De Veldleeuwerik Alauda arvensis als broedvogel in akkerbouwgewassen

In 2003 and 2004, 55 nests of Skylarks were found and observed in the provinces of Drenthe and Groningen. Nests were situated in root crops (8), winter cereals (10), summer cereals (11), grass seed (7), fallow land (16) and in other crops (3). Sequence of crop use is shown in Fig. 1 and main characteristics of growth and management of the crops in Table 1. Crops are suitable for breeding as long as they provide sufficient shelter, but are avoided as soon as the vegetation becomes too high or too dense ("closing" in Table 1). Skylarks produced bigger clutches in the course of the breeding season (Fig. 2), which largely explains the differences in clutch size as mentioned in Table 2. The proportion of successful breeding attempts was highest in grass seed and summer cereals. The high success rate in root crops is partly caused by two nests being rescued from destruction during hoeing. Without this operation the success rate in root crops would have dropped to 56%. The poor breeding success in fallow is mainly caused by high predation. Fallow occurs as islands amid otherwise inhospitable farmland, and probably attracts predators. Winter cereals provided suitable breeding habitat early in the season only, but then food abundance was low. However, many Skylarks started their first clutch in this crop, resulting in poor reproductive output partly because of nestling starvation. Since the 1980s, the breeding population of Skylarks in The Netherlands suffered a decline of at least 45%. This may have been the result of an increase of maize acreage at the expense of cereals, a shift from summer- to winter cereals and increased usage of herbicides. Moreover, in the study area the few fields with summer cereals were used for nesting till the start of June only, whereas in England this crop is used through July. Possibly, summer cereals in The Netherlands are sown more densely, which renders the crop unsuitable for nesting at an earlier stage of growth. It is argued that present-day farming in The Netherlands does not allow Skylarks to produce more than one successful brood per season, which is insufficient to maintain a stable population.