Learn Spanish (Latin America): Hay más bandadas migratorias de las que pensábamos. - There are more migratory flocks than we thought.

When you compare a noun (here, bandadas) with a conjugated verb phrase (here, pensábamos), you cannot just use que. Instead, you use de + a definite article + que. Because bandadas is feminine and plural, the article becomes las, giving us de las que.

Would it be wrong to say más bandadas que pensábamos?

Yes, that would be incorrect in Spanish. You use más... que when comparing things like two nouns (for example, "more dogs than cats"). But when the second half of the comparison is an entire clause with a conjugated verb ("than we thought"), you must use the de las que structure to link back to the noun.

What exactly does las agree with in de las que?

It agrees with bandadas migratorias (migratory flocks). Since bandada is a feminine noun and it is plural here, the article has to match it, making it las. If the sentence were about pájaros (masculine plural birds), it would change to de los que.

Why use the imperfect tense pensábamos instead of the preterite pensamos?