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The accuracy of urban nesting gull censuses
Coulson, J.C.; Coulson, B.A. (2015). The accuracy of urban nesting gull censuses. Bird Study 62(2): 170-176.
In: Bird Study. British Trust for Ornithology: Oxford. ISSN 0006-3657, more
Peer reviewed article  

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  • Coulson, J.C.
  • Coulson, B.A.

    Capsule The vantage point survey methods used in the 1999–2002 surveys of urban nesting large gulls in Britain and Ireland appreciably underestimated numbers, resulting in lower national totals and exaggerated national population changes, because they did not account for a substantial proportion of undetected nests.Aims To evaluate the efficiency of various survey methods for urban nesting large gulls.Methods Vantage point and street surveys of Herring and Lesser Black-backed Gulls Larus argentatus and Larus fuscus nests were made in six urban conurbations and used to compare detection efficiencies. Some nests missed by both methods were later identified by the presence of unfledged chicks. In later years, the numbers of nests which were actually missed was determined by using a ‘cherry-picker’ which allowed all nests to be located in Dumfries. The proportion of nests on industrial and commercial buildings was recorded.Results Vantage point and street surveys missed an appreciable number of nests and had average maximum nest detection rates of 78% and 48%, respectively. Combining the two methods raised the efficiency to a maximum of 88%. The detection rate varied inversely with the proportion of nest sites that were on commercial or industrial sites. The complete census of nests in Dumfries in 2013–14 showed that vantage point surveys detected only 75% of nests, while the combination of the two survey methods increased the detection rate to 84%.Conclusion Vantage point surveys markedly underestimate numbers of nesting large gulls. Surveys were less effective on conurbations where nesting gulls used a high proportion of industrial and commercial properties. Therefore the 1999–2002 national surveys in Britain and Ireland underestimated the numbers of urban nesting Herring and Lesser Black-backed Gulls and, as a result, may have exaggerated the national trends in abundance of both species. More intensive methods are required in surveys of urban nesting gulls and correction factors need to be established from very high vantage points by use of cherry-pickers or aerial surveillance.

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