Spatial and temporal distribution of breeding skylarks Alauda arvensis in relation to crop type in periods of population increase and decrease

The seasonal variation in Skylark Alauda arvensis occurrence, in terms of b oth presence and abundance, was examined in relation to crop type on a numb er of survey sites in English lowland farmland in two periods, 1965-70 and 1990-95. These two periods were selected to coincide with periods of popula tion increase and decline respectively. We were able to consider the effect s of changes in cropping regime on the length of the breeding season for Sk ylarks, a factor implicated in the cause of the Skylark population decline. The overall occurrence of Skylarks was significantly lower in the 1990-95 period, in accord with national trends. At the farm level, Skylark abundanc e increased significantly with crop diversity. There was little difference between spring and winter cereals in occupancy rates or seasonal trend in t he 1965-70 period, but in the 1990-95 period, territories were abandoned mu ch later on spring cereals than on winter cereals. Spring cereals were aban doned later in the season in the 1990s than the 1960s whereas winter cereal s were abandoned earlier. There was thus an indication that the suitability of winter cereals had decreased between the two periods. However, in the 1 990-95 period, farms with spring cereals tended to have more diverse croppi ng than those without, so the results for this crop may be confounded by fa rm-level effects. Set-aside, which only occurred in the 1990-95 period, had the highest rates of Skylark occurrence, but showed no seasonal trends. Th ese results broadly support the suggestion that increases in winter cereals and loss of farm habitat diversity have contributed to the Skylark decline .