What quality does the nightingale display in the poem by William Cowper?

In the poem "The Nightingale and Glow-Worm" by William Cowper, the nightingale displays the quality of perseverance in its song, despite facing perceived competition from the glow-worm. The nightingale's dedication to its natural instincts and expression remains undeterred, showcasing a resilient and unwavering spirit.