The simplification and homogenisation of agricultural landscapes has led to dramatic losses of farmland biodiversity. This decline includes passerine populations and one of the mechanisms that may be contributing toward this may be increased nest predation. Ground covers are agri-environmental measures whose objective is to halt the loss of biodiversity by creating high-quality habitat patches embedded in a homogeneous landscape by maintaining herbaceous ground cover within crops. However, although such patches are selected by breeding birds, they may also attract predators, and an increased nest density in small habitat patches would thus result in increased predation rates. We have therefore performed nest predation experiments with artificial nests in order to investigate whether the effect of ground cover on nest predation is density dependent. The experiments were carried out during the breeding season in 2014, in two areas of olive groves in southern Spain. We used two nest density levels, both in olive groves with ground cover and in those where the ground was bare. A total of 420 nests were placed, 226 of which (53.81%) suffered predation. Our results showed that 1) nest predation was density dependent in olive groves with ground cover only with respect to tree nests, while there was no density-dependent predation in bare-ground olive groves of either ground-based or tree nests, and 2) artificial nests placed on the ground were more frequently depredated (65%) than those placed in trees (29%), irrespective of either the presence of vegetation cover or nest density. This study suggests that variations in patterns of nest predation are determined by the identity of nest predators and their specific foraging behaviours in different landscapes and habitats. It also underscores the need to implement agri-environmental measures aimed at the conservation of farmland birds.
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Vol. 63 • No. 2