They are used in conjunction with other methods of estimation or by themselves, either by choice or necessity. "Visibility bias" results from animals being missed and is exacerbated by factors such as dense vegetation, bad weather conditions, and observer fatigue. Beyond the logistics and dangers of flying surveys precisely, the principal difficulty in aerial censuses lies in estimating the visibility bias to establish a correction factor. Caughley (1977: 35) presents a table showing the visibility bias in a wide range of aerial surveys. Biologists tempted to minimize this problem should study this table where the visibility bias is often in the range of 30-50% animals missed! If absolute density estimates are to be reported there will always be the need for a correction factor. Here we review the various methods of estimating visibility bias from an applied statistician's perspective. Neither of us claim to be experts in the practicalities of running aerial

[1]  K. Pollock,et al.  Correction of Visibility Bias in Aerial Surveys Where Animals Occur in Groups , 1981 .

[2]  W. Rice,et al.  APPLICATION OF MULTIPLE AERIAL SAMPLING TO A MARK-RECAPTURE CENSUS OF WHITE-TAILED DEER , 1977 .

[3]  J. Packard,et al.  Variation of visibility bias during aerial surveys of manatees , 1985 .

[4]  David R. Anderson,et al.  Estimation of Density from Line Transect Sampling of Biological Populations. , 1982 .

[5]  Graeme Caughley,et al.  Experiments in Aerial Survey , 1976 .

[6]  G. Caughley,et al.  IMPROVING THE ESTIMATES FROM INACCURATE CENSUSES , 1972 .

[7]  L. L. Eberhardt,et al.  Transect Methods for Population Studies , 1978 .

[8]  G. Seber,et al.  The Estimation of Animal Abundance , 1975 .

[9]  C. Henny,et al.  The 1973 distribution and abundance of breeding ospreys in the Chesapeake Bay , 1974 .

[10]  Daniel W. Anderson,et al.  Osprey distribution, abundance, and status in western North America: III. The Baja California and Gulf of California population , 1979 .

[11]  G. Caughley,et al.  A Correction Factor for Counting Emus from the Air, and its Application to Counts in Western Australia , 1982 .

[12]  G. Caughley,et al.  Analysis of vertebrate populations , 1977 .

[13]  David R. Anderson,et al.  Wildlife Monographs: Estimation of Density from Line Transect Sampling of Biological Populations , 1981 .

[14]  R. D. Cook,et al.  A design for estimating visibility bias in aerial surveys , 1979 .

[15]  R. Routledge.  The Unreliability of Population Estimates from Repeated, Incomplete Aerial Surveys , 1981 .

[16]  R. Dennis Cook,et al.  A Model for Quadrat Sampling with “Visibility Bias” , 1974 .

[17]  Gordon C. Grigg,et al.  A Double-Survey Estimate Of Population Size From Incomplete Counts , 1978 .

[18]  G. Seber.  The estimation of animal abundance and related parameters , 1974 .

[19]  D. G. Chapman.  Some properties of the hypergeometric distribution with applications to zoölogical somple censuses , 1951 .

[20]  R. Routledge,et al.  The Method of Bounded Counts: When Does It Work? , 1982 .

[21]  Jon M. Gerrard,et al.  Aerial-Visibility Bias and Survey Techniques for Nesting Bald Eagles in Northwestern Ontario , 1981 .

[22]  David R. Anderson,et al.  The need for distance data in transect counts , 1984 .