Final answer:
The correct verb form to complete the sentence is 'caminábamos.' It agrees with the subject 'nosotros' and indicates an ongoing action in the past.
Step-by-step explanation:
The correct verb form to complete the sentence is caminábamos.
In the sentence, 'Mientras nosotros caminábamos por la selva, vimos muchas aves con pelajes coloridos,' the subject 'nosotros' indicates a plural form, so we need to use the verb form that corresponds to 'we'.
The verb 'caminábamos' is the correct form in this case because it agrees with 'nosotros' and indicates an ongoing action in the past.
Learn more about Verb forms in Spanish