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Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	Evaluation of aerial sampling methods for detecting waterbird colonies
Auteurs :	M. Clay Green, Auteur ; Paul Leberg, Auteur ; Margaret Luent, Auteur
Type de document :	Article
Dans :	Journal of Field Ornithology (81(4) DEC 2010)
Article en page(s) :	411-419
Langues:	Anglais
Résumé :	Estimating the number of waterbird colonies in a given area can have important conservation implications, including assessment of the regional or global importance of an area and the impacts of conservation efforts (e. g., habitat restoration) and human disturbance (e. g., oil spills). Our objective was to examine differences in estimates of the number of waterbird colonies determined using strip-transect (ST) surveys, distance sampling, and adaptive cluster sampling (ACS), and to compare these estimates to the minimum number of known colonies (MNKC) obtained using point-to-point surveys. We conducted aerial surveys in May 2004 and May and June 2005 at two sites in southern Louisiana: the Atchafalaya Basin (AB), a large forested wetland, and the Barataria-Terrebonne estuary (BTE), a large coastal marsh with isolated clumps of woody vegetation suitable for nesting. In AB, we detected nine and eight colonies using the ACS and ST/distance sampling methods, respectively. Neither ACS estimator of number of colonies (Horvitz-Thompson and Hansen-Hurwitz) was within the 95% confidence interval of the estimate determined from ST; ST estimated-two to three times more colonies than either ACS estimator. The MNKC for the AB was 33, well within the 95% confidence interval of (Y) over cap by ST sampling. For the BTE, ACS estimators ((Y) over cap (HT) = 20.49, CI = 9.3-31.7; (Y) over cap (HH) = 14.15, CI = 2.3-26.0) were similar to the MNKC (20), whereas the ST ((Y) over cap = 87.94, CI = 82.9-92.9) and distance sampling ((Y) over cap = 60, CI = 31-113) methods produced much larger estimates. Our results suggest that the ACS method performed better when waterbird colonies were spatially clumped (BTE) and the ST method performed better in areas where colonies were more uniformly distributed (AB). Depending on management objectives, a complete, systematic survey of a study area may be required if the potential for missing large colonies is unacceptable. If surveying an area with no previous information about colony location or dispersion, we recommend a coarse-scale analysis of the availability and contiguity of habitat likely to contain waterbird colonies; this analysis will help determine the most appropriate survey method.

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