The mother bird bought food for her kids to the nest

food is bought by the mother bird for her kids to the nest

food was bought by the mother bird for her kids to the nest

food was bought by the mother bird for their kids to the nest

None

Convert the object of the Active Voice Sentence into the subject of the Passive Voice Sentence.

Use helping verb was/were according to the subject of the Passive Voice sentence.

Use 3rd form of the verb.

Then use the word “by”, to show the doer.

Then convert the subject of the Active Voice Sentence into the object of the Passive Voice sentence.

In the end, add the remaining words if there are any in the Active Voice Sentence.