Where to Find and Catch Tranquill
From Pokemon Go Wiki
Tranquill are found and captured in Nest and Habitat locations.
Nests
Tranquill Nests are locations that consistently spawn Tranquill.
Habitats
Tranquill Habitats are spawn locations where Tranquill 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: