What seabirds eat and how they obtain their food has, until recently, been one of the least studied aspects of Antarctic bird ecology (Ashmole, 1971; Ainley, 1977; Croxall and Prince, 1980; Croxall, 1984; Harper etal., 1985; Schramm, 1986; Harper, 1987).
Croxall (1984) reviewed the feeding methods of seabirds and gave examples of feeding techniques based upon the terminology of Ashmole (1971) and the morphology of bill structure. Although some species scavenge food from ships and fishing vessels, most Procellariiformes appear to catch their prey at night by taking advantage of the vertical migration to the surface of zooplankton and their attendant predators such as squid and fish. During a two and a half year research period aboard U.S.N.S. Eltanin, one of the authors (P.C.H.) made 4,926 observations on 20 species of petrels feeding at sea. Most of these observations were made at night during oceanographic stations, when the ship's moveable signalling lights and other techniques could be effectively used to study the feeding behaviour of seabirds. Eleven feeding methods were distinguished, the most common of which were surface seizing (49.1%, used by 14 species); dipping (25.2%, 9 species), and surface plunging (c. 6%, 6 species). Seven species foraged entirely at night, and five fed by day only. Live food seen to be caught at the surface was chiefly crustaceans and squid (for further details, see Harper, 1987).
Nearly all studies on what oceanic birds eat have analyzed food samples brought by adults to feed their chicks by regurgitation (Croxall, 1984). Recent techniques of water offloading to obtain seabird stomach contents (Wilson, 1984) have lessened the need to kill birds, which therefore can be resampled as necessary (e.g., penguins (van Heezik, 1988) and Cape Pigeons (Green, 1987)).
There may be biases in data collected by both regurgitation and water offloading techniques, however. In his study of three species of gadfly petrels breeding at the Prince Edward Islands, Schramm (1986) showed that the percentage of the total weight of stomach contents regurgitated by chicks was 69% (range 0-100%) for Pterodroma macroptera, 29% (33-73%) for P. brevirostris, and 7% (0-89%) for P. mollis. He stated that "it was therefore necessary to kill chicks to obtain representative samples of stomach contents."
On the basis of such studies, some authors (e.g., Abrams, 1985) have classified seabirds as planktivores, squid-eaters, piscivores, or "mixed". Such classifications are open to error for the following reasons.
1. There is a lack of sufficient seasonal data to determine whether birds take the same types of food prey throughout the year. Intuitively, one would expect this notion to be extremely unlikely, and that winter prey ought to be quite different from summer prey. For example, breeding birds, because of their restricted foraging zones, may be less choosy in their food preferences than they are at other times. A counter argument to this might assert that because more prey species are available to birds in summer they merely take advantage of this abundance of available food. Which, if either, of these hypotheses is correct is not known.
As an example, the Broad-billed Prion (Pachyptila vittata) takes copepods and other crustaceans during summer, but catches small squid at other times, especially during winter when crustaceans are much less plentiful. The bill of P. vittata is remarkably structured to sieve small plankton such as copepods from the water (Murphy, 1936) yet is also efficient at grasping and holding squid approximately 50 mm in length (Harper, 1987). Using Abram's terminology, P. vittata could be regarded as a planktivore in summer, and a squid-eater in winter.
In another important corollary to the above, Hunter (1983) showed that differences in diet are greater between sexes than between species in the sexually dimorphic Giant Petrels (Macronectes giganteus and M. halli) at South Georgia where males take more carrion and females more crustaceans. If this occurs in other sexually dimorphic procellariids, then the true nature of interspecific and intersexual competition, and niche partioning among such oceanic birds may prove more challenging to researchers than is currently realized.
2. In analyzing regurgitated food samples and gizzard contents, there is a strong bias towards hard-bodied prey. Some soft-bodied food can pass through the gut of prions and shearwaters quickly, such that the glandular stomach is empty about five hours after food intake (P.C. Harper, pers. observ.).
3. Adult birds may collect different foods for their chicks than for their own energy needs (e.g., Bradstreet and Brown, 1985). This can be tested experimentally by analyzing the foods of non-breeding birds and comparing them with those of breeding birds.
4. Pterodroma petrels are often cited as squid-eaters (e.g., Imber, 1973; Abrams, 1985; Schramm, 1986) which, of course, they are, but some species are also extremely adept at catching crustaceans from the sea surface (Harper, 1987). Which of these prey species is the more important component in the diet of these birds? And does the important prey change throughout the year? Data are lacking.
5. Griffiths (1983) suggested that Snow and Antarctic Petrels are restricted to areas of sea ice because of their non-specialized, flapping flight which, although more expensive energetically than the gliding flight of most pelagic procellariids (Pennycuick, 1972) allows them to exploit the generally unpredictable wind gradients associated with polar sea ice and around icebergs (Fig. 10.6). A lack of diet specialization enables Snow and Antarctic Petrels to feed opportunistically on crustaceans, fish and squid. Is this lack of diet specialization a summer feature, or is it prevalent throughout the year in these species?
Precisely what the correlations are between bird abundance at sea, their energy requirements, and their "principal food types" will remain open to speculation until much more information on foraging behaviour and the seasonal nature of foods taken by birds is available.
Whether the Antarctic Sector of the Pacific will be the place for such oceanic bird studies appears unlikely. Future prospects for ornithological research in the Antarctic Sector of the Pacific look certain to be largely landbased, with radio telemetry used to follow some birds during their foraging to sea. Improvements in miniaturization and placement of transmitters on birds, which can supply data to satellites passing overhead, may provide the vital key to discovering where birds go during the Antarctic winter. The formidable costs of putting scientific research ships into remote areas appears, at least for the present, to mean the end of prolonged scientific journeys such as those of the U.S.N.S. Eltanin in the 1960s. Ornithological observations from an increasing number of tourist ships frequenting southern seas (e.g., Rogers, 1980) will continue to provide a useful, if tenuous, link to our understanding of the distribution of marine birds at sea.
Fig. 10.6. Snow Petrel in the Ross Sea.
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