1.
birds, rabbits
A counter for birds and rabbits. Historically, rabbits were counted with this counter because monks could eat rabbit meat if they called them 'birds.'
羽 is the counter for birds and rabbits.
Counting readings
Note the sound changes: ば after ん, ぱ after っ.
Etymology
The origin of counting rabbits with 羽 is traditionally attributed to Buddhist monks: since eating meat was forbidden, they classified rabbits as "birds" (鳥) so they could eat them. While this folk etymology is well known, the actual origin is debated.
Usage
Used for all types of birds, from small songbirds to large birds like cranes. Also used for rabbits but not for other small mammals.