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 vocalization and prevent an energetically costly fight . In birds with song repertoires, individuals may share the same song type and use these song types for more complex communication. Some birds will respond to a shared song type with a song-type match (i.e. with the same song type). This may be an aggressive signal, however results are mixed. Birds may also interact using repertoire-matches, wherein a bird responds with a song type that is in its rival's repertoire but is not the song that it is currently singing. This may be a less aggressive act than song-type matching. Communication through bird