Changes in breeding wader populations of the Uist machair and adjacent habitats between 1983 and 2022

J. CALLADINE, R. FULLER, D. HODKINSON, S. FRANKS & J. BOYLE

Periodic extensive surveys of machair and associated habitats on the west coast of North Uist, Benbecula and South Uist have documented marked changes in the composition of an important breeding wader assemblage. Within a study area of 123 km2 there had been a 25% decline in the total number of breeding waders recorded between 1983 and 2022. Dunlin and Ringed Plover had shown the most marked declines while Oystercatcher and Redshank both increased. Changes in breeding numbers up to 2014, the date of the previous extensive survey, had tended to be more negative in a southern zone (South Uist and Benbecula) where introduced Hedgehogs are important predators of wader eggs. However, changes between 2014 and 2022 were less clearly defined between areas with and without Hedgehogs, with some marked declines in a northern zone without Hedgehogs (North Uist and Berneray) and also in Benbecula. By comparison, there was a general pattern of increasing or stable numbers on South Uist. At a time when populations of breeding waders are declining across much of Britain, the assemblage remains exceptionally important with an average of about 66 apparent pairs of breeding waders per km2 across the survey area in 2022. There is a need to better understand the different drivers of change, for example land-use and predation.

Introduction

Relative to their area, islands can support disproportionately high populations of breeding waders, especially where there are suitable habitats and a restricted suite of predators (Calladine et al. 2022). This holds true for the west coast habitats of South Uist, Benbecula, North Uist and islands in the Sound of Harris, which continue to support exceptionally large concentrations of breeding waders at a time when breeding waders are in serious decline in many other parts of Britain (Fuller et al. 2013, Harris et al. 2022). A first extensive survey of the machair and associated habitats in this region (hereafter referred to as ‘Uist’) in 1983 suggested it held between a third and a quarter of the UK breeding populations of Dunlin Calidris alpina and Ringed Plover Charadrius hiaticula respectively (Fuller et al. 1986). Subsequent repeat surveys in 2000, 2007 and 2014 recorded widespread losses of both those species but mixed fortunes for Oystercatcher Haematopus ostralegus, Redshank Tringa totanus, Lapwing Vanellus vanellus and Snipe Gallinago gallinago (Jackson et al. 2004, Fuller et al. 2010, Calladine et al. 2015). Hedgehogs Erinaceus europaeus were introduced to the southern part of South Uist in 1974 and subsequently spread throughout the west coastal areas of that island, to Benbecula, Grimsay and onto the southern part of North Uist (Jackson et al. 2004, Jackson 2007). Hedgehog exclusion experiments, the recording of nest predation events, and contrasting changes in populations between areas with and without Hedgehogs, provide compelling evidence that population declines were at least partly driven by predation of eggs by these introduced mammals (Jackson & Green 2000, Jackson 2001, Jackson et al. 2004, Calladine et al., 2015, 2017). However, changes in soil chemistry, moisture, salinity and vegetation characteristics within the machair systems, potentially linked with their management may also be involved, possibly acting through alterations to food supplies (Calladine et al. 2014, Fuller 2018).