1 Wandering Albatross
2 Black-browed Molly mawk
3 Grey-headed Mollymawk
4 Light-mantled Sooty
5 Giant Petrel
6 Cape Pigeon
7 Antarctic Fulmar
8 Antarctic Petrel
9 Snow Petrel
10 Blue Petrel
11 Thin-billed Prion
12 Sooty Shearwater
13 Grey Petrel
14 White-headed Petrel
15 Kerguelen Petrel
Fig. 10.5. The effect of the Polar Front on the distribution of seabirds in the Antarctic Sector of the Pacific (Eltanin Cruises 20 and 21,1966, see text). Note the effect of a sharp-zoned Polar Front on the Thin-billed Prion (11) and how, despite its intensity, the Polar Front can effectively halt species such as the Wandering Albatross.
general southward drift of the surface layer, and the krill's power of locomotion would scarcely allow for one mile a day even if it moved continuously in one direction. It must live under the ice, waiting, as it were, to be uncovered." Whether this actually occurs remains unclear, but it has been known for some time that seabirds are attracted to the vicinity of the ice edge (e.g., Routh, 1949).
In their review of the influence the Antarctic ice edge has on seabird distribution, Fraser and Ainley (1986) suggested that the ice edge is a region of overlap between two distinct seabird communities, one associated with the pack ice (Emperor and Adelie Penguins, and Snow and Antarctic Petrels) and the other with waters generally free of ice, but still under its influence (Antarctic Fulmar, Cape Pigeon, Wilson's Storm Petrel, and Mottled Petrel). The ice edge was regarded as important to both bird communities because of the intense algal blooms which occur in the Antarctic spring in the numerous open leads and polynyas. As the ice melts, the associated algal communities seed the water, which are gradually exposed to increasing sunlight as the ice retreats. It is the coalescing of small algal blooms into larger ones that eventually produces extensive oceanic blooms so characteristic of the Antarctic. According to Fraser and Ainley (1986), recent information suggests that seabirds congregate not at the edge of the ice but rather 7-10 km further north, reflecting the time lag between ice disintegration and the surge in productivity of the water previously covered by ice.
In addition, not merely the presence of ice but also its age and general condition are possibly important in the distribution of polar seabirds. Pack ice surveys by Zink (1981) and Ainley et al. (1984) have shown birds are directly associated with intrusions of extremely thick, densely concentrated, multi-year ice. "Seabird numbers peaked over these intrusions, which were laced with floes showing both extraordinary algal and bacterial communities and advanced stages of decomposition. These were also the only sites within the pack where euphausiids, decapod crustaceans, and other prey regularly occurred. Again, heavy ice cover, infrequent foraging behaviour, and abrupt discontinuities in seabird abundance that corresponded with the boundaries of these intrusions all suggested that the birds were responding to physical features of the ice, not to prey availability" (Fraser and Ainley, 1986).
Antarctic birds live in an environment of low temperatures, unpredictable winds, and ice-strewn seas. Zink (1981), in his study of oceanic birds during a high-latitude voyage from Ross Island to Anvers Island between 16 January and 7 February 1976, sighted Snow Petrels over pack ice on all but two days and suggested an ice cover of 3-5 oktas (ice concentration) was their habitat preference. In recognition of their limited distribution, Shuntov (1974) classified the Snow and Antarctic Petrels as neritic-ice species rather than pelagic ones. Zink (1981) emphasized the need for future studies to determine the type of ice as well as its concentration. Older ice is thinner and has a more irregular surface that not only provides wind shelter for resting or moulting birds but also develops numerous small holes and crannies in which marine invertebrates are to be found (Watson, 1975; Zink, 1981).
The Antarctic Fulmar appears to frequent more ice-free waters in the Pacific
(Zink, 1981; Ainley et al., 1984; P.C. Harper, pers. observ.), although this may not be so in the Australian Polar region (Johnston and Kerry, 1974) or the Scotia Sea (Murphy, 1936; J.P. Croxall, pers. comm.). Dissimilarities in bird behaviour between resident populations of the three major polar basins surrounding Antarctica emphasize the need for careful behavioural studies and the recognition that, despite being contiguous, populations of birds can remain separate entities rather than forming a homogeneous whole within the Antarctic domain.
The distances that breeding birds can forage away from their eggs and chicks depends on food availability, incubation duties, chick feeding, and the more subtle limitations such as competition, predators, and the oscillations of convergences where birds tend to congregate. Antarctic species have the added problem of flying considerable distances over pack ice to reach open water.
During summer, the breeding season for most oceanic birds, the great influx of birds to the waters surrounding their breeding islands tends to obscure distributional patterns of other birds in the region, while at the same time exposing the migration patterns and movements of sexually immature and non-breeding birds which range far to sea. Whereas breeding birds are restricted in how far away they can forage from nest-sites, non-breeding birds have no such limitations. Because young Procellariiformes delay breeding for several years, pre-breeders make up a substantial percentage of at-sea populations.
But what exactly are these non-breeding birds doing at sea for perhaps a third of their total life-span? The answer lies initially in determining the status of birds seen at sea. This is not an easy task because an at-sea observer rarely has any way of determining the status of an observed bird unless specific age-groups can be conspicuously marked. Are, for example, the Mottled Petrels (Pterodroma inexpectata) which forage along the Antarctic ice edge in summer non-breeding birds; or are they, as Warham et al. (1977) suggested, breeding adult birds which can make the 4,000 km round trip during the week-long stints between incubation or chick feeding duties? Are squid and crustaceans in such short supply in the New Zealand region that breeding Mottled Petrels must fly such distances? We doubt it.
The age structure of New Zealand storm-driven birds such as Antarctic Fulmars, prions and the Kerguelen Petrel clearly shows that fledglings have a much greater range than adults (Harper, 1980; Veitch, 1980; Imber, 1984). If it could be shown that the majority of zoneless procellariids are only five or six years old, and that the adult birds tend to remain within their adopted life zones, then we would be much closer to understanding the real nature of marine bird distributions.
The so-called "tendency to wander" by juvenile birds appears to have been largely misunderstood for many years. Baker (1978,1980,1982), in his challenging discussions on bird migrations, argued that post-fledglings do not "disperse" but use "exploratory migration" instead; the need to search for new feeding areas and possible places to breed which do not necessarily match those of adult birds.
Baker believes that a bird's process of familiarization with its environment consists of three parts : "(1) exploratory migration (i.e., the act of moving along unfamiliar routes to unfamiliar destinations but always with the intention of eventually returning to a familiar site); (2) habitat assessment (i.e., judging the suitability of a route or place, identifying what resources it can offer, and ranking it against other places already discovered); and (3) navigation (i.e., negotiating the way back to a familiar site)." Baker's hypotheses on bird migration are, at the very least, intriguing, and worthy of testing. His hypotheses could be usefully tested using colour-banded fledgling Giant Petrels, a programme initiated by Dr S. Hunter (British Antarctic Survey) as part of the International Survey of Antarctic Seabirds (ISAS). Giant Petrels are large, abundant, and easily identified birds, an excellent species with which to begin seeking some answers.
In describing the pelagic distribution of marine birds in the eastern Bering Sea, Hunt et al. (1981) remarked: "The general pattern of bird distribution in the area is one of highly mobile units, frequently single birds, scattered over the ocean, coalescing into small or large assemblages for short periods, and then dispersing. This produces a permutating web of high and low densities over the surface waters of the eastern Bering Sea." This description typifies the birds of the Antarctic Sector of the Pacific, for without island refuges, birds can use this vast habitat only as a place to feed and as a migration corridor.
Many southern Procellariiformes engage in west-east migrations. Examples include Wandering and Royal Albatrosses, which move from their New Zealand breeding grounds to the region around the Horn and Patagonia (e.g., Robertson and Kinsky, 1972); whether these birds have a circumpolar distribution remains unproved. Subtropical Buller's Mollymawks probably travel well to the north of the Polar Front to winter in waters adjacent to the Chilean coast (Murphy, 1936). Warham (1982) reported a bird banded as a chick on The Snares being recovered on a fisherman's long-line 7,460 km away at 12°25'S, 105°06'W, some 2,000 km southwest of the Galapagos Islands. Hedgling Giant Petrels (Macronectes sp.) were encountered by the Eltanin from 21 March 1966 in position 58°06'S, 103°55'W all the way west to near the Chatham Islands (44°00'S, 176°30'W) on 25 April 1966, a distance encompassing most of the polar Pacific (P.C. Harper, pers. observ.). These birds were migrating from the New Zealand region eastwards. Diving Petrels are extremely difficult to see at sea at the best of times, but observations made in the polar Pacific between New Zealand and Chile during 1965-66 clearly indicate that some birds are migrating eastwards in the 52°-57° latitudes (Watson et al., 1971; P.C. Harper, pers. observ.). Their identity remains unknown.
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