Swifts and Swallows Thrive Where Olive Groves Keep Their Green Cover
Study in southern Spain finds herbaceous cover increases abundance of aerial-feeding birds, with implications for biodiversity and pest control
Testing the value of herbaceous cover
Olive groves dominate much of the Mediterranean landscape, yet how they are managed can make a profound difference to wildlife. A new case study from Andalusia, southern Spain, has shown that maintaining herbaceous ground cover beneath olive trees strongly benefits aerial-feeding birds such as swifts and swallows.
Researchers compared three plots of traditional olive groves, each managed differently: one with chemically treated herbaceous cover, one with mechanically treated herbaceous cover, and one kept as bare soil. Across seven months of surveys in 2019, they recorded nearly 20,000 birds from 22 insectivorous species.
Winners among aerial feeders
Six aerial-feeding species were the focus of the study: Common Swift Apus apus, Pallid Swift A. pallidus, Alpine Swift Tachymarptis melba, Barn Swallow Hirundo rustica, Red-rumped Swallow Cecropis daurica, and Common House Martin Delichon urbicum. These birds were consistently more abundant in the plots where herbaceous cover was retained, with numbers three times higher than in the bare-soil plot.
Swifts and House Martins were among the dominant species in covered plots during the breeding and post-breeding migration periods, while they were largely absent from bare-soil plots. Overall, aerial-feeding birds represented 11–12% of insectivorous birds in covered plots but just 3% in the bare-soil treatment.
Vegetation and insects as the link
The researchers argue that herbaceous cover boosts flying insect populations, providing vital food for aerial feeders. This link has been overlooked in previous studies of olive groves, which have focused on ground-dwelling birds. The new evidence suggests that swifts and swallows, often associated with urban habitats, can make extensive use of farmland habitats when insect prey is abundant.
Seasonal patterns were also clear. Numbers of aerial feeders increased steadily from May to August before declining sharply in September, tracking the peaks of insect availability in the groves.
Implications for farming and biodiversity
The findings underline that maintaining ground vegetation in olive groves not only supports biodiversity but may also benefit farmers. Insectivorous birds are known to reduce pest insects such as olive fruit fly Bactrocera oleae, which causes severe losses to olive yields and oil quality. By supporting aerial feeders, herbaceous cover could enhance natural pest control.
The authors caution that their study was limited to single plots of each treatment, but the results align with broader work showing that vegetation cover increases bird and insect diversity in Mediterranean farmland. They argue that herbaceous cover management should be promoted within agri-environmental schemes and olive farming policy as a tool for both conservation and sustainability.
September 2025
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