Study: Pigeons Use Smell to Find Home

Aug. 14, 2006 — Italian researchers may have solved the puzzle of what makes homing pigeons such legendary navigators­ — they simply follow their noses.

In a real life homing experiment, Anna Gagliardo of the University of Pisa and colleagues tested the birds’ magnetic sensing and olfactory systems in order to establish how they make their extraordinary navigations across hundreds of miles.

The research, published in the current issue of the Journal of Experimental Biology, followed a 2004 laboratory study by Cordula Mora and colleagues from the University of Auckland, New Zealand.

In this research, Mora conditioned pigeons to detect an anomaly in a magnetic field. Mora showed that pigeons detected a magnetic stimulus in their upper beaks by using the ophthalmic branch of the trigeminal nerve (the largest cranial nerve).

The 2004 study reinforced the theory that homing pigeons navigate by using tiny magnetic particles in their beaks to map changes in the Earth’s magnetic fields.

But Gagliardo argues just because the pigeons have the ability, doesn't mean they always use it.

"They do have the ability to detect magnetic fields, but this doesn’t mean they use it to navigate," Gagliardo told Discovery News.

To test how much the birds use this sense, Gagliardo cut a section of the olfactory nerve in 24 homing pigeons and a section of the ophthalmic branch of the trigeminal nerve in another 24. A third group of 24 birds underwent sham operations and served as a control group.

Gagliardo then released the three different groups of inexperienced homing pigeons 30 miles from their loft.

All but one of the birds with the severed trigeminal nerve were home the next day, suggesting that the ability to detect magnetic fields is not used to navigate. Among the control group, only one pigeon was lost.

Meanwhile, most of the birds deprived of their sense of smell got totally lost. Only four made it home.

According to Gagliardo, homing pigeons create odor maps of the areas they fly over and use them to navigate — basically reading landscapes as a patchwork of odors.

"In my view this study certainly ends the debate for homing pigeons," Verner Bingman, a behavioral neuroscientist at Bowling Green State University, Ohio, told Discovery News. "It is now as clear as can be that information about the spatial variation in atmospheric odors detected by the olfactory nerve is the primary sensory basis of the homing pigeon navigational map."

However, Martin Wild, a neurobiologist at the University of Auckland who performed the surgical procedures for both the Mora and Gagliardo studies, cautioned that the Pisa experiment should not be considered the definitive study on on how pigeons navigate.

"Nature does not yield up her secrets so easily," Wild told Discovery News. "The birds will use whatever sensory cues are available at the time. Showing that pigeons actually use a magnetic sense is extraordinarily difficult."

Wild further points out that homing pigeons are thought to have two magnetic senses, not just one. One is in the beak and the other in the photoreceptors of the eye.

"Perhaps both are used under different circumstances," he said.