Estimating rat predation on Humboldt Penguin colonies in north-central Chile

Abstract

Rats (Rattus spp.) are among the most successful alien predators brought to islands by humans and have had devastating impacts on numerous seabird populations, but studies demonstrating rates of consumption and ecological impacts on penguins are scarce and mostly based on anec- dotal evidence. We investigated the effects of rat predation on Humboldt Penguins (Spheniscus humboldti) by simulat- ing unattended clutches with domestic Chicken eggs. Experiments were independently set at two Humboldt Pen- guin colonies in north (Pajaros Island, 29S) and central Chile (Algarrobo Island, 33S). At both colonies, eggs were primarily predated by rats (Rattus rattus = 70.8 % at Paja- ros and Rattus norvegicus = 52.6 % at Algarrobo), and secondarily by Kelp Gulls (Larus dominicanus = 9.7 % at Pajaros and 15.8 % at Algarrobo). Significantly more eggs were predated at night. At both colonies, rates of rat and gull predation were highest within the first 12 h. Our study con- stitutes the first quantification of rats as important alien predators at Humboldt Penguin colonies. We suggest that rat presence at Humboldt Penguin colonies coupled with events that can cause temporary nest abandonment, such as human perturbation and El Nino events, may impact on the species' breeding success. Eradication of rats is suggested to improve the nesting habitat of this and other threatened and endemic seabird species in the region.