Comment by pyrale

European swallows migrate to Africa in winter [1], they move through Spain and the middle-east, and a few more adventurous through Italy and Sicily ; and that's why they "bubble up" there first in this view. But you can see that, in the dead of winter, barely any swallow is left in the peninsula.

It would be nice to see the dataset extended to Africa and the middle-east.

[1](swallow migration patterns): https://tbhpartnership.org.uk/news/migration-marvels-part-2-...