Apparent Nonstop Seacrossings

Apparent non-stop sea-crossings provide further striking examples of the flight capabilities of birds. They enable some journeys to be made extremely rapidly, providing the journey time is taken from take-off to landing, and no allowance is made for the pre-flight fattening period. Most data on the time taken for sea-crossings derive from comparing the departure dates of birds from coastal staging sites near their breeding areas with their arrival dates at their destinations, a method applicable only to conspicuous species (see above). On this basis, the Bar-tailed Godwits Limosa lapponica baueri that migrate 10 400 km from Alaska to New Zealand were estimated to cover this distance in about 175 hours (7.3 days) of uninterrupted flight, giving a mean speed of 1512 km per 24-hour day (or 63 km per hour). This is the longest and fastest overwater flight known from any land-bird (Gill et al. 2005). Similarly, Great Knots Calidris tenuirostris were estimated to fly 5420 km between northwest Australia and Chongmen Island at the mouth of the Yangtze River in China in about four days, or 1355 km per day (equivalent to 56.5 km per hour) (Battley et al. 2001). Both these species lost around half their body weights over the journey.

Autumn migrations of Brent Geese Branta bernicla over the Pacific Ocean from Izembek Lagoon, Alaska, to Baja California, Mexico, were studied in the same way (Dau 1992). Observers at both sites documented departures and arrivals in three different years, estimating times en route to be 60, 60 and 95 hours respectively, which implied average speeds of 1992, 1992 and 1344 km per 24-hour day (or of 83, 83 and 56 km per hour) over the 5000 km journey. Variability in mean route times between years was associated mainly with differences in wind speed and directions, and possibly also with times spent resting on the sea. This migration is also energetically costly, with birds of both sexes losing an estimated one-third of their total pre-departure body weights during the 2-4 day journey. On a somewhat shorter journey, Canada Geese Branta canadensis minima were estimated to take 48 hours to fly 2800 km over the sea from the Alaska peninsula to the Klamath Basin in Oregon, averaging 1392 km per 24 hours, or 58 km per hour (Gill et al. 1997). One can never be certain in such studies that the birds did not rest on the sea for a time, but their journey times were so short that any such rests cannot have lasted long.

Other information on sea-crossings has come from the tracking of radio-tagged birds. Ten Whooper Swans Cygnus cygnus tracked by satellite took 12.7-101 hours to cross the sea between Iceland and the British Isles, a journey of 962-1700 km in different individuals (Pennycuick et al. 1999). These figures relate to the sea-crossing alone, and exclude times spent over land, as well as the initial fattening period. The great variations in journey times were due mainly to variations between individuals in the amount of time spent sitting on the sea. Two birds were blown far off course and waited on the sea until conditions improved, when they flew back on course. Two others made the journey with no obvious stops, one reaching 1856 m above sea level, the maximum height recorded.

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