What words does the poet use to show the birds' strength in the third stanza?

The strength of the thrush is defined by, and enriched by, its "joy." The intensity of the thrush's joy at existing seems to touch the poet, despite his general air of melancholy; note that the thrush's joy seems to have no end—it is "illimited," infinite. This word is extremely emphatic. Other words showing the strength of the thrush pertain to its song, which is "full-hearted." This description once again suggests that the thrush has committed its entire self to expressing its joy; another word which works in conjunction with this descriptor is the active verb "fling." The bird, in thus "fling[ing]" its soul into its song, is committing a dynamic act. Although the thrush itself is small and old, then, its song, born of its happiness, is intense; the bird has committed to it entirely.