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^–1 in corn yield gain, and a disservice valued at approximately $348 ha^–1 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 2 crop systems suggest a range of bird impacts within widespread agroecosystems and demonstrate the importance of quantifying net trophic effects.