Investigating brown rat Rattus norvegicus egg predation using experimental nests and camera traps

Introduced rat species have been implicated in the decline and local extirpation of numerous seabird species from islands across the globe, leading to widespread eradications as a conservation tool. However, little conclusive evidence has been established to determine the direct mechanisms in which rat and seabird species interact. This study aimed to quantify rates of egg predation by brown rats Rattus norvegicus using automated trail cameras at seabird nests baited with domestic quail and hen eggs to represent different-sized seabird eggs. The trail cameras were in situ for a total of 915 days during June and July 2011. Evidence for rats visiting the experimental nests was only observed at one location, where 19 visits were recorded, and no evidence of rat predation on hen or quail eggs was observed. Low levels of rat activity were observed during the study; therefore, it is not possible to conclude that rats never predate seabird eggs as predation may be more likely when rat densities are higher and pressure on food resources greater. This study does however highlight that where rats occur at low densities, predation of eggs is unlikely. Measures aimed at maintaining low abundance of rats in and around vulnerable seabird colonies may therefore be useful where complete eradication is not feasible, although potential for predation of chicks should also be considered.