Here's a great read about homing pigeons where they do that sort of test [1]. The answer appears to be that they use multiple cues so it's hard to isolate them.

Here's a great read about homing pigeons where they do that sort of test [1]. The answer appears to be that they use multiple cues so it's hard to isolate them. This seems likely to me, since there just doesn't seem to be enough information in knowing the magnetic field at your location to determine your home location, which the this article claims they do (they don't need information from the trip out to find home, and they fly roughly straight home so they must know the home direction immediately upon start). Interestingly, they can't return home if you knock out their sense of smell. But maybe that accidentally knocks out 'compass sense' as well. This article has some great snippits: "Older pigeons transported to the release site inside sealed metal containers, supplied with bottled air, anesthetized and placed on rotating turntables, all of which should make it hard for them to keep track of their outward journey, still home." They can home from 1800km away! And after being away for 7 years! They can home even if they never left the home loft (presumably until being sedated and carried away to test their homing ability), though the success rate is low. Edit: more directly to your question is this quote: "Keeton reasoned that if his pigeons were well oriented under overcast they must be using a cue other than the sun compass.He found that pigeons with magnets fastened on their backs were often disoriented when released under overcast." and "Under a sunny sky, the coils, whatever their polarity, only deflect the pigeon’s vanishing bearings by a few degrees. If a pigeon wearing an NUP coil starts its homeward journey under overcast, it will head away from the release point in the direction away from home. Should the sun appear, even if only momentarily, through a hole in the clouds, it will reverse course and head directly for home." [1] https://www.researchgate.net/publication/13905901_Pigeon_Homing_Observations_Experiments_and_Confusions https://www.researchgate.net/publication/13905901_Pigeon_Hom...