Fill in the blank with the correct form of the imperfect tense.
los pájaros (volar) sobre los árboles.

In Spanish (Castilian) the imperfect tense (indicative mode) is used to:  

1. talk about past habitual actions (adding used to in its different forms)

2. describe what someone was doing when it was interrupted by someone (or something) else. In fact, is more a combination of the past tense and a continuous or repeating aspect. Frequently, this includes a sense of incompletion in the verb, but not always. (Is the equivalent to the Past Progresive Tense in english)

In other words:

It is specially used to describe what used to happen or what was happening.

Now, in this sentence we are talking about birds (pájaros/aves) which is a plural noun.

Then, the imperfect tense of the spanish verb volar (to fly) with the third person in plural ellos (they) is volaban.  

Therefore the complete sentence is:

The birds were flying over the trees