Use of preferred nesting habitat by stone curlews Burhinus oedicnemus in relation to vegetation structure

Abstract

The stone curlew is a nocturnal bird which nests and forages on sparsely vegetated ground. Its population and geographical range in Britain have declined throughout the 20th century. Estimates of the size of a sub‐population in the Breckland region of eastern England show that a decline which began in the 1940s or earlier continued up to the mid‐1980s, but that the population was stable from 1985 onwards. A survey in 1987 indicated that stone curlews nested at the highest densities on fragments of formerly much more extensive short semi‐natural dry grasslands and heaths (referred to throughout as heathland). However, most pairs bred at much lower density on spring‐sown tilled farmland. An important part of the decline in the Breckland stone curlew population between 1968 and 1991 is attributable to a decline in the number of pairs nesting on heathland.Observations of stone curlews marked individually with colour rings showed that individuals did not nest exclusively on heathland or arable farmland. Some birds nested in both habitats in the same year and there were frequent moves between habitats from one year to the next. Year‐to‐year changes in the number of pairs of stone curlews nesting on heathland during a period of population stability were associated with variation in both the rate of return to heathland of ringed birds which previously nested on heathland and the proportion of surviving young adults with no previous breeding experience which nested on heathland. Some of the birds which were displaced from, or failed to recruit to, heathland nesting sites nested on arable land, but there was also evidence that a proportion of them did not attempt to breed.Measurements of sward height at heathland sites with a recent history of occupation by nesting stone curlews show that those sites which had been abandoned had taller swards than those where the birds remained. An index of the biomass of green vegetation was derived from satellite imagery obtained before and after a sharp decline in the number of heathland nesting stone curlews. It was found that the index had increased significantly more at heathland sites which had ceased to be used by stone curlews than at sites which continued to be used for nesting.The long‐term decline in the number of stone curlews nesting on heathland is believed to have begun with the conversion of large areas of heathland for arable agriculture and forestry. However, it has continued during a period when little heathland was lost. Changes in vegetation structure and species composition associated with a reduction in grazing by rabbits and livestock are the most likely cause. Since 1985 breeding success has been enhanced by protection of eggs and chicks in arable crops from destruction by farming operations. Beginning in 1989 efforts have also been made to change the management of Breckland heaths. These include shallow ploughing and increased grazing by livestock, supported by payments from government under the Environmentally Sensitive Areas scheme. The number and proportion of stone curlews nesting on heathland has increased and the decline in the total population size has ceased. The relative contribution of different management factors to this change is not yet clear.