The moment scientists had been dreading arrived late last year, when H5N1, or bird flu, was found for the first time in the Antarctic. Last week a king penguin on the island of South Georgia became the first in the region to be suspected to have died from the disease. The Guardian’s biodiversity reporter, Phoebe Weston, tells Ian Sample why researchers have said the spread of bird flu through the Antarctic’s penguin colonies could signal ‘one of the largest ecological disasters of modern times’