Breeding skylarks Alauda arvensis on Environmentally Sensitive Area arable reversion grass in southern England: survey-based and experimental determination of density.
Abstract
In the South Downs and South Wessex Downs (southern England), designated an Environmentally Sensitive Area (ESA) in 1987 and 1993, respectively, farmers may undertake to conserve existing areas of downland turf (chalk grassland rich in plants and invertebrates) and to create new areas of grassland out of previously cultivated land (arable reversion), in exchange for annual payments. These areas are subject to strict management prescriptions. Singing skylarks were surveyed on 40 km2 of downland turf, arable reversion, non-ESA grass and arable land between 1994 and 1996 (April-July) in the two ESAs. The emphasis was on two types of arable reversion: permanent grassland sown with agricultural grass species, and chalk downland sown with traditional chalk grass species. The surveys showed that singing skylark density was greater on chalk downland arable reversion than on permanent grassland reversion, in swards above 15 cm in height and in fields without boundary scrub. In general, permanent grassland reversion did not support more skylarks than winter wheat or grass not entered in the ESA schemes; highest densities were in undersown spring barley. Singing skylark density decreased after cutting in May-June in the South Wessex Downs ESA. In 1995 and 1996, a fully randomized experiment was carried out on 12 fields (each about 5 ha) at Plumpton, Sussex, to verify the survey findings. Six fields were grazed by sheep to maintain a sward below 10 cm in height (short-grazed), and the others grazed to maintain a sward between 15 and 25 cm (long-grazed) during the skylark breeding season (April-July). On average, singing skylarks were six times as abundant, and non-singing ones twice as abundant, in experimental long-grazed fields as in short-grazed fields. The total number and number of taxa of chick-food invertebrates were about twice as large on average, and the number of grass seed heads recorded in July was two to 15 times as great, in long-grazed than in short-grazed fields. Grazing-exclusion cages within a randomized block experiment at New Erringham, Sussex, were sampled for invertebrates in mid-May 1995 and 1996. Ungrazed cages (sward up to 40 cm tall) supported over five times as many invertebrates and over three times as many invertebrate taxa as grazed areas (sward <2 cm tall). These results contributed to revised management prescriptions for the South Downs (1997) and South Wessex Downs (1998) ESAs. It is suggested that prescriptions to encourage taller swards and to prevent cutting during the nesting season in the South Wessex Downs, and removal of the 10-cm limit on sward height, allowance of fertilizer on some arable reversion grassland, and an arable tier encouraging undersown spring cereals and overwinter stubbles in the South Downs, would benefit grassland birds.