Each spring and autumn, hundreds of millions of birds across the Northern Hemisphere undertake journeys that ought, by ordinary measures, to be impossible. A garden warbler weighing less than an ounce navigates from sub-Saharan Africa to a particular hedge in Sweden; an Arctic tern circles between polar regions, recovering its bearings without map or instrument. That birds find their way is uncontroversial; how they do so has occupied biologists for more than half a century, and the answer that has emerged is stranger than the question first invited.
A range of cues are now known to be in play. The position of the sun by day, the pattern of stars at night, polarised light at dusk and dawn, infrasound from distant coasts, and the smell of the air all appear to contribute, in proportions that vary by species and by stage of life. Most striking among these is the magnetic sense, which allows certain birds to perceive the direction and inclination of the Earth's field. The mechanism by which this perception is accomplished is still debated, but a leading proposal locates it in the bird's eye, where pairs of light-sensitive molecules are thought to undergo brief quantum-coherent states whose statistics depend on the orientation of the local field. The retina, on this view, is doing double duty as a compass.
The proposal is striking precisely because it pushes a delicate quantum process into a warm, wet biological setting, where such processes were, until recently, thought to be too fragile to last. Cryptochrome molecules — found in the retinas of many migratory species — appear capable, in laboratory analogues, of producing the kind of signal required. Yet whether the in-vivo behaviour matches the in-vitro promise remains to be settled, and several rival accounts, including iron-based mechanisms in the upper beak, are not yet excluded. The honest position is that something interesting is happening, that part of it is likely magnetic, and that the precise molecular story is, for the moment, more conjectured than confirmed.
None of this diminishes what the migrating bird accomplishes. The various cues, however generated, must be integrated into a single representation of which way to go, and that representation must survive cloud cover, contrary winds, and the bird's own fatigue. The investigation will continue to surprise, as it has so far, and the most interesting surprises are likely to come at the meeting points of fields — biology, chemistry, physics — that until recently had little reason to address one another.
Which one of the following best captures the main idea of the passage?