This is the 5474th most frequent Japanese word.
ぅ
The character "ぅ" is the small form of "う" used in Japanese to modify or extend vowel sounds in combination with other kana.
In this sentence, 'ぅ' is used as part of an interjection ('わぁぅ'), expressing surprise or a reaction.
びっくりしたとき、「わぁぅ！」と言うことがあります。
When surprised, one might say 'Wow!'
Here, 'ぅ' is part of an onomatopoeic representation ('ピィぅ') that mimics a bird's sound.
小鳥が「ピィぅ」と鳴いていました。
The small bird chirped 'piiu.'
In this example, 'ぅ' is used in an alternative form of 'う,' enhancing the word's phonetic nuance.