How Migrating Birds Find Their Way

AUntil well into the twentieth century the standard explanation of bird migration was a social one: young birds were assumed to learn the route by flying in the company of adults who had made the journey before, much as an apprentice learns a coastline from a pilot who already knows it. For a handful of large species that travel in family parties, cranes and geese among them, the assumption happens to be correct. It cannot, though, be stretched to cover the great majority of songbirds, which cross continents alone, at night, in their first autumn, and frequently set out a fortnight or more after the last experienced adult of their population has left. A juvenile pied flycatcher weighing twelve grams leaves a Finnish forest without a companion and arrives some five thousand kilometres later in a strip of West African woodland. Nothing about that performance can have been shown to it beforehand. The question that has occupied ornithologists since the 1960s is therefore not whether navigational information is inherited, but how much of it is, and what must still be supplied by the bird own experience.

BThe classical answer, assembled from birds kept in circular cages during the season when they would otherwise be flying, is a mechanism usually called clock-and-compass: an inherited heading, maintained for an inherited length of time, with a change of course written in at the point where the route bends. Hand-reared warblers that had never seen a migrating adult, placed in funnels whose paper lining records the direction of every hop, oriented south-west throughout September and then, during the final week of October, turned towards the south-east, exactly the dog-leg their free-flying relatives make around the eastern Mediterranean. Between 1979 and 1986 the ringing station at Trelleborg logged that autumn switch in 214 of 240 birds raised in isolation. The programme is elegant, and it is certainly inherited. Its limitation is just as plain: a heading held for a fixed number of nights delivers a bird to the intended place only if nothing disturbs the flight, and something usually does. A single night of easterly wind may carry a bird of twelve grams two hundred kilometres from its track, and no degree of loyalty to the original bearing will retrieve those kilometres.

CWhat a displaced bird needs is not a compass but a map, some means of establishing where it now is in relation to where it ought to be. Ingeborg Halvorsen, of the Nordvik Institute of Avian Physiology, maintains that for most long-distance migrants the map is magnetic, read from two properties of the earth field that vary along different axes: the angle at which the field lines meet the ground, which changes steadily with latitude, and total intensity, which changes far less regularly. Her group tests the claim by simulating a displacement rather than performing one, placing a bird in a coil room where the field of a site far to the east is reproduced while the bird itself never leaves the building. Of 96 adult reed warblers treated in this way in the springs of 2015 and 2017, 71 reoriented towards the west-north-west, the bearing that would have cancelled the displacement had it been real. Juveniles on their first journey did nothing of the kind. The response, Halvorsen concedes, emerges only in birds that have already completed at least one return migration, which she takes as evidence that the map is calibrated by experience rather than issued at hatching.

DDermot Rylance, who runs the field station at Carrickfergus, is sceptical of conclusions drawn inside coil rooms and contends that a simulated displacement is not a displacement at all. A caged bird, he argues, is deprived of everything that would normally accompany a change of position, from unfamiliar odours to a shifted sunset azimuth, so an altered direction of hopping shows only that the animal noticed a change in the field, not that it derived a position from it. His own experiments point elsewhere. Warblers whose nasal passages had been temporarily blocked failed to compensate for a genuine displacement of 300 kilometres, whereas birds carrying magnets on the head compensated as well as untreated controls. Rylance therefore attributes the map to smell, a conclusion his critics have found hard to dislodge over land. At sea it is another matter, and he concedes the difficulty, since birds crossing eight hundred kilometres of open water have no gradient of vegetation odour to work with and yet arrive. His trials, moreover, used a single species, ran for two spring seasons, and released every bird within one stretch of coastline.

EThe disagreement is narrower than it looks, because the two accounts answer different halves of the problem. Nobody doubts that the compass is inherited and that a first-year bird runs a programme; the dispute concerns the map, which both sides accept must be assembled after the animal has moved through the world at least once. The most defensible reading of the evidence at present is that both kinds of gradient are used, in proportions that depend on where the bird happens to be. Over continental interiors, where odour plumes are rich and the magnetic field is distorted locally by rock, smell probably carries most of the load; over the ocean, where the mechanism Rylance favours has nothing to act upon, the magnetic gradients are all that remain. Halvorsen and Rylance now agree at least on the sequence: whatever the map is made of, it is put together during the first journey and refined on every one that follows. The juvenile flycatcher, on that view, is not navigating at all. It is gathering the information with which it will navigate the following year.