Different bird species also vary greatly in the distances they can fly without needing to rest. Near one extreme are certain gallinaceous birds that can fly less than 1 km before having to land (Palmer 1962). Near the other extreme is the Common
Swift Apus apus which outside the breeding season normally spends both day and night on the wing. As roosts are unknown from winter quarters, it is possible that, after leaving the nest in August, young swifts remain on the wing until they re-enter nest-sites as pre-breeders in the following or a later year (C. M. Perrins, in Wernham et al. 2002). Frigatebirds also remain on the wing continuously when away from their nests, including their pre-breeding years, but they may come to roost on land (Weimerskirch et al. 2003).
Consideration of the non-stop flight capabilities of migrating birds has usually centred on the need for fuel or water, rather than on the need for rest. Such flights are clearly limited by the body reserves available to fuel them, but well inside this limit, some birds may have to come down periodically in order to rest. This is difficult to prove, because migrants come to ground for various reasons, including adverse flying conditions (winds, rain, mist or heat), and not necessarily merely to sleep. However, many birds appear tired on arrival from migration, and may break their journeys for several hours without apparently feeding. Individuals that arrive low over the sea are sometimes seen to collapse onto the beach or into the first patch of vegetation available. Those that are caught are often found to have substantial body reserves, so have not run out of fuel (Chapter 6).
In many birds just after landing from a long flight, the need for sleep may take precedence over most other activities (Schwilch et al. 2002b). Some Pink-footed Geese Anser brachyrhynchus arriving at a lake in eastern Scotland after a long flight from Iceland first drank a good deal, but then spent many hours sleeping, standing on one leg in shallow water. At times they also waded into deeper water, splashed and preened for long periods. On the first day after arrival, most birds probably took no food at all, and not until the next morning did they fly to feed in nearby fields (Newton & Campbell 1970). Other observations describe shore-birds after long flights falling asleep within minutes after arrival on coastal mudflats, and sleeping for several hours before starting to feed; or passerines after arrival from long overwater flights resting under coastal bushes for much or all of the following day before moving on, apparently without feeding (Schwilch et al. 2002b).
Many radio-tagged birds have now been tracked on their journeys, but because individuals are located from satellites only at intervals of several to many hours, it is not often possible to tell whether they have settled for brief rest periods. However, some Barnacle Geese Branta leucopois that were tracked on migration also had their heart rates monitored continuously during the journey. Heart rates were faster when the birds were flying than at other times. Records showed that birds settled about every 11 hours or so (maximum non-stop flight 13 hours) during the journey, even though this often entailed the birds resting on the sea (Butler et al. 2000). Similarly, some Brent Geese B. bernicla travelling between the Netherlands and the Taimyr Peninsula of Siberia were found to stop about every 6.5-7 hours, on average, which gave flight lengths of 450-500 km. However, some non-stop flights by these birds lasted 14-19 hours, and covered up to 1300 km. From satellite-based records of Brent Geese migrating from Europe to northern Canada, it was also surmised that birds settled to rest on the Greenland ice cap, at least on the upward part of the journey (Chapter 4; Gudmundsson et al. 1995). Long rests on the sea were detected by satellite-based tracking of Whooper Swans Cygnus cygnus, but these stops may have been in response to poor weather or complete darkness rather than to tiredness (Pennycuick et al. 1999). Shorebirds may also settle on the sea from time to time, at least under adverse flying conditions (Piersma et al. 2002). During stops on the sea and ice, and some of those on land, the need for feeding can be ruled out, as in many cases can the need for drinking or sitting out bad weather, leaving the need for rest as the most likely explanation. Of course, most landbirds could not settle on the sea and, despite the many records of birds resting on ships and offshore structures, most landbirds are assumed to fly even their longest overwater journeys non-stop. Nevertheless, the possibility that the need for sleep may help to shape the migratory behaviour of birds, and influence the routes they take, warrants further study.
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