Geneva ? European governments are bolstering their guard against bird flu, faced with a growing number of dead swans and the risk that migratory birds – which begin returning north next month – could bring the disease from Africa.

Song birds are expected to start arriving in southern Europe from Africa in two weeks. They are judged to be less likely carriers of bird flu than ducks and other water fowl because song birds fly longer distances and tend to stay away from wetlands, where the disease is more prevalent. The migration period is heaviest in March and April, but runs into May for some species.

France, Germany, the Czech Republic, Switzerland and Sweden all took steps Wednesday to try to prevent the spread of the deadly H5N1 strain, ordering that domestic fowl be kept in screened, ventilated buildings, where they are less likely to come in contact with wild birds. Britain and the Netherlands have yet to record bird flu outbreaks, but ordered similar precautions.

The fear of birds migrating from Africa has been augmented by the deaths of swans from the Baltic Sea to the southern tip of Italy. Because of their susceptibility to bird flu, swans are among the first birds to die from the disease, giving an indication of where it is present, experts told The Associated Press.