。
Observing wild birds is really interesting.
Literal
Wild-bird [obj-を] observe-[nom-のは] very interesting.
Expresses the activity-is-fun idea with the verbal frame [verb]+のは+adjective rather than a nominal frame [noun]+は+noun+だ. Both are grammatical, but the verbal frame puts the activity itself in focus — 'observing wild birds' as an action — rather than 'birdwatching' as a category.