The long-term impacts of upland livestock grazing pressure on the breeding productivity, nestling diet and nutrition of a common insectivorous passerine

Abstract

Changes in grazing management of upland habitats during the last decades have contributed to declines of many bird species. In order to determine drivers of population change of upland birds, a mechanistic understanding of how land management affects breeding conditions is needed. Using a long-term, landscape-scale experiment, this study examined the effect of livestock grazing intensity and type on: a) the breeding productivity of the meadow pipit Anthus pratensis, a common insectivorous passerine in the British uplands; b) the abundance of arthropod groups common in upland bird diets; c) nestling diet composition using DNAmetabarcoding; and d) nutrient quality of provisioned prey. The grazing experiment started in 2003 and had four treatments: I) intensive sheep; II) extensive sheep; III) extensive mixed sheep and cattle and IV) ungrazed. Meadow pipit nests were monitored and arthropods were sampled in 2004/5 and 2015/16, to compare differences between early and late stages of the experiment. Faecal sacs of nestlings were used to identify prey DNA and estimate diet nutrient quality. Egg-stage nest survival was highest in plots with extensive sheep grazing but no statistically significant change in nest survival between grazing treatments was detected over time. Total arthropod mass and abundance was highest in extensively sheep grazed and ungrazed plots. Nestling faecal samples contained a higher concentration of an aggregated measure of micro-nutrients in intensively sheep grazed plots, where the diet analysis also suggested that nestlings were fed prey from a wider range of invertebrate orders. The higher breeding abundance, diet diversity and concentration of some essential nutrients in intensely grazed plots may be an indication of more favourable foraging conditions. However, this was not reflected in nest survival, which was mainly affected by predation. The applicability and forthcoming obstacles with DNA-based methods for upland bird diet assessments are discussed.