Observations made directly or with radar have shown that birds prefer to migrate under clear skies, when sun or stars are visible (Chapter 4). Nevertheless, those that do fly in overcast sometimes seem no less well orientated in that their flight relative to the ground and air mass appears 'as straight, level and fast as comparable birds flying on clear nights' (Able 1982a). Such behaviour might be expected if birds were migrating on magnetic cues. However, at other times nocturnal migrants seem to become disorientated when they enter low cloud, mist or rain and when neither sky nor ground is visible to them. When over land they normally settle, but when over the sea they mill around in various directions gradually drifting downwind (as revealed by radar). However, a brief break in the cloud to expose the sky is enough to enable them to get back on course. Such observations suggest that birds depend on celestial navigation and are not normally guided by cues from the earth's magnetic field alone. The additional fact that migrating birds caught in mist or rain are attracted to lighthouses and other illuminated structures further implies that nocturnal migrants respond to visual cues.
The perception of a horizon may also be important to migrating birds. Some radio-marked Whooper Swans Cygnus cygnus travelling between Iceland and the British Isles tended to keep going when over the sea, providing that the altitude above the horizon of either the sun or moon was higher than -4° (giving sufficient light to see), and also that the visibility was greater than about 2 km; otherwise these birds tended to sit on the water and wait until conditions improved (Pennycuick et al. 1999). When out of sight of land, they may have needed a visible horizon to navigate by, as expected if they used celestial cues.
That flying birds, both day and night, are influenced by the landscape below is well accepted, partly from the observation that migrants often follow coasts or other leading lines. Birds might maintain direction in the same way as humans, by projecting a compass direction onto a series of landmarks, and heading successively towards them. This would enable birds to compensate for drift by cross-winds, and so remain on course, at least over land. It has been suggested that birds crossing extensive tracts of water could compensate for the drifting effects of wind by using the wave pattern below them to maintain a constant heading, but drift is generally more marked in birds migrating over water than over land (Chapter 4). Use of visual cues also provides continuity between night and day and allows the recognition of familiar areas.
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