Soil management of olive groves has contrasting effects on nest densities and reproductive success of tree-nesting passerines

Agri-environmental measures aim at mitigating the negative impacts of modern agriculture on farmland biodiversity. For example, soil management practices can positively influence the abundance and diversity of songbirds in olive groves by enhancing habitat and food availability. However, little is known about their potential implications on the breeding success of these species. We monitored nest density and breeding performance of tree-nesting birds in 17 olive groves of southern Spain under two contrasting soil management regimes (with and without herbaceous cover) over three years. We analyzed breeding success by examining the transitions between different stages of the breeding cycle for three common cardueline finches: Common Linnet (Linaria cannabina), European Greenfinch (Chloris chloris), and European Serin (Serinus serinus). Breeding success of songbirds was low (19% of 88 nests for L. cannabina, 16% of 196 nests for C. chloris, and 38% of 234 nests for S. serinus). Many nests were abandoned prior to egg laying and nesting failure was highest between egg laying and hatching. Nest predation (56.6%) was the main cause of nest loss. Although soil management regimes did not influence breeding success, the presence of herbaceous cover had opposing effects on nest densities of the three species: groves with herbaceous cover had higher nest densities for greenfinches, lower for serins, and this treatment had no effect on Common Linnet. Other factors, like disturbances due to management activities or interannual weather variation may have a larger impact on nesting success than soil management regimes, at least for some species and for specific stages of their breeding cycle. The fact that nest densities of some birds were negatively affected by soil treatments associated with agri-environmental actions aimed at benefiting biodiversity raises the question of the general applicability of these measures for conservation.