Seabirds of the GBR can be classified into four major foraging guilds; coastal, inshore, offshore and open-ocean or pelagic. Species in each of these guilds travel different distances to access food and are away from young for different periods of time. Offshore and pelagic species may travel very long distances to obtain food (see Box 31.1) and because of this usually regurgitate food for young. Inshore foragers travel much shorter distances, forage in and around reefs adjacent to colonies and carry whole fish in their bill to feed young. Foraging mode also affects both the number of eggs laid and chick growth rates, with more pelagic foraging species having single egg clutches and much slower growing chicks.
Inshore, offshore and pelagic foragers show consistent differences in the size of breeding colonies. Inshore foragers usually have small, more densely packed colonies of up to a few thousand birds, while pelagic species can have enormous colonies spread over multiple closely spaced islands. For example, ~500 000 wedge-tailed shearwaters breed in the 13 islands of the Capricorn and Bunker groups.
Such high population abundance means that where large seabird colonies occur the quantity of marine resources consumed by seabirds is significant. Such high consumption rates also mean that seabirds play a number of important functional roles in marine ecosystems. Most seabird prey items are either pelagic fish or squid. Therefore, seabirds play a major role in transferring nutrients from offshore and pelagic areas to islands and reefs. For example, black noddies deposit an estimated 4 tonnes of guano on Heron Island per annum. Some species of seabirds also play an important role in seed dispersal and in distributing organic matter into lower parts of the developing soil profile (e.g. burrow-nesting species such as shearwaters).
Seabirds can forage alone, but most forage in large mixed species aggregations or flocks. The likely reason for this is that flocking offers advantages in both finding and accessing prey. Different species forage in different ways. Species that capture prey within a metre or so of the ocean surface usually just dip their beaks underwater either after landing, or while still in the air. Other species that forage to depths of up to 20 metres may plunge dive at speed using just the momentum of the dive to carry them to prey, or they may extend the pursuit of prey underwater using slight movements of their wings. A third group more actively pursues prey by swimming underwater using their wings or feet for propulsion. Regardless of foraging mode most seabirds are also heavily reliant on the activity of subsurface predators to drive prey within reach. For this reason the presence and activity of large predatory fishes such as tuna and mackerel, as well as other subsurface predators, are extremely important to seabirds. This is particularly so in the relatively resource poor foraging environment of the tropics.
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