Some strains of bird flu are coming ever closer to developing the traits they need to cause a human pandemic, a study released Monday said. Researchers who analysed samples of recent avian flu viruses found that a few H7 strains of the virus that have caused minor, untransmissible infections in people in North America between 2002 and 2004 have increased their affinity for the sugars found on human tracheal cells. Investigators said the evidence suggests that the virus could be evolving toward the same strong sugar-binding properties of the three worldwide viral pandemics in 1918, 1957 and 1968. The authors said that if the viruses continue to evolve in this direction, the avian flu viruses could travel more easily between other animals and humans. They called for strict surveillance of avian flu viruses and continuing federal preparations for a possible future pandemic.