Seacrossings

Landbirds that migrate over oceans provide some of the most extreme examples of endurance flight and precise navigation when they travel, without opportunity to feed, drink or rest, over vast areas of open water devoid of helpful landmarks. Nor can they stop and take shelter, as birds do over land, when the weather turns against them. While some landbirds on overwater flights take whatever opportunities for rest are available, stopping on ships, oil-rigs and other installations, or even on mats of floating vegetation or other flotsam, these individuals probably represent an exhausted minority, and almost certainly most landbirds make water-crossings non-stop.

In all long sea-crossings, the behaviour of the birds with respect to weather is paramount. Birds often accumulate in coastal areas, sometimes waiting for days at a time for conditions to improve. Even then, they often seem reluctant to leave, flying along the coast or turning back inland. To accomplish a long sea-crossing successfully, birds need a good tailwind, and an adequate supply of body fuel, sufficient not only for the journey, but also for any emergencies that might arise. Even with favourable weather at take-off, birds have no means of predicting how it might change on a flight lasting up to several days, on a journey of up to several thousand kilometres. Any long-distance migrant would therefore benefit from having sufficient reserves to cope with adverse conditions which a human pilot, benefiting from weather forecasts, could anticipate and avoid. Landbirds making long overwater flights therefore need generous fuel reserves, and it is little wonder that such species accumulate before they set off some of the largest reserves of body fat recorded among birds.

A second problem for landbirds making overwater flights concerns navigation. They have no ground features below to help keep them on course, but must rely entirely on celestial or magnetic cues. If they encounter mist or rain, they often become disoriented, and mill around for hours or drift far off course, as radar observations confirm. Nor can they shelter from rain, for if they come down on the water, they usually perish. In sea-crossings, natural selection acts harshly against any kind of mistake or mishap, and abundant evidence points to heavy mortality of migrating landbirds over water (Chapter 28). One way round these problems is to fly high enough to avoid mist and rain, and on long overwater flights birds have often been detected by radar at heights exceeding 4 km above sea level, but high-altitude flights bring other problems (see later).

Despite the difficulties, all the major seas and oceans of the world are crossed regularly by some landbird species (Figure 6.1 and Table 6.1). Each year, millions of birds of a wide range of species cross on a broad front the Mediterranean Sea in the Old World or the Gulf of Mexico in the New World (Moreau 1961, Gauthreaux 1999). At their widest points both these crossings involve flights of more than 1000 km. One of the species that crosses the Gulf is the tiny Ruby-throated Hummingbird Archilochus colubris, which doubles its normal 3.5 g body weight with extra fat in order to do so. Frequent storms make the trans-Gulf crossing one of the most hazardous in the world, especially in the autumn hurricane season. It is unlikely that any small birds could withstand a hurricane at sea. Smaller numbers of other birds also cross all the major inland waters, such as Caspian and Baikal in the Old World and the Great Lakes in the New World.

Figure 6.1 Some major sea-crossings undertaken by landbirds. Some journeys are made only in one direction, while the return is made over a more overland route. Mainly from Williams & Williams (1990).

Crossing these waters by direct flight saves considerable energy over the alternative of flying around them.

Smaller numbers of species also make long flights over parts of the Atlantic, Pacific and Indian Oceans. These journeys run up to several thousands of kilometres, and involve an estimated 50-100 hours of non-stop flight, which can be lengthened in adverse conditions. They include the most extreme long-distance endurance flights performed by birds.

Some species make the journey from eastern North America, across the Atlantic direct to South America. Judging by their appearance on Bermuda, which lies 900 km east of the North American coast, more than 80 species make this journey each autumn, and others occasionally (e.g. Wingate 1973). Ships have reported that most birds appear within 600 km from the coast, but others appear at more than 2000 km, mainly warblers and shorebirds (McClintock et al. 1978). Small birds flying from Nova Scotia over the Atlantic face a nonstop 2400 km flight to the West Indies, or 3700 km to South America. Small Blackpoll Warblers Dendroica striata, with a fat-free weight of about 11 g, can double their total body weight before departure as they accumulate sufficient fuel (Nisbet 1963b, Nisbet et al. 1963). Some shorebirds that breed on the northern tundras set off from staging areas in Newfoundland and Nova Scotia and strike out over the Atlantic to South America. They make the longer journey of up to 3700 km, but can fly twice as fast as the Blackpoll Warbler (60-70 km per hour versus 30-35 km per hour), so accomplish the flight in around half the time (Nisbet 1970, McClintock et al. 1978). They include the American Golden Plover Pluvialis dominica, Hudsonian Godwit Limosa haemastica and White-rumped

Table 6.1 Some long overwater journeys of migratory landbirds. Flight durations are calculated assuming still air, and for flight speeds of 30-35 km per hour (typical of small songbirds) and of 60-70 km per hour (typical of small shorebirds). In practice, most overseas migration occurs with the benefit of a tailwind, which reduces the flight times

Journey

Species

Distance (km)

Flight duration (hours) at

30-35 km per hour

60-70 km per hour

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