premise: The domestic poultry were likely to have caught the virus after coming into contact with migratory birds, officials said.
  The cases are near the Sivash saltwater lake in Crimea's northeast, a popular stopover route for millions of migratory birds from Russia's Siberia and Volga regions, as well as Asia, Verbitsky said.
  "The virus is behaving differently than in Russia and Romania," Ukraine's neighbors where bird flu has also been detected, said Verbitsky.
  "The sickness passes without any symptoms," he said. "In the evening the birds are healthy and in the morning they are dead."
  Although the virulent H5N1 strain has not yet been detected in Ukraine, the country has banned the import of poultry and its derivatives from Croatia, Romania and Turkey, as well as several regions of Russia, along with imports of live wild and pet birds.
   What does "behaving" refer to?
hypothesis: coming into contact with migratory birds