A non-native species of house mice that have evolved to be 50% larger than average house mice have adapted to Tristan da Cunha. They are thought to have been accidentally introduced by 19th century seal hunters who would dock on the islands. These mice have adapted by consuming sea bird eggs and chicks (as they nest on the ground), killing an estimated 2 million chicks annually pushing the species to extinction. Gathering at night in groups of 9 or 10, the mice gather at the bird's nest to feast. With no natural predators, the invasive mice population is able to expand by producing new generations twice a year. With no natural predators and a high generational output rate, there is no way to control the growth of the invasive mice population. The native bird species have slower generational output and exposure to the islands.