Increasing perceived predation risk through playbacks reduces Red-winged Blackbird abundance in agriculture late in the breeding season

Lay Summary & BULL; The presence of predators and brood parasites can cause behavioral avoidance by birds. & BULL; We conducted an experiment to determine whether broadcasting predator or brood parasite vocalizations can reduce Red-winged Blackbird numbers in agricultural nesting habitat. & BULL; We also included playback of the "Sonic Net" in our experiment, which increases predation risk by disrupting blackbird communication systems with broadcast of frequencies overlapping blackbird vocalizations. & BULL; We found that predator vocalizations and the Sonic Net reduced Red-winged Blackbird numbers at sites late in the nesting season, but brood parasite vocalizations did not. & BULL; Playbacks could mitigate human-wildlife conflicts with avian pests nesting in agricultural habitat in some ecologically relevant contexts. Perceived predation and brood parasitism risks strongly influence nesting habitat selection in several bird species. Here, we report on a playback experiment evaluating whether perceived predation or brood parasitism risk can reduce Red-winged Blackbird (Agelaius phoeniceus) abundances in agricultural nesting habitat. We broadcast Cooper's Hawk vocalizations (Accipiter cooperii, a predator of adult blackbirds and nests), Brown-headed Cowbird vocalizations (Molothrus ater, a brood parasite of many passerine species, including blackbirds), and the "Sonic Net" as treatments, the latter of which is broadcast of frequencies that overlap with blackbird vocalizations and prevent blackbirds from accessing intraspecific communication informing of predator and brood parasite risks. Neither the hawk, cowbird, nor Sonic Net treatments reduced blackbird abundances at sites early in the breeding season (April to May), when blackbirds were selecting nesting habitat. In contrast, late in the breeding season (July to August), hawk vocalizations and the Sonic Net reduced blackbird abundances at sites, but cowbird vocalizations did not. Our late-breeding season results suggest that blackbirds may flexibly change responses to perceived predation risk based on their stage of reproductive investment. Perceived predation risk could potentially be used to manage pest birds that nest in agricultural landscapes, at least for crops that are vulnerable to birds late in the breeding season.