ScienceWeek

ANIMAL BEHAVIOR: ON ANIMAL NAVIGATION

The following points are made by James L. Gould (Current Biology 2005 15:R653):

1) Juvenile birds regularly migrate thousands of kilometers. Most fly at night, without their parents to guide them. The mystery of how birds manage this apparent impossibility inspires ever-more-heroic attempts to defeat them at this crucial task. New work [1] reports a dramatic new way to confound south-bound sparrows: take them along or above the Arctic Circle aboard an icebreaker; fly them to experimental sites by helicopter; and see what directions they choose. Only time will tell whether, once again, the birds have another layer of yet-to-be-deciphered finesse in their navigation repertoire, or if they finally have been driven to the computational wall and are responding in a consistent "does-not-compute" manner.

2) Exactly what does a migrating species require? First, the birds need a compass to know which direction they are going, and they come supplied with several: the earth's magnetic field (indicating magnetic north); the celestial pole (indicating true north, about which the stars appear to rotate at night); and the Sun's location (as inferred from patterns of sun-centered polarization which, with a suitable time sense, also specifies true north). Second, they need to know at least roughly -- and in some instances quite precisely -- where they are relative to their goal. In the case of homing pigeons, this ability is known as a map sense, and has a resolution of a very few kilometers [2].

3) The map sounds quite mysterious compared to the compass, but they are both daunting challenges. Consider the problem from the bird's point of view. First of all, it is cloudy a lot, so much of the time you can forget about using celestial cues. But then, why not just use magnetic north? If you are born at a high latitude -- where large numbers of species breed -- there is often a large discrepancy between magnetic and true north -- the declination error, which arises in part from the 1400 kilometer separation of this point from the geographic pole. Worse, declination generally changes as you fly south. And even if the evening is clear, the stars and patterns of polarized light change with both latitude and date.

4) Birds dispose of these problems by periodically calibrating one compass against the other [3;4]. Recent evidence has shown that when the sky is clear, the recalibration occurs daily, and takes only about an hour [5]. The accuracy and sensitivity of this system is astonishing: in tests performed near the conventional north magnetic pole -- where the earth's field lines plunge vertically into the planet, providing no directional information at all -- birds are well oriented just a few dozen kilometers away, where there is only a 1.1 deg deviation from vertical. (There is another "pole", one of magnetic intensity, located about 2000 kilometers farther south near the western edge of Hudson's Bay, which is critically important in magnetic-map theories.)

5) But how do the birds know which way to fly in the first place. Most species have an innate starting direction: put them as juveniles or adults in a cage during migration season, and they will all try to escape in roughly the direction they ought to fly to get to their wintering grounds. But there is a danger here in assuming that the birds know only as much as their behavior suggests: some species display accurate departure directions in cages, while others select a consistent but quite incorrect one (west into the sunset). But release the same "misguided" birds with tracking beacons, and they set off in the direction they ought to have chosen in the cage. The results of Åkesson et al. [1] take on more meaning in this light. They found that sparrows moved gradually east above the Arctic Circle completely altered their migration strategy after encountering the massive natural change in declination near the magnetic pole.

References (abridged):

1. Åkesson, S., Morin, J., Muheim, R., and Ottosson, U. (2005). Dramatic orientation shift of White-crowned Sparrows displaced across longitudes in the high arctic. Curr. Biol. 15:1591

2. Gould, J.L. (1980). The case for magnetic sensitivity in birds and bees (such as it is). Am. Sci. 68, 256-267

3. Able, K.P. and Able, M.A. (1990). Calibration of the magnetic compass of a migratory bird by celestial rotation. Nature 347, 378-380

4. Able, K.P. and Able, M.A. (1995). Interactions in the flexible orientation system of a migratory bird. Nature 375, 230-232

5. Åkesson, S., Morin, J., Muheim, R., and Ottosson, U. (2002). Avian orientation: effects of cue-conflict experiments with young migratory songbirds in the high Arctic. Anim. Behav. 64, 469-475

Current Biology http://www.current-biology.com

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NATURAL HISTORY: ON THE MAGNETIC COMPASS OF SONGBIRDS

The following points are made by W.W. Cochran et al (Science 2004 304:405):

1) Billions of songbirds migrate between continents twice each year, but their orientation capabilities are almost exclusively studied in the laboratory. The authors presented birds with experimentally altered orientation cues and followed their subsequent migratory flights in the wild. Avian navigation capabilities are very precise (1), with many individuals returning to the same breeding sites year after year (1-3) after a voyage of up to 25,000 km (4, ).

2) Migratory songbirds can orient on the basis of compass information from the sun and its associated polarized light patterns, the stars, the earth's magnetic field, and the memorization of spatial cues en route. However, the interactions and relative importance of these cues remain unclear and a source of much debate. Our knowledge about the orientation mechanisms of songbirds relies almost exclusively on data from cue-manipulated captive migrants tested in various orientation cages, on vanishing bearings based on the first few hundred meters of flight, and to a much lesser degree on field data (ringing and radar and visual observations) from unmanipulated natural migrants.

3) On clear evenings, the authors fitted Catharus thrushes with radio transmitters and placed them in outdoor cages in an artificial eastward-turned magnetic field from about sunset until the sun was 11 deg or more below the horizon when they were set free. The authors then radio-tracked the birds in flight to obtain heading data. Because Catharus thrushes do not compensate for wind drift but individuals maintain nearly constant preferred headings from night to night, the authors used measured headings for orientation analyses.

4) In summary: Night migratory songbirds can use stars, sun, geomagnetic field, and polarized light for orientation when tested in captivity. The authors studied the interaction of magnetic, stellar, and twilight orientation cues in free-flying songbirds. The authors exposed Catharus thrushes to eastward-turned magnetic fields during the twilight period before takeoff and then followed them for up to 1100 kilometers. Instead of heading north, experimental birds flew westward. On subsequent nights, the same individuals migrated northward again. The authors suggest that birds orient with a magnetic compass calibrated daily from twilight cues, and that this could explain how birds cross the magnetic equator and deal with declination.

References (abridged):

1. P. Berthold, E. Gwinner, E. Sonnenschein, Eds., Avian Migration (Springer, Berlin, 2003)

2. J. P. Hoover, Ecology 84, 416 (2003)

3. P. O. Dunn, D. W. Winkler, Proc. R. Soc. London Ser. B. 266, 2487 (1999)

4. D. C. Outlaw, et al., Auk 120, 299 (2003)

5. W. L. Engels, Biol. Bull. 123, 94 (1962)

Science http://www.sciencemag.org

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ZOOLOGY: ON ANIMAL NAVIGATION

The following points are made by James L. Gould (Current Biology 2004 14:R221):

1) Nearly all animals move in an oriented way, but navigation is something more: the directed movement toward a goal, as opposed to steering toward or away from, say, light or gravity. Navigation involves the neural processing of sensory inputs to determine a direction and perhaps distance. For instance, if a honey bee were to seek food south of its hive, it would depart from home with the sun to its left in the morning, but to its right in the afternoon.

2) Several trends reflecting favorably on natural selection and poorly on human imagination characterized early studies of navigation. One tendency was the assumption that animals sense at most the same cues as we do. Thus, being blind to our own blindness, it came as a total surprise when honey bees and many other species were found to be able to see UV light. As navigation depends on the processing of such cues, the number of "new" senses uncovered in the past fifty years has greatly expanded our thinking about what may be going on in the minds of animals -- and there is no reason to assume the list is complete. To UV must be added polarized light, infra-red light, special odors (pheromones), magnetic fields, electric fields, ultrasonic sounds and infrasonic sounds.

3) The second crippling propensity is what navigation pioneer Donald Griffin called our innate "simplicity filter": the desire to believe that animals do things in the least complex way possible. Experience, however, tells us that animals whose lives depend on accurate navigation are uniformly overengineered. Not only do they frequently wring more information out of the cues that surround them than we can, or use more exotic or weaker cues than we find conceivable, they usually come equipped with alternative strategies -- a series of backups between which they switch depending on which is providing the most reliable information.

4) A honey bee, for instance, may set off for a goal using its time-compensated sun compass. When a cloud covers the sun, it may change to inferring the sun's position from UV patterns in the sky and opt a minute later for a map-like strategy when it encounters a distinctive landmark. Lastly, it may ignore all of these cues as it gets close enough to its goal to detect the odors or visual cues provided by the flowers. This is not to say that animals do not often rely on approximations and neural shortcuts to simplify these daunting tasks.

5) A third stumbling block has been our presumption that because the earliest cases studied involved "imprinting" (irreversible one-trial learning), animals must have simple navigation programs, which need merely to be calibrated to the local contingencies. This is just what at least some relatively short-lived animals do -- like honey bees for instance, who rarely forage for more than three weeks. But most animals live longer, and in consequence many need to recalibrate themselves.

6) Finally, most researchers deliberately ignored or denigrated the evidence for cognitive processing in navigating animals. This legacy of behaviorism (the school of psychology that denied instinct) puts a ceiling on the maximum level of mental activity subject to legitimate investigation. There are many navigating animals whose behavior lacks any hint of cognitive intervention. However, the obvious abilities of hunting spiders and honey bees to plan novel routes make it equally clear that phylogenetic distance to humans is no sure guide to the sophistication of a species' orientation strategies.(1-3)

References:

Able, K.P. and Able, M.A. (1995). Interactions in the flexible orientation system of a migratory bird. Nature 375, 230-232

Gould, J.L. (1980). The case for magnetic-field sensitivity in birds and bees. Am. Sci. 68, 256-267

Walker, M.M. (1998). On a wing and a vector. J. Theor. Biol. 192, 341-349

Current Biology http://www.current-biology.com

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