Christmas Island is home to threatened bird species including the red-tailed tropicbird and the Christmas Island thrush, both of which are preyed upon by feral cats and rats. We investigated whether an existing eradication program for feral cats would cause black rat populations to increase, reducing the conservation benefit of the cat eradication. Using a variety of surveys and modelling methods, we assessed the current population size and spatial variation of rats on Christmas Island, and estimated their rates of predation on bird nests. From these models, we investigated the potential for rat populations, and their impacts to native birdlife, to increase following the removal of cats.