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

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 four 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 carnivorous bird populations in areas where natural wetlands are scarce; 3) 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.

Methods
We identified 9 alfalfa fields that utilized flood irrigation in Yolo County, in California’s Central Valley. We visited fields during irrigation and visited each site 1-4 times in May-July in 2017 or 2018.  Each visit consisted of a 30-minute period split into six consecutive 5-minute observation intervals. Within each interval, we counted any ‘rodent-eating bird’: species that include rodent prey in their diet, including raptors, wading birds, and corvids. We observed foraging of birds present in fields and counted all rodents consumed by birds in the same time interval. Rodent species were not discerned between, however fossorial rodents commonly impacted by flood irrigation in this region are Botta’s pocket gopher  and California vole.  Only rodents that were sighted being consumed were counted. Wading birds held prey items in their beaks prior to swallowing them, whereas birds of prey including B. swainsoni generally appeared to consume prey items by holding them on the ground and tearing pieces off to eat. We therefore believe we have underestimated prey consumption rates by raptors in particular.