Urban nesting Herring Gull Larus argentatus and Lesser Black-backed Gulls Larus fuscus population estimates: devising species-specific correction models for ground-based survey data (JNCC21_01)

Thumbnail image of publication cover.

Joint Nature Conservation Committee was commissioned by Natural England to produce two reports that present provisional estimates of Herring Gull and Lesser Black-backed Gull populations in England and how these estimates were calculated. The accompanying report on Access to Evidence is linked below. Both reports have been independently reviewed by the BTO and ‘urban gull sub-group’ of the Seabird Census Steering Group.

This report summarises survey methodologies and the analysis and modelling to produce correction factors with which to then use in calculations to produce current breeding population estimates for Herring Gull (HG) and Lesser Black-backed Gull (LG) in England. Survey methodologies for ‘Seabirds Count’ differ greatly from the ‘vantage-point’ methodology used for ‘Seabird 2000’, which was subsequently discovered to be unreliable (Coulson & Coulson, 2015). Ground-based surveys (GBS) and digital aerial surveys (DAS) 2018 – 2020 were performed to find suitable conversion factors to account for detectability issues with GBS. Different combinations of environmental predictors, such as urban fabric type and proportion of urban coverage in survey squares (PROP), were used in ten candidate models. At various stages of analysis, a number of statistical, graphical, modelling and verification tools were employed. This study found the strongest count type relationship was between DAS ‘apparently occupied nests’ (AON) and GBS ‘individual adult gulls present on suitable nesting habitat’ (IND). This relationship improved with inclusion of environmental variables and validation tests on conversion models for HG and LB indicate good fit. However, weaknesses exist in the ability of these models to adequately predict lowest and highest nesting densities for both species, particularly LB, and some small sample sizes of squares with certain physical and gull population characteristics was an exacerbating factor. A number of recommendations for improvement are presented.

Downloads available for this record

File Uploaded
Urban nesting Herring Gull Larus argentatus and Lesser Black-backed Gulls Larus fuscus population estimates: devising species-specific correction models for ground-based survey data (JNCC21_01), PDF, 601.0 KB 2021/02/25

Related Access to Evidence records

Title
Population estimates for urban and natural nesting Herring Gull Larus argentatus and Lesser Black-backed Gull Larus fuscus in England (JNCC21_02)