aInclude Lesser Sand Plover Charadrius mongolus, Whimbrel Numenius phaeopus, Pacific Golden Plover Pluvialis fulva, Bar-tailed Godwit Limosa lapponica, Lesser Yellowlegs Tringa flavipes, Wandering Tattler Heteroscelus incanus, Grey-tailed Tattler H. brevipes, Long-billed Dowitcher Limnodromus scolopaceus, Ruddy Turnstone Arenaria interpres, Sanderling Calidris alba, Least Sandpiper C. minutilla and ducks such as Northern Pintail Anas acuta and Northern Shoveller A.. clypeata, although the bulk of their populations winter elsewhere. Only the Bristle-thighed Curlew Numenius tahitiensis winters entirely on Hawaii and other Pacific Islands. For the Bar-tailed Godwits that migrate from Alaska or Siberia to New Zealand see text. bAt least eight shorebird species: White-rumped Sandpiper Calidris fuscicollis, Least Sandpiper C. minutilla, Lesser Yellowlegs Tringa flavipes, Hudsonian Godwit Limosa haemastica, Red Knot Calidris canutus, Short-billed Dowitcher Limnodromus griseus, Semipalmated Sandpiper Calidris pusilla (McNeil & Cadieux 1972). At least 13 parulid warblers, notably Blackpoll Warbler Dendroica striata (Nisbet et al. 1963). Other warbler species make this Atlantic flight occasionally, for at least two dozen species turn up frequently in autumn in Bermuda which lies 900 km east of the North America coast at that latitude (Scholander 1955, Wingate 1973). Studies using a network of radar sites agree with the estimates that small passerines take more than 80 hours to accomplish the 3700-km non-stop flight between northeast North America and northern South America (Williams et al. 1978).

Sandpiper Calidris fuscicollis. Shorebirds and passerines on this journey have been found by radar to reach heights of 4-6 km (Williams et al. 1977)), taking advantage of the northwest winds that follow a front. Flight speeds measured by radar gave estimates of the flight times of passerines at 18 hours to Bermuda, 64-70 hours to the Caribbean and 80-90 hours to South America (Williams et al. 1978). Waders could presumably cover these distances in half the time. This overwater 'shortcut' saves over 1000 km on the more roundabout overland coastal route. In the spring, however, when winds are against them, these same species return by the overland route, northward through eastern North America.

Other long sea-crossings by North American birds are undertaken by shorebirds and waterfowl that migrate direct from Alaska over the eastern Pacific to make landfall at various sites between southern Canada (2500 km) and Baja California (up to 5000 km), depending on species. Again, some species make this flight only in autumn when winds are favourable and return in spring by the longer landward route. Other long overwater journeys are flown by shorebirds and ducks that migrate between Alaska and Hawaii (>4000 km) or between Alaska and south Pacific islands without stopping on Hawaii (6000-9000 km) (Thompson 1973, Johnson et al. 1989, 1997, Williams & Williams 1990, 1999, Marks & Redmond 1994). These journeys are remarkable, not only for the distances involved, but because of the great precision of navigation required to find such tiny wintering areas in the vastness of the Pacific. Participants include the Bristle-thighed Curlew Numenius tahitiensis, which is the only migratory bird species that breeds on a continent and winters entirely on Pacific Islands.

The most impressive of all overwater migrations by a landbird, however, is undertaken by the Bar-tailed Godwits Limosa lapponica baueri from eastern Siberia and western Alaska, which apparently fly an astonishing non-stop 10 400 km to eastern Australia and New Zealand. The shortest (Great Circle) distance from coastal Alaska to the North Island of New Zealand is 10 260 km, requiring in typical winds an estimated 175 hours (7.3 days) of flight. This flight requires enormous fat reserves, and considerable shrinkage of other body organs to reduce weight. When the birds depart, they are about twice their normal weights (Chapter 5). Astonishingly, by the time they reach New Zealand, birds have already flown more than 10 000 km without feeding, but some birds killed soon after arrival still had sufficient fat to fly another 5000 km, enough to reach the South Pole (Pennycuick & Battley 2003). The birds evidently travel with a substantial safety margin, in case they are forced down, or blown off course by adverse winds. The main advantage of this oversea migration is the saving in distance, for if the birds migrated round the coasts of eastern Asia (as they do in spring), they would have to travel about 16 000 km one way, a journey almost 40% longer than the direct trans-Pacific route. The single flight may also be safer (fewer predators) and healthier (fewer pathogens, Piersma 1997), as well as being accomplished in a much shorter period of about seven days in favourable winds. The reason that birds take the longer route in spring is that the winds are unfavourable for a northern transoceanic flight.

This is such a remarkable migration that it is worth examining the evidence that the majority of individuals do indeed fly non-stop (Gill et al. 2005). The god-wits involved are a distinct race L. l. baueri, and many have been colour-marked, so that they can be identified from a distance without being captured. Yet virtually none has been reported from along the eastern Asia coasts during the autumn migration period (a time when other marked shorebirds are commonly reported from that region). Second, the peak southward departure from Alaska is followed about a week later by the peak arrival in New Zealand. Third, considering that 150 000 L. l. baueri godwits make this journey every year, very few are sighted on Pacific Islands where they might be expected to stop. Such sightings occur mainly in the September-November migration period, on islands that lie on a direct route between Alaska and eastern Australia/New Zealand, and especially near journey's end where most fallout would be expected. Fourth, the bird appears energetically and mechanically able to complete such a flight non-stop. Flight simulation models, the bird's extreme fat loads, and the selection of suitable wind conditions from Alaska, all support the notion of a direct flight. With the benefit of a tailwind, godwits can travel at more than 100 km per hour, and by analogy with other shore-birds, they should be capable of remaining airborne for the required period. Lastly, known departures from Alaska occur on winds favourable for a southerly flight, rather than on winds that would take them on a southwesterly course round the coast of Asia. All these lines of evidence point to a direct non-stop flight.

On their northward migration, baueri godwits are thought to undertake a two-step journey, the first flight of 8000-10 000 km from New Zealand and eastern Australia to staging areas around the Korean peninsula, Japan or the north coast of the Yellow Sea, where they refuel before the shorter second stage which takes them to Siberia or Alaska. Birds that leave New Zealand in spring have fat reserves not much smaller than those that leave Alaska in autumn, but the autumn birds have shrunk their internal body organs to a much greater degree, giving them a much larger proportion of fat in the body. This greater 'fat fraction' enables them to make a much longer journey than with the same weight of fat in a heavier body. Other Bar-tailed Godwits perform long-distance migrations elsewhere in the world, but none as long as the baueri birds.

Other long sea-crossings of note are made by the Amur Falcons Falco amurensis which in autumn fly the 4000 km direct from India to southern Africa. These are the longest overwater flights regularly performed by a raptor. Similarly, Northern Wheatears Oenanthe oenanthe in autumn cross 2000-3000 km of the North Atlantic between Greenland and Europe before moving on to Africa. These are some of the longest overwater flights performed by a passerine. Both species take somewhat different routes on their return journeys when winds are less favourable for the most direct route. In this respect they resemble many of the species mentioned above.

Examples of long overwater migrations by southern hemisphere landbirds are provided by three species that breed in New Zealand, namely the Double-banded Plover Charadrius bicinctus, which crosses 2000 km of sea to Australia, the small Shining Bronze Cuckoo Chrysococcyx lucidus, which crosses 2500 km of sea to the Bismarck and Solomon Islands, and the much larger Long-tailed Cuckoo Eudynamys taitensis, which crosses 2700 km to Samoa and Fiji and other islands of the central Pacific east to the Caroline, Line and Phoenix Islands. Even within the tropics, some landbird species move up to 3000 km between breeding and non-breeding areas, part of which may involve a sea-crossing: for example, the Broad-billed Roller Eurystomus glaucurus migrates between Madagascar and the Congo, the Roller (Dollarbird) Eurystomus orientalis migrates between northern Thailand and Borneo-Java, and the Pied Cuckoo Clamator jacobinus and others between India and East Africa.

Was this article helpful?

0 0

Post a comment