How Do Homing Pigeons Find Their Way Home?

Fri Oct 30 · Science

Short answer: probably several senses at once, layered together: the sun read against an internal body clock, a map that may be built from smells on the wind, the familiar landscape near home, and Earth's magnetic field as a backup on cloudy days. [1][2]

Put a homing pigeon in a box, drive it hundreds of kilometers to a place it has never been, open the lid, and it will fly home. [1]

It doesn't have a map. Nobody showed it the route. It may circle once or twice, pick a direction, and go. Scientists still argue about exactly how it works.

That's the delightful part. The bird on your windowsill eating a french fry carries one of biology's longest-running unsolved puzzles.

Map and compass

To get home from somewhere new, you need two things. A map tells you where you are relative to home. A compass tells you which way to fly once you know. Researchers studying pigeons generally break the problem into those two pieces. [1]

The compass part is the better understood of the two.

The sun is a clock problem

Pigeons use the sun as a compass. [1] But the sun moves across the sky all day, so using it for direction means you also have to know what time it is. The pigeon needs a clock as well as a compass.

Scientists tested this in a wonderfully sneaky way. They kept pigeons under lights shifted by several hours, so the birds' internal clocks were out of sync with the real sun. Then they released them. In unfamiliar places, the clock-shifted birds headed off in the wrong direction, off by roughly the angle the sun would have moved in that many hours. [2]

So the bird reads the sun, checks its body clock, and works out a heading. On cloudy days, research suggests it can fall back on Earth's magnetic field as a backup compass. [1]

The map might be made of smell

The map is where it gets strange.

In the early 1970s, the Italian researcher Floriano Papi and his colleagues put forward the idea that pigeons work out where they are by smell, reading the trace chemicals carried on the wind. [4] A 1988 review summed up the idea: the map part of the system uses the smell of airborne compounds that carry positional information "up to several hundreds of kilometers from home." [1]

In Papi's experiments, pigeons whose sense of smell was cut off were much worse at getting home. [4][5] A bird building a map of the world out of what the wind smells like is still one of the most surprising ideas in animal navigation, and it's been argued over for decades.

Near home, where the landscape is familiar, pigeons also seem to steer by what they can see: the familiar landscape itself. [2]

The compass in the beak that wasn't

For years, a leading idea was that pigeons had a magnetic sensor in their beaks: clusters of iron-rich cells, packed with magnetite, wired into the nerves. Earlier studies described six specific spots in the upper beak, and the idea was "widely accepted in the field." [3]

Then, in 2012, a team led by David Keays looked closely and published in Nature. Those iron-rich cells, they found, were macrophages, immune cells that collect iron, not magnetic sensing neurons. Their number and position varied from bird to bird, which a dedicated sense organ shouldn't do. [3]

They didn't say pigeons can't sense magnetism. They said the search for the real sensor had to start over. [3] It's still going.

The lone bird and the long argument

There's something very True Detective about a mystery this old being this unsolved. Generations of researchers have chased the same bird through the same sky, built elaborate experiments, published in the best journals, and still disagreed about the most basic question: what is it actually sensing?

And there's something very Tyson about the answer so far: probably several things at once. Sun, clock, smell, sight, magnetism, layered together into a sense of direction we don't have a word for.

Some of us know what it's like to navigate by cues nobody else seems to notice. The smell of a place. The angle of the light. A pull toward home that's hard to explain and never wrong.

The pigeon just does it with wings.

Sources

1. Wallraff, H. G. (1988). "Navigation by means of an olfactory map and a sun compass: the homing ability of pigeons" [review, in German]. Naturwissenschaften 75(8). PubMed: https://pubmed.ncbi.nlm.nih.gov/3065647/
2. Wallraff, H. G. et al. (1994). "The Role of Visual Familiarity with the Landscape in Pigeon Homing." Ethology 97. https://onlinelibrary.wiley.com/doi/10.1111/j.1439-0310.1994.tb01025.x
3. Treiber, C. D. et al. (2012). "Clusters of iron-rich cells in the upper beak of pigeons are macrophages not magnetosensitive neurons." Nature 484. https://www.nature.com/articles/nature11046
4. Papi, F. (1982). "Olfaction and Homing in Pigeons: Ten Years of Experiments." In Avian Navigation, Springer. https://doi.org/10.1007/978-3-642-68616-0_15
5. Gagliardo, A. (2013). "Forty years of olfactory navigation in birds." Journal of Experimental Biology 216(12). https://pubmed.ncbi.nlm.nih.gov/23720797/