Opie learns about responsibility after killing the mother of three baby songbirds ("Opie the Birdman," Episode #101). He names the three birds Wynken, Blynken and Nod.
With Barney's help in translating bird talk, Opie raises the birds until they're old enough to fly away on their own. The cage seems empty, but as Andy points out, "Don't the trees seem nice and full."