A Sixth Sense

ORIGINAL: The Scorpion and the Frog

ancient Egyptians used carrier pigeons, the domesticated descendants of wild rock doves, to carry urgent messages to distant lands. They proved to be cheaper, faster and more efficient than human messengers and their use spread throughout the Mediterranean, central and northern Europe, and then throughout the world. Yet it wasn’t until the mid-1800s that scientists began to ask how they do it. To this day, how animals accurately navigate on long migrations is still one of biology’s great mysteries. 

A modern day rock dove.
Photo by Ingrid Taylar at Wikimedia.

That’s not to say science hasn’t made a lot of headway on this investigation. Scientists have found that some animals learn landmarks when they travel in one direction, and use those landmarks to find their way back. Some animals follow odor cues. But one of the more intriguing theories is that animals have an internal map and compass… but not a literal map and compass. The “map” is how the brain knows where things are in relation to each other and the “compass” is how the animal knows what direction it is facing with respect to where it wants to be.

How might such a compass work? One internal compass is a sun compass, in which an animal can use the position of the sun and the time of day to determine what direction it is facing. Some of the most convincing evidence supporting the existence of such a compass is that pigeons that are kept in a room with a time-shifted light cycle will fly in a predictably wrong direction on a sunny day. They will fly this wrong direction for long distances and even when they can see known landmarks. So pigeons clearly rely on the sun to achieve their great navigational feats… But what do they do on cloudy days… or at night? 

A cartoon of the Earth's magnetic field by Zureks at Wikimedia. In reality, the directions of magnetic pull are not this straight and uniform, but you get the idea.

Maybe pigeons have another compass based on the Earth’s magnetic field. Over 30 years ago, researchers found that racing pigeons (the new profession of decedents of carrier pigeons after mailmen in trucks and airplanes took their previous jobs) arrive at their destinations a little bit later when there have been recent magnetic storms due to sunspots. Pigeons also get disoriented in places with magnetic anomalies, such as areas with lots of iron ore. But experiments in which researchers have placed magnets or magnetic coils on the backs, wings, necks, heads or legs of pigeons have not had consistent effects, particularly on sunny days (when the sun compass likely comes into play).

Enter Cordula Mora and Michael Walker from the University of Aukland, New Zealand. Cordula and Michael reasoned that because the magnetic field gets weaker with distance and because we think that magnetoreception (the ability to perceive magnetic fields) occurs in or around the head, maybe these previous studies had inconsistent results because the magnets used were too far away and/or too weak to affect the receptors in a consistent way. So they did their own study with smaller, stronger magnets applied to pigeon beaks. 

Cordula and Michael glued magnets to the cere of pigeons. The diagram on the left shows how they did it and the photo on the right is a pigeon showing off his new nosepiece. (Check out the rock dove image above to see what a naked cere looks like). Diagram and image from Mora and Walker 2012 Animal Behaviour paper.

Cordula and Michael glued either a magnet or a brass weight (as a control) to the cere (the fleshy upper-part of the beak that contains the nostrils) of experienced racing pigeons right before a flight. In one experiment, researchers released the pigeons 11 consecutive times from the same place (called Gernsheim), and alternated whether they had a magnet or a brass weight glued to their cere. In a second experiment, the researchers released every bird once from each of 25 different places, each time with either a magnet or a brass weight glued to their cere. The birds were always flying to the same place (their loft), but the direction and distance they needed to fly was different for each of the release sites. Thus, the first experiment provides the birds with an opportunity to learn and compensate for any effect of the magnet, while the second experiment does not. All of the flights were done on sunny days. 

The places the researchers released the pigeons from were all different directions and distances from their home loft (at the center). Diagram from Mora and Walker 2012 Animal Behaviour paper.

For each flight, the researchers watched the bird through binoculars until it vanished from view, at which point they recorded the vanishing bearing (the direction the pigeon was flying before it vanished from view). They also timed how long it took the bird to return to the loft and recorded any instances in which the bird did not return to the loft.

The pigeons with magnets consistently flew just a little to the right of pigeons with brass weights. The effect was very small (ranging from 11° to 22°), but it almost always happened, regardless of the bird or the release site. The effect was also consistent over consecutive years, even in birds tested repeatedly from the same release site. However, although the vanishing bearing of birds with magnets was regularly to the right of the birds with brass weights, the magnets did not prevent the pigeons from finding their loft and did not even cause them to take longer to get home. This shows that some time after the vanishing distance, the pigeons with magnets compensated for their originally slightly-off bearing.

The fact that the pigeons with magnets almost always started off flying too-far right suggests that they do have and use a magnetic compass, or perhaps even a magnetic map. But the fact that they always got back to the loft just as fast as their brass weight carrying counterparts shows that they also rely on other mechanisms, like a sun compass and landmarks. Perhaps magnetoreception is only important to determine take-off direction. Or alternatively, maybe the birds learn to ignore the confusing signals of the magnets after awhile.

We still have a lot to learn about how animals use magnetic fields. And how does magnetoreception even work? How animals navigate over long distances is still a great mystery, but scientists are on the case.

Want to know more? Check these out:

1. Mora, C.V., & Walker, M.M. (2012). Consistent effect of an attached magnet on the initial orientation of homing pigeons, Columbia livia. Animal Behaviour, 84, 377-383 DOI: 10.1016/j.anbehav.2012.05.005

2. Wiltschko, R., & Wiltschko, W. (2003). Avian navigation: from historical to modern concepts. Animal Behaviour, 65, 257-272 DOI: 10.1006/anbe.2003.2054

3. Bingman, V. P., & Cheng, K. (2005). Mechanisms of animal global navigation: comparative perspectives and enduring challenges. Ethology Ecology & Evolution, 17, 295-318 DOI: 10.1080/08927014.2005.9522584

4. Winged Migration, a fantastic movie by Jacques Perrin