Learn Italian: L'aquila è volata al nido. - The eagle flew to the nest.

This is the core rule for verbs of motion like volare (to fly), correre (to run), or saltare (to jump). When the sentence includes a specific destination—in this case, al nido (to the nest)—you must use essere because the focus is on the change of location. If the eagle were just flying around as an activity without a stated destination, you would use avere (ha volato).

Why does the past participle end in an "a" (volata)?

Whenever a compound tense is formed with the auxiliary verb essere, the past participle must agree in gender and number with the subject. Since aquila (eagle) is a feminine singular noun, the participle changes from volato to volata.

Words starting with l' can be masculine or feminine. How can I tell which one aquila is here?

The grammar in the rest of the sentence gives it away! Because this sentence uses the auxiliary essere, the past participle matches the subject's gender. The -a ending on volata tells you definitively that aquila is feminine.

Why do we use al nido instead of just a nido for "to the nest"?