This is the 14056th most frequent Russian word.
одну
"Одну" means "one" in the accusative feminine singular form in Russian.
Here, 'одну' is used to emphasize the number and singularity of the bird as a direct object in the accusative case.
Я видел одну птицу в небе.
I saw one bird in the sky.
In this sentence, 'одну' highlights the act of selecting a single item from a group, showing its accusative case agreement with 'книгу'.
Она выбрала одну книгу из списка.
She chose one book from the list.
Here, 'одну' is part of a descriptive phrase emphasizing a single girl, in the accusative case modifying 'девочку'.