Northern rockhopper penguin mice ("Mus musculus") have been introduced into their environment by human sea expeditions. The mice have proven to be invasive, and consume northern rockhopper eggs, as well as hunt their young. In order to preserve the birds, a culling of the mice is being considered. The northern rockhopper penguin is classified as endangered because of the decline in numbers over the last three generations (or 30 years). Population Trends by Year and Island On March 16, 2011, the Maltese-registered freighter MS "Oliva" ran aground on Nightingale Island, spilling tons of heavy crude into the ocean. The crew was rescued, but
Bird-of-paradise lek-type mating system. Others, such as the "Cicinnurus" and "Parotia" species, have highly ritualised mating dances. Males are polygamous in the sexually dimorphic species, but monogamous in at least some of the monomorphic species. Hybridisation is frequent in these birds, suggesting the polygamous species of bird of paradise are very closely related despite being in different genera. Many hybrids have been described as new species, and doubt remains regarding whether some forms, such as Rothschild's lobe-billed bird of paradise, are valid. Birds-of-paradise build their nests from soft materials, such as leaves, ferns, and vine tendrils, typically placed in a tree