De. Chamberlain et al., Effects of habitat type and management on the abundance of skylarks in thebreeding season, J APPL ECOL, 36(6), 1999, pp. 856-870
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.