The world's most southerly bird has become a threat to planes in Antarctica after developing a love for sitting on warm, snow-free airstrips.Air traffic experts are seeking ways to scare off the south polar skuas, a large and aggressive brown seabird, but without harming them. The birds are protected by the 47-nation Antarctic Treaty, which declares the frozen continent a nature reserve.At the British Rothera research station on the Antarctic Peninsula, about 100 skuas often sit on the 900 meter (3,000 ft) gravel runway. The odd penguin or seals can also be hazards.As part of the skua scaring ritual, large bangs are set off and then a runway worker drives a six-wheeled vehicle up and down the runway, swerving toward any remaining birds. Sirens wail to alert both skuas and people."It seems to be working -- about 15 minutes prior (to takeoff or landing) we've driving up and down with bells and whistles to get them out of the way," Steve King, a pilot and flying safety officer at Rothera, told Reuters.At Davis, an Australian Antarctic base, staff take a different tack by feeding the birds to try to lure them away from the airstrip.Skuas are a hazard because bird strikes can down aircraft. The U.S. jetliner that made an emergency landing on the Hudson River on Thursday -- in which all 155 abroad survived -- had apparently struck a flock of birds.No birds have been hit this season at Rothera but there were minor strikes last year. The skuas are apparently attracted by the dark gravel surface -- warmer than sitting on snow.Antarctic airstrips can get a license to kill the birds in extreme cases.The south polar skua is a marauder that often eats other birds' eggs or steal other birds' food, behavior scientists call "klepto-parasitism."Anyone who goes too close to a skua nest risks attack. They sometimes strike but usually veer off at the last moment. "It's like someone dropping a chicken on your head," said John Loines, a skua expert at Rothera.Not everyone dislikes the skuas, however.A photograph of Bubba hangs in a corridor below the Rothera control tower as the official "base skua." "Bubba is 29 years old," said Loines. He succeeded "George" and "Mildred" as official base skua.