Birds provide ecosystem services (pest control) in many agroecosystems and have neutral or negative ecological effects (disservices) in others. Large-scale, conventional row crop agriculture is extremely widespread globally, yet few studies of bird effects take place in these agroecosystems. We studied indirect effects of insectivorous birds on corn and soybean crops in fields adjacent to a prairie in Illinois (USA). We hypothesized that prairie birds would forage for arthropods in adjacent crop fields and that the magnitude of services or disservices would decrease with distance from the prairie. We used bird-excluding cages over crops to examine the net effect of birds on corn and soybean grain yield. We also conducted DNA metabarcoding to identify arthropod prey in fecal samples from captured birds. Our exclosure experiments revealed that birds provided net services in corn and net disservices in soybeans. Distance from prairie was not a significant predictor of exclosure treatment effect in either crop. Many bird fecal samples contained DNA from both beneficial arthropods and known economically-significant pests of corn, but few economically-significant pests of soybeans. Song Sparrows (Melospiza melodia), one of our most captured species, most commonly consumed corn rootworms, an economically-significant pest of corn crops. We estimated that birds in this system provided a service worth approximately US $275/ha in corn yield gain, and a disservice valued at approximately $348/ha in soybean yield loss. Our study is the first to demonstrate that birds can provide substantial and economically valuable services in field corn, and disservices in soybean crops. The contrasting findings in the two crop systems suggest a range of bird impacts within widespread agro-ecosystems and demonstrate the importance of quantifying net trophic effects.

Exclosure data presented are indices of corn and soybean crop yield from control and bird-exclosure plots in northern Illinois, USA, 2016. Measurements were taken for five plants within each plot. Exclosures were in place for approximately 3.5 months before the crops in exclosures and controls were harvested by hand and measurements taken.

Molecular data includes raw read counts of arthropod OTUs as rows, and bird fecal samples as columns. Each row is an OTU, and the assigned taxonomy is included in the first column. Data have not been normalized by total read counts per sample.

For exclosure data:Variables for corn include "ID" (letter from A-F indicating plot); "distance" (close is 5m from field edge, far is 55m from field edge); "num.kernals" (count of corn kernals on the primary ear); "all_kernals" (total number of corn kernals per plant from both primary and any secondary ears); "primary_dry_mass" (dry mass of all kernals removed from cob from primary ear only); "all_dry_mass" (combined dry mass of all kernals removed from cob from all ears per plant); "ave_kernal" (all_dry_mass/all_kernals). 

Variables for soybeans include "ID" (letter from A-F indicating plot); "distance" (close is 5m from field edge, far is 55m from field edge); "count_pods" (number of soybean pods per plant); "count_beans" (number of soybeans per plant, counted after removing from pods); "dry_mass" (total dry mass of all soybeans per plant); "wet_mass" (mass of soybeans per plant measured upon harvest but before being oven dried); "ave_bean" (dry_mass/count_beans)

For DNA data: Only OTUs assigned to phylum Arthropoda are included here. Because these data include read counts per OTU by sample, we have included species here that did not pass our minimum read count to be included in further analyses. We also included samples here that were later thrown out due to poor data quality.