Accuracy of ’total counts’ of waterbirds from aircraft in coastal waters
Estimating ‘total counts’ of waterbirds from aircraft is a widely used survey method, and we assessed the effectiveness of this method for geese, ducks, waders and gulls by comparing the count results from aircraft with ground counts in the Danish Wadden Sea during 1984-1998. In total, 47 counts were carried out in 12 counting sites and the results were compared for 18 waterbird species, which varied in abundance, flock size and degree of aggregation. Significantly greater numbers of waterbird species were identified from the ground than from aircraft (mean number: 16.1 species versus 10.6 species). Depending on the accuracy in aerial counts compared to ground counts the species were divided into three categories. A) Brent goose, shelduck, mallard, eider and oystercatcher had a high level of correspondence between densities obtained from ground and aerial counts (detection rate > 80%, no statistical difference between slopes and intercepts of the observed regression lines and the ideal lines (x = y), differences between mean densities of ground and aerial counts < 15%). B) Wigeon, teal, grey plover, dunlin, bar-tailed godwit, black-headed gull, common gull, herring gull, great black-backed gull and common/arctic tern had a medium correspondence between densities obtained from the two platforms (detection rate > 50% with differences between the mean densities of ground and aerial counts < 30%. C) Redshank, greenshank and curlew had a low correspondence between the densities obtained from the two platforms (detection rate < 50% and differences between the mean densities of ground and aerial counts > 30%). Species with a high and medium level of correspondence between the two platforms are mostly species that are numerous, of widespread occurrence and found in large flocks. Species with a low correspondence are minority species, occurring scattered or in small flocks. It is recommended to supplement aerial counts with ground counts in sites with mixed flocks of more dabbling duck species present to increase the accuracy of the count results.
Key words: aerial survey, assessment, detection rate, gulls, species identification, waders, waterfowl