Where to Find and Catch Swellow
From Pokemon Go Wiki
Swellow are found and captured in Nest and Habitat locations.
Nests
Swellow Nests are locations that consistently spawn Swellow.
Habitats
Swellow Habitats are spawn locations where Swellow is likely to be encountered, but not guaranteed the way a Nest is.
Additional Tips and Information
Best Nests
If you know a really good nest or any of the nests above is really good, list them here: