Homing pigeons commonly fly hundreds of kilometers (even as much as a thousand kilometers) during races from remote release sites back to their home lofts. They perform amazing feats of navigation, and while we have lots of theories about how they do it, the best explanation may be a method that's entirely new to our experience: infrasound.
It's been reasonably well established that homing pigeons, like many other birds, have iron particles in their beaks that they can use to detect and follow magnetic field lines, like a compass. They also probably use the sun to point them in the right direction. But direction is only half of the homing pigeon equation: if you've ever used a map, you know that there's two important pieces of information that you need to know to figure out how to get home from somewhere: you need to know which directions are which (with something like a compass), and you need to know where home is relative to where you are (with something like a map). So no matter how good homing pigeons are at picking out direction cues from magnetic fields and the sun, it's not going to do them any good unless they have both a "compass sense" and a "map sense."
Pigeon Flight School
Now, it's not like you can just grab a homing pigeon and put it in a box and take it anywhere and it'll be able to get back home. The birds have to first be trained, which involves releasing them at steadily increasing distances from their home loft, from a variety of directions. This suggests that the birds have to incrementally create some sort of map: in other words, pigeons don't operate on an absolute GPS coordinates principle, but rather something more relative, like landmarks.
When we say "landmarks," you're probably thinking that the birds use the same sorts of features to navigate that a human might: mountains, rivers, forests, and even buildings and roads. And you'd be right, because research has shown that the birds definitely do this over areas that they're familiar with, to the extent of making 90 degree turns to follow highway interchanges. However, research has also shown that if you outfit a homing pigeon with frosted contact lenses that prevent it from seeing much more than the horizon line, it'll still be able to find its way home without much trouble, so there's obviously more going on here.
A variety of other localization methods have been suggested for homing pigeons, but none of them seem to really explain everything that these birds can do. Maybe the pigeons create a map of gradients in the Earth's magnetic field, but they seem to have no issue flying around or through areas with magnetic anomalies or during geomagnetic storms. Or maybe the pigeons use their sense of smell, but the ability to get home from hundreds of miles away with a consistent tailwind seems to make that unlikely. So, we've been over just about all of a pigeon's senses, here: they seem to be able to navigate without relying exclusively on sight or smell or magnetism, so what does that leave?
How about sounds?
Picking Up The Bass Line
Initially, it seems sort of nutty: with a few possible exceptions, specific places tend not to have unique sounds that we can hear, and even if they do, those sounds don't travel far enough to be useful in navigation. But pigeons can hear sounds that we can't: specifically, they can hear extreme low frequency sounds, called infrasounds. You can think of an infrasound as a sort of ultra deep bass, with a frequency so low that you can't hear it, but you can sort of feel the vibration. And as you've probably noticed with bass, the deeper the note (the lower the frequency), the farther it can travel, which is why the bass line of music cuts through structures like buttah. Human hearing bottoms out at about 12 Hz, but pigeons can hear down to 0.05 Hz.
So that's great, pigeons can hear low frequency sounds. But that's not going to help them develop a map sense unless there's some sort of correlation between these sounds and places on a map. Probably not surprisingly (since we bothered to write this article), it turns out that this may be the case.
It would work like this: the Earth has a constant seismic background "hum" at about 0.14 Hz, caused by ocean waves interfering with each other. This hum (it's called a microseism) is detectable thousands of kilometers from the oceans, and you can pick it up pretty much anywhere on Earth. In of itself, the hum doesn't contain much in the way of information that a pigeon could use to navigate, but it does get effected by topographic features: things like mountains and valleys, with their steep sides, may directionally amplify the Earth's microseism in such a way as to provide infrasonic navigational cues. In other words, homing pigeons may be be able to hear subtle differences in the background hum of the Earth caused by topographic features, and then home in on those sounds from tens or hundreds of kilometers away, like radio beacons, to get where they're trying to go.
So it may very well be that homing pigeons seem to follow landmarks because the landmarks emit sounds, and not because they're visible during flight, a suggestion that's backed up by the fact that the birds can somehow find their way around when flying between layers of clouds that completely block the view of the ground.
As with any theory, it's not going to be very convincing without some way of testing it, but the dude who came up with this idea, a United States Geological Survey geologist named John Hagstrum, has some compelling evidence.
To try and figure out whether homing pigeons were indeed navigating with infrasound, Hagstrum took at look at some of the more notable times in which homing pigeons failed to reach their destination en masse. One example happened in 1997, when some 60,000 pigeons from England were released all at once from Nantes on the western coast of France. A trip across the English channel is usually hatchling's play to a pigeon, but in this case, virtually none of the birds made it home, and Hagstrum thinks that the reason why is that just as the birds were crossing over the water, the Concorde was accelerating towards New York from Paris in the same area. The shockwave from the supersonic aircraft could easily have distrupted the sensitive low-frequency hearing of the birds, causing most of them to become disoriented and return either very late or not at all.
Hagstrum also found several other homing pigeon races that were partially or completely disrupted when a Concorde was in the area, both in Europe and on the west coast of the United States. Correlation doesn't imply causation, of course, but it certainly suggests that infrasound is involved, so Hagstrum did more research. In a forthcoming paper in the Journal of Experimental Biology, Hagstrum used acoustic modeling based on weather and topography to investigate homing pigeon release sites known to be especially confusing for the birds.
He found that certain combinations of weather patterns and topography can consistently create acoustic "shadows" that block infrasound from a given direction, causing homing pigeons who are trying to navigate in that direction to become lost. In one particular case, Hagstrum was able to show through his model that the reason why birds that always got lost from one site — but didn't on one specific day (August 13, 1969) — was because a lucky combination of wind and temperature that allowed for perfect infrasound transmission through the air. Hagstrum's model also explains why pigeons would depart in the wrong direction from some sites: topography and wind can combine to make infrasounds from one direction seem as though they're coming from another direction, and the pigeons will always fly directly towards them.
Other Observations Of Infrasound
There are some other reasons why the general idea of animals using infrasonic sounds to navigate would help explain a lot of things. For example, infrasonic navigation would work underwater, which would explain how whales and sea turtles can get around in an otherwise featureless ocean. And infrasonics would also explain why animals often freak out before earthquakes: they can actually hear them coming in extremely low frequences.
This is a fascinating theory that's taken Hagstrum decades to find evidence for, and it's remarkable to think that these animals have a method of long-distance navigation that's so effective and yet so entirely unlike anything we've ever even considered. It'll take some additional testing (and likely quite a few confused pigeons) before the infrasound theory is conclusively proven, but as far as we can tell, it's the best explaination yet of how homing pigeons can do what they do.