In olden times, when people tended to spend their whole lives in one small area, people didn't know yet that birds migrated south for the winter. So they came up with a lot of ideas. Swallows lived under the sea for half the year. Other birds moved to the moon during that time. But my personal favorite was the claim that geese died off every year, and then grew again on trees in the spring.

Gerald of Wales even claimed to have seen this himself, bc the 1100's were good times for shitposting.