Some birds follow roads instead of flying direct.
Researchers may have discovered how pigeons find their way along familiar routes. Instead of heading straight for their destination, they follow main roads, railways and rivers.
Tim Guilford and colleagues from Oxford University fitted more than 50 homing pigeons with tiny tracking devices. They then monitored them, second by second, as they made the familiar journey back to their loft.
Some of the birds that used landmarks did so again and again on separate occasions, says Guilford, following a set path to theirloft. "One pigeon flies along the road to the first roundabout, takes the third exit, goes along the dual carriageway to the next roundabout, then leaves the road and goes cross-country," he says.
The birds can add an extra 20% or more to their journey by following these features, says Guilford. It may be more demanding physically, he says, but easier mentally.
Other birds probably use a similar strategy. Even crows may not fly 'as the crow flies', says Guilford.
Compass or landmarks?
Pigeons are thought to use a combination of the Sun, the Earth's magnetic field and their sense of smell to find their way around, says Roswitha Wiltschko from Frankfurt University, Germany, who studies pigeon navigation. The birds have tiny particles of magnetite - an oxide of iron - in their beak, which may allow them to use the Earth's magnetic field to find their way.
They may also have 'magnetoreceptors' in their eyes - specialized proteins that can detect the direction of the Earth's magnetic field. Other birds, such as the European robin, use such proteins to navigate.
Guilford argues that pigeons probably use different navigational tools during different parts of their journey. He and colleague Stephen Roberts developed a mathematical model that predicts when pigeons will swap from one technique to another1. The model, based on the flights of 12 birds, reveals that pigeons are most likely to use landmarks in the middle of their journey.
Pigeons may rely more on compass-like tools when starting their trip, says Guilford. This may help to get them going in the right general direction. They could then switch to using landmarks when they need to reassess their route and make corrections to their bearings.
But the results are controversial. Wiltschko says she has never observed pigeons flying over roads.
People use different methods to track pigeon flight, counters Guilford. Some follow the birds in a helicopter. Others measure their heading as they disappear over the horizon. Some methods aren't that precise, he says, so other researchers may have missed the effect.
Guilford, T., Roberts, S. & Biro, D. Positional entropy during pigeon homing II: navigational interpretation of Bayesian latent state models. Journal of Theoretical Biology, published online, (2004).
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Current Biology (2004)