Congregations of rodent-eating birds during flood irrigation of crops exploit novel prey pulses and potentially provide pest control services

Abstract In California’s Central Valley, we have observed large congregations of rodent-eating birds- including wading birds, raptors, and corvids-hunting for rodent prey during flood irrigation of alfalfa fields. A pilot study in 9 fields revealed a mean of 23.7 rodent-eating birds, and a maximum of 102 birds, hunting simultaneously in a field. We observed a total of 293 rodent prey captures in 570 minutes of monitoring and calculated a mean of 1.03 rodent prey captures per hour of avian hunting effort. We believe these initial observations provide insights into the responses of rodent-eating birds to an intensive farming practice that echoes natural fluctuations in surface water. We propose 4 testable hypotheses for others exploring the role of irrigation practices for carnivorous birds: (1) rodent-eating birds use social and environmental cues to exploit prey during short-term flood irrigation events; (2) rodents make up a larger proportion of the diet of Ardeidae species in regions that use short-duration flood irrigation compared to regions where this practice is not used and flood-irrigated fields may therefore sustain rodent-eating bird populations in areas where natural wetlands are scarce; (3) rodent-eating birds provide significant rodent pest control services within flooded alfalfa fields; and (4) nocturnal predators respond to short-term flood irrigation events in a similar pattern as diurnal birds.