Demography and productivity of woodlarks Lullula arborea in Breckland
Woodlarks Lullula arborea are ground nesting, multi-brooded passerines of European
Conservation Concern. Around 30 % of the UK population breeds in the Breckland region
of East Anglia. Woodlark populations have increased dramatically in the region during the
past 35 years. This thesis investigates the mechanisms and causes of the observed changes
in woodlark population size since 1970.
W oodlark breeding habitat in Breckland has varied historically between heathland,
plantation forest clear-fells and, more recently, farmland set-aside fields. Currently all
three habitats are occupied. Farmland set-aside could support a significant proportion of
the Breckland population. There was no significant difference in reproductive output
between heathland and forest clear-fells. However clutch sizes may be smaller on
farmland, perhaps an indication that farmland is sUb-optimal compared to the other habitat
types and that a buffer effect is operating.
Habitat occupancy within Thetford Forest was investigated, in order to determine the
factors influencing woodlark abundance in clear-felled coupes, including stand age,
vegetation, soil type and coupe area.
Factors affecting productivity were considered for birds breeding in Thetford Forest. Nest
success declined significantly over the 35 years of the study. Clutch size was reduced at
higher population density, and woodlarks laid larger clutches later in the breeding season.
Nestling survival rates were lower on younger stands.
Woodlarks bred earlier in warmer springs. Breeding success was controlled mainly by
predation, but also by weather. Simulation modelling of repeat nesting attempt5
demonstrated that changes in predation rates had a greater effect on annual productivit)
than the combined effects of weather. Mortality was higher in cold winters, but annual
recruitment rates were higher after 1988 compared to the period up to 1988 (controlling for
the effect of winter temperature). The availability of winter stubble fields on farmland fron
1989 (due to set-aside) may have allowed survival rates to increase.