Decline in the numbers of Eurasian Oystercatchers Haematopus ostralegus on the Exe estuary Special Protection Area

Author(s): Goss-Custard, J.D., Austin, G.E., Frost, T.M., Sitters, H.P. & Stillman, R.A.

Published: November 2024

Journal: Ardea Volume: 112

Digital Identifier No. (DOI): 10.5253/arde.2023.a23

The Exe Estuary in Devon is a nationally important site for Oystercatchers wintering in the UK. However, the proportion of this species found in south-west England and wintering on the Exe declined from 60% in the late 1980s to 35% by the late 2010s. This study uses 45 years of data collected by volunteers taking part in the Wetland Bird Survey (WeBS) to investigate why.

The study found that kleptoparasitism by Carrion Crows and Herring Gulls could help to explain the trends observed on the Exe Estuary. Here, Carrion Crows and Herring Gulls steal the Oystercatchers’ food, specifically Mussels.

Juvenile Oystercatchers were more affected by this kleptoparasitism than adults, increasing their likelihood of starvation. Juvenile Oystercatchers were also found to be spending time on sites away from the Exe in order to meet their food requirements.

The Oystercatcher is a declining species, classified as ‘Near Threatened’ by the International Union for Conservation of Nature. This study demonstrates the value of long-term monitoring schemes like WeBS in understanding trends of species in specific areas. Comparisons of regional and national data allow for the formulation of targeted conservation measures to prevent future population declines.

Abstract

According to the monthly counts of the Wetland Bird Survey (WeBS), the numbers of Eurasian Oystercatchers Haematopus ostralegus wintering in Great Britain has gradually decreased since the late 1980s/early 1990s. Although numbers also decreased in the South West region of England, the decline was much steeper in the Exe estuary population, suggesting that site-specific pressures may have affected this species in this regionally-important Special Protection Area. By combining data from the WeBS with those from 45 years of research on Oystercatchers by the Centre for Ecology and Hydrology, we tested five hypotheses that could explain the relative decline on the Exe estuary: (1) a gradual improvement in estimating Oystercatcher numbers as counting methods were refined, (2) a deterioration in the main food supply, the Blue Mussel Mytilus edulis, (3) disturbance from people on and alongside the estuary, (4) disturbance from Peregrine Falcons Falco peregrinus and (5) an as yet unexplained increase in the frequency of stealing of mussels from Oystercatchers (kleptoparasitism) by Carrion Crows Corvus corone and European Herring Gulls Larus argentatus. The data are consistent only with the fifth hypothesis. Individual-based modelling suggested that kleptoparasitism at the increased frequency that occurred on the Exe estuary could have reduced the foraging success of Oystercatchers sufficiently to have (1) reduced the overwinter survival of the numerically dominant mussel-eating adults and (2) deterred prospecting immatures from choosing the estuary as their future wintering site.