Effects of nest predators on ground nesting birds in intensively used arable fields

Agricultural intensification may increase an impacts of predators on the reproductive performance of declining populations of farmland birds. Still, there is little definitive evidence of nest success and predator identity in intensive arable fields. In order to clarify whether nest predators really contribute to declines in farmland ground nesting birds, I used videomonitoring to identify nest predators and quantify nest success in the Skylark (Alauda arvensis) and Lapwing (Vanellus vanellus). Both species share common nesting habitats in sparsely vegetated arable fields, but their life histories suggest different vulnerabilities to nest predation. Results showed very low nest success in the Skylark, but relatively high success in the Lapwing. Skylark nests were vulnerable to all local predators, while it seems that Lapwings can avoid avian predators. The species composition of predators and patterns in nest predation rates mostly differed from those reported from Western Europe. Because of expected differences in predator identity and nest survival among nesting habitats and regions, I further quantified nest success and identified nest predators for a high-density population of Skylarks as well as Woodlarks (Lulllula arborea) breeding in more natural heath and grassland habitats in the Netherlands. Populations of both species co-occur in this area and their nests are similar targets for local predators; even so their nest predators might differ, because these larks differ in the selection of their nest sites. My results suggest that Skylark nests located in open sites were preyed upon mainly by red foxes (Vulpes vulpes), while the main predators of Woodlark nests, located generally closer to trees, were Carrion Crows (Corvus corone). Changes in agricultural practices, especially shifts from spring-sown to autumn-sown crops, can limit the number of breeding attempts of Skylarks. Under these circumstances, Skylarks are forced to shift to different breeding sites or habitats. In spite of the seasonal shift in nest sites in my study, nest predation rates did not show a clear seasonal trend; however, the proportion of predation attributed to birds decreased along with vegetation growth. My results suggest that the recently increasing area of maize fields in particular provide an attractive, yet risky, nesting habitat for Skylarks, especially late in the season when autumn-sown crops are too dense. Nest proximity to field edges may also have a negative influence on breeding productivity, with nests placed closer to edges experiencing higher rates of nest predation. I found that Skylarks seem to avoid areas close to field edges in spite of the comparatively low predation cost associated with nesting there. Variation in nest predation risk during the breeding period may be an important source of natural selection on parental behaviour. Skylark females spent more time attending clutches in the early morning and evening, with more frequent recesses during the afternoon. Moreover, this diurnal variation was dependent on (i.e. interacted with) ambient temperature and vegetation characteristics at the nest site (height and concealment). This suggests that the way incubation time is allotted during the day may be equally as important as the total amount of time spent incubating.