Cattle egrets regurgitate house mouse carcasses onto a mouse-free island: implications for rodent eradications

Abstract

Abstract ContextEradication of invasive rodents on islands typically results in positive conservation gains, and maintaining a rodent-free island requires elevated biosecurity, including prevention of assisted rodent arrival via watercraft, aircraft and animals such as birds. Cattle egrets (Bubulcus ibis) are widespread, and often fly several kilometres daily to roost and forage. They frequently swallow insects and vertebrates (including rodents) whole, and some regurgitate prey. Cattle egrets have been regularly observed flying between the Hawaiian Islands of Ni’ihau (where non-native mice and rats are established) and Lehua (where one species of non-native rat is established and was targeted during a recent eradication attempt). Aims The objectives were to identify the species of rodent that cattle egrets regurgitate following transport between Ni’ihau and Lehua islands, and to determine if any of the rodent individuals regurgitated were alive once deposited onto Lehua Island following 1.1-km oversea flights. Methods Eighty-five individual rodent carcasses (regurgitated by cattle egrets) were collected, preserved and identified to species using morphological characteristics and DNA sequencing. Key results All rodents regurgitated by cattle egrets were dead upon collection on Lehua Island. Although the Pacific rat (Rattus exulans) is the only rodent species on Lehua Island, and field staff suspected the regurgitated rodents were R. exulans, all 85 carcasses were identified as house mice (Mus musculus). Conclusions This is the first evidence (that the authors know of) showing movement of rodent carcasses, via cattle egrets, between islands. Implications Cattle egrets that deposit rodent carcasses onto rodent-free islands, or segments of islands, may confuse land managers and biosecurity professionals who are unaware of this phenomenon. House mice did not survive cattle egret ingestion, >1-km flight and regurgitation; therefore it is unlikely that live rodents would be introduced to rodent-free areas via cattle egrets.