Final answer:
The second key question geographers would ask about bird migration, besides "where are the birds going?" is "why are they going there?", which helps explain migratory patterns and informs conservation efforts.
Step-by-step explanation:
Geographers use two key questions to study bird migration: "Where are the birds going?" and "why are they going there?" The correct answer to complete the sentence in the question is D. "when are the birds traveling?" By understanding the "why" behind the location, geographers can infer the reasons behind migratory patterns, such as seeking warmer climates, finding food, or mating. Migratory birds have well-established routes known as flyways, and these patterns are influenced by various environmental cues ranging from landmarks to the position of the sun. This knowledge is vital for conservation efforts and understanding global changes.