If they laid eggs you would be able to find them someplace. People were created from dirt, birds fly close to heaven, they appear after the winter thay when the mud is exposed. It is equally as reasonable to a person who couldn't imagine a world as large as the range those birds migrate.
The scientific method hasn't been invented yet.
We got people today that you can't convince of the conservation of matter and energy. Good luck convincing anyone else of that.