Territory Distribution and Breeding Success of Skylarks Alauda arvensis on Organic and Intensive Farmland in Southern England

Abstract

Skylark numbers declined by 51% between 1968 and 1995 on UK lowland farmland; a loss of approximately 3 million breeding birds. This study examined whether distribution and breeding success of skylarks varied with the cropping of organically and intensively managed fields in southern England in accordance with the hypothesis that changes in agricultural land-use and intensity of management have contributed to this decline. Density was lowest on fields surrounded by tall boundary structures or unsuitable habitat, and those with tall, dense vegetation cover. After controlling for these effects, set-aside and organically-cropped fields supported significantly higher skylark densities throughout the breeding season than intensively cropped fields or grazed pasture. Nests were usually built in crops between 20 and 50 cm tall. In fast-growing broad-leaved crops (e.g. oilseed rape, legumes), skylarks held territories, but no nesting activity was observed. Rapid crop growth probably allows too little time for nesting to begin. Breeding success was higher on set-aside than on intensively managed cereals. Predation caused most nest failures, but did not vary in frequency with crop type. Silage cutting and trampling caused many failures on grass fields, and all cases of apparent brood starvation occurred in cereal fields. These breeding success data, together with published estimates of survival rates, suggest that skylark pairs must make 2-3 nesting attempts per season in order for populations to be self-sustaining. A single crop type rarely provides a suitable vegetation structure for nesting throughout the breeding season. Skylarks therefore require structurally diverse crop mosaics in order to make multiple nesting attempts without territory enlargement or abandonment. Mixed farms are more likely to fulfil these requirements than those dominated by winter cereals and broad-leaved crops. These results are consistent with the hypothesis that loss of mixed farming and rotational cropping, and concomitant increases in autumn sowing of crops, agrochemical inputs, multiple silage cuts and grazing intensities since the 1950s have reduced the breeding productivity and population density of skylarks on lowland farmland in southern England. The following recommendations are made for changes in farming systems that would assist the conservation of breeding skylark populations on lowland farmland. Organic farming systems, set-aside and habitat management for gamebirds are all likely to improve nesting and feeding conditions for skylarks. More generally, breeding skylark populations are only likely to increase on farms that reduce agrochemical inputs, reduce grazing intensity and frequency of silage cutting, and increase the structural diversity of field vegetation by adopting mixed rotations of winter and spring cereals, root crops and grass. Traditional mixed farming systems of this kind are now rarely economically desirable. Only agricultural policy reforms motivated in part by environmental concerns rather than solely by production control are likely to direct subsidy support to reduced-intensity. mixed farming enterprises of this kind, and thus help to restore populations of breeding skylarks on lowland farmland.