The distribution of birds at sea is a complex issue made more difficult by our limited understanding of what controls it. Distributional studies are, at the very least, reliant upon comprehensive oceanographic data being available and understandable to the ornithologist at sea. Sadly, this is often not the case. As Brown (1980) has noted, an appreciation by ornithologists that the ocean is not merely a wet provider of food but is in fact a myriad of patches and pathways affected by water temperature, salinity, colour, currents, depth, wind and weather has been a long time in coming. The modern-day field observer needs to have a thorough background in marine biology as well as knowing something of the physiology, metabolic rates, and ecological relationships of the birds under study. For many, it is a daunting prospect.
Attempts to integrate bird distributions with a few of the above parameters has begun (e.g., Szijj, 1967; Pockington, 1979; Brown, 1980; Ainley et al., 1984; Abrams, 1985) but these studies reveal just how elusive meaningful results are. Preliminary findings (Manikowski, 1971; Brown et al., 1975) suggest that any correlation between oceanographic and meteorological factors and seabird distribution is probably subtle and complex, involving permutations of the above, together with biotic factors such as food availability, the nature of seabird communities, and the age structure of birds at sea. Intuitively, one would expect seabirds to seek out large and conspicuous physical features, such as frontal systems or ice edges, first and to then seek prey associated with such boundaries. Designing experiments to test these speculations will prove interesting.
The Polar Front has long been cited as being an important boundary zone both to plankton and to birds (e.g., Murphy, 1936). According to Mackintosh (1960, fig. 75), for example, the distribution of Euphausia triacantha straddles the Polar Front and he further stated that "the Antarctic Convergence is not necessarily a sharp boundary to the total range of many species, but there are many contrasts in the planktonic fauna to the north and south of it."
Recent research has shown the Polar Front to be a complex zone, with up to three "fronts" separating different water masses. These can migrate laterally to form wavelike disturbances that frequently close on themselves, forming current rings and eddies which may have lifetimes from days to years (Foster, 1984). The effect of the Polar Front on ocean birds depends on whether it forms sharp discontinuities of temperature and salinity. In some places outside the Antarctic Sector of the Pacific, such as the Scotia Sea in the South Atlantic, the Polar Front forms over shallow submarine ridges where deflected currents and eddies can disrupt, displace and mix the water. There, the Polar Front can be hard to detect at the sea's surface. Whether oceanic birds are capable of discerning the Polar Front under these conditions is unknown, but certainly many species of subantarctic birds cross the Polar Front in the Scotia Sea area with seeming impunity (e.g., Harper, 1973). Kock and Reinsch (1978) and Ainley et al. (1984) considered the
Polar Front to be greatly overrated as a faunal barrier to birds in the Scotia Sea.
In the Antarctic Sector of the Pacific, the Polar Front forms over deep waters (3,000-6,000 m) and can thus form sharply. In calm weather, dense sea-mists frequently occur above it. In rough weather, when considerable mixing of surface waters takes place, its effect on birds is an enigma. To give an example, during a voyage from Auckland, New Zealand, to Valparaiso, Chile, in 1965, the Antarctic research ship U.S.N.S. Eltanin while heading due south along longitude 144°32'W encountered heavy sea-mists and a clear Polar Front on 28 September 1965 between 55°21'S and 55°41'S in the western Pacific. Between these two positions, the sea surface temperature dropped from 3.7°C to 0.7°C. This occurrence of the Polar Front was very sharp and it abruptly halted the southern movement of seven subantarctic species (Wandering Albatross, Black-browed Mollymawk, Grey-headed Mollymawk, Light-mantled Sooty Albatross, Thin-billed Prion, Grey Petrel, White-headed Petrel). It similarly affected the northward movement of Blue Petrel, Antarctic Petrel, Antarctic Fulmar, and interestingly, the Kerguelen Petrel (Pterodroma breuirostris ) (Fig. 10.5). Proceeding north again in a stormy ocean in the eastern Pacific between longitudes 104°W and 120°W the ship did not detect the Polar Front at the surface on 18 October 1965 (56°38'S, 105°W) but six of the above seven subantarctic species all reappeared on this date.
When the Eltanin returned to the area in relatively calm weather on 25 November, 55 days later, the Polar Front had moved some 460 km south to 60°S (at 120°W longitude) and the subantarctic species had all followed it south. They vanished as soon as the Eltanin passed south over the Polar Front and reappeared immediately after the ship returned northwards en route to Punta Arenas in the Magellan Straits. The two most reliable subantarctic water indicator species in the polar Pacific proved to be the Grey Petrel (Procellaria cinerea) and the White-headed Petrel (Pterodroma lessoni). TTiey were better at indicating the Polar Front than most of the instruments onboard ship.
Clearly, one of the greatest gaps in our understanding of seabird ecology is the effect of faunistic barriers such as the Polar Front on bird distributions, particularly since the behaviour of the birds in the Pacific appears to be quite different from that of birds frequenting the South Atlantic. Here, numerous species, including the Grey Petrel, apparently cross the Polar Front freely (Eakin et al., 1986). Why they should do so may be related to the availability of krill in the region, but more information is needed.
Adjacent to the Antarctic coastline and its land-fast ice is a belt of shifting pack ice which extends well northwards in winter and spring to cover more than half of the ocean between the continent and the Polar Front. During late spring and early summer, the ice melts and retreats south, and in doing so uncovers a substantial area of sea to sunlight, which results in rapid primary production by phyto-plankton and a resulting large jump in zooplankton numbers. Mackintosh (1970) reported krill hundreds of kilometres south of the spring position of the ice edge, and noted that it "cannot have travelled there with the open water, for there is no
September at 145"West SST°C
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