Effects of habitat type and management on the abundance of skylarks in thebreeding season

1. There is increasing evidence to link major declines in skylark populatio ns in Britain to agricultural intensification. However, whether causal mech anisms identified through localized studies can be generalized to the natio nal scale remains unknown. The abundance of breeding skylarks was determine d by surveying singing males in over 600 randomly selected 1-km squares thr oughout Britain, in which skylarks recorded were assigned to homogeneous ha bitat patches. A more intensive survey of skylarks was carried out on lowla nd farmland sites in England. Singing males were assigned to specific crop types, and data on crop height and field boundary features were recorded. 2. Skylark occupancy (presence/absence) and density where birds were presen t (i.e. omitting zero counts) were analysed in relation to habitat type, ha bitat diversity and time of year, using generalized linear modelling. 3. Set-aside, moorland and winter cereals had high rates of skylark occupan cy at the national scale. Set-aside had consistently high rates of occupanc y and high densities across the breeding season at different spatial scales . Apart from set-aside, there was little difference in density between habi tats in the early half (March to mid-May) of the breeding season. In the la ter half of the breeding season (mid-May to July), density declined signifi cantly on winter cereals, which showed significantly lower density than a n umber of habitats at this time, including spring cereals, legumes and moorl and. 4. Within lowland farmland, there were significant effects of crop height o n skylark occupancy, with crops of greater than 30 cm in height being occup ied at relatively low rates. Winter cereals reached this height significant ly earlier in the breeding season than a number of other crops, including s pring cereals and legumes. 5. Skylark density increased with increasing habitat diversity across the w hole sample of 1-km squares and in lowland 1-km squares in England. However , within the lowland farmland plots in England, skylark density showed a si gnificant decrease with increasing habitat diversity. These conflicting res ults suggest that crop type rather than habitat diversity per se is importa nt. 6. The effects of vegetation height on skylark abundance support the hypoth esis that increases in winter cereal, and simultaneous loss of spring cerea l, have had an adverse effect on skylark populations by reducing the number of breeding attempts made per year. These results support findings from sm aller scale studies showing the generality of these habitat effects at diff erent spatial scales. The extent of the British skylark population associat ed with agricultural land suggests that sympathetic changes in farming prac tice are likely to provide the best mechanism for improving the status of t his species. The inclusion of options, such as spring cereal or fallow land (an equivalent to set-aside), in agri-environment schemes is likely to ben efit skylarks breeding on farmland by providing suitable nesting habitat th roughout the breeding season. In addition, reductions in the intensity with which cereals are managed, such as reduced pesticide and fertilizer input under approaches such as precision farming, and the creation of sparser pat ches of cereal sward, are also likely to increase the suitability of winter cereals for nesting skylarks.