Final answer:
'Beautifully,' in the given sentence, is an Adverb. It modifies the verb 'chirping' by indicating how the chirping was done.
Step-by-step explanation:
In the sentence, 'The birds were chirping beautifully,' the word 'beautifully' serves the role of an Adverb.
Adverbs modify verbs, adjectives, or other adverbs, and in this case, 'beautifully' modifies the verb 'chirping' by indicating how the birds were chirping. So, 'beautifully' is an Adverb.
Learn more about Adverbs