 Desbouvrie kept his swallows in a cage and trained them to fly in and out of the cage . Another challenge he overcame was the birds ' natural migratory behavior . Desbouvrie believed that swallows migrated southward because the supply of insects for them to feed upon dwindled in winter . Desbouvrie refused to make public disclosure of what he fed the birds in wintertime and regarded that as a trade secret . Later attempts to train homing behavior into migratory birds determined that the birds have no instinctive knowledge of where to fly when the season changes , so if the normal migratory pattern is disrupted during the first year of life the individual does not migrate afterward . 
