Pelagic birds

Pelagic seabirds travel over vast stretches of featureless ocean, yet they can return unerringly to their tiny nesting islands after long migrations or foraging trips. The use of satellite tracking has enabled the movements of individual birds to be followed. Albatrosses leave their breeding islands on flights that take them over distances of several thousand kilometres, foraging in seas all around the nesting colony (Weimerskirch et al. 1993, 1994). After wandering in all directions over days or weeks, they can then return on a straight line to the breeding island. Penguins are similarly efficient oceanic navigators, even though they remain on or below the water surface while at sea. Foraging King Penguins Aptenodytes pat-agonicus extend to the polar front, with individuals foraging up to 1500 km from their nesting places, and with total journey lengths up to 4000 km (Jouventin et al. 1994). Again, the inbound part of such trips usually has a straight course. Emperor Penguins A. forsteri can also walk in a straight line over featureless sea-ice for distances exceeding 100 km to their nesting areas (Ancel et al. 1992). These various findings imply that marine birds, on returning to their distant nesting places, know the direction they should go from any point in the surrounding ocean.

While we have no reason to suppose that pelagic birds navigate differently from landbirds, using celestial and geomagnetic cues, other mechanisms have been suggested based on olfactory cues (smell-gradients) or on 'route-based navigation'. Among petrels, in particular, the brain structure suggests a well-developed olfaction sense, and undoubtedly these birds use their keen sense of smell to find food sources and nest-sites; but whether they use olfaction for long-distance orientation remains an open question. For route-based navigation, an animal must encode its home location with respect to its current position. When moving, it must continuously update the home-pointing vector by subconsciously processing information collected en route about its changes of direction and location. Such an updating process, called path integration, works independently of the presence of landmarks, and could be useful in featureless seascapes. Although known in insects, the process has not been tested in birds. However, seabirds passively displaced from their nesting colonies can get back again fairly efficiently, as shown in experiments with Manx Shearwaters Puffinus puffinus, Laysan Albatrosses Diomedea immutabilis and others (Kenyon & Rice 1958, Matthews 1968), and more recently using satellite-based radio-tracking of Cory's Shearwaters Calonectris diomedea (Dall'Antonia et al. 1995). It seemed that these birds did not depend on path integration, but on some other kind of site-dependent mechanism.

Cory's Shearwaters carrying magnets on both wings and heads (to disrupt perception of the earth's magnetic field) were released 160 km and 900 km from the breeding colony (Massa et al. 1991). They homed with the same success as control birds released without magnets. The same held for nine Wandering Albatrosses Diomedia exulans that had magnets fixed to their heads, along with back-transmitters to reveal their movements (Bonadonna et al. 2005). On their normal foraging flights over several thousand kilometres, these birds showed no impairment in their ability to return to specific nest-sites compared with control birds, equipped only with transmitters. The two groups showed no differences in trip duration or length or in directness of the homeward flight, implying that these birds did not require magnetic cues to navigate back to their nesting colonies. The same held for White-chinned Petrels Procellaria aequinoctialis that were caught and displaced 300-360 km from the nesting colony (Benhamou et al. 2003). From these and other experiments, geomagnetic information does not seem to be crucial in such species, if alternative cues are available.

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