It is not easy for birds to get all the food they need from the sea. Except for along the shoreline, primary productivity (photosynthesis) is carried out by tiny phytoplankton. These are eaten by small zooplankton, which in turn are eaten by larger invertebrates or fish – only then is food available to seabirds. Photosynthesis requires light and only occurs in the upper layers of the water, and because nutrients (including mineral and organic compounds) tend to sink to the dark sea floor, oceans are much less productive than the ecosystems on land. Nutrient-rich patches (where algae can grow) are created where nutrients are stirred towards the surface by currents, convergences, wind, abrupt changes in depth, or combinations of these. Out in the open sea these oases tend to be far apart, and seabirds have to forage widely, often travelling for hundreds or even thousands of kilometres.
Flying versus diving
Flying and diving require very different types of bodies. Birds that fly well enough to cover large areas in search of scattered food resources are generally unable to dive deeply, and are restricted to feeding in the top few metres of the ocean. Albatrosses, the most adept of the seabirds at flight, float like corks when they alight on the water and can manage only shallow dives at best. On the other hand, birds that dive well such as penguins and diving petrels have compromised the ability to fly. Penguins’ heavy bodies and reduced wings allow them to dive to greater depths and swim more efficiently than other birds, but to achieve this they have lost the power of flight altogether.
Shearwaters are perhaps the consummate seabirds. Of medium size (weighing 300–800 grams) and with stiff, oily feathers, some species can dive to about 60 metres – as deep as some penguins – and forage as far out to sea as albatrosses.
Breeding on land
No bird can incubate its egg at sea, so the places where seabirds breed are inevitably separated from where they find food. This means that vast numbers of breeding seabirds make use of the islands which are close to rich feeding areas. The Snares Islands are strategically located within foraging distance of feeding zones, and the birds which breed there include millions of sooty shearwaters (muttonbirds or tītī), thousands of Buller’s albatrosses (Thalassarche bulleri), Snares crested penguins (Eudyptes robustus), various small petrels and smaller numbers of Antarctic terns (Sterna vittata), subantarctic skuas and gulls.
Likewise, a variety of seabirds breed tightly packed on the predator-free ground and rock stacks of the Chatham Islands, the Bounty Islands and some islands around the New Zealand mainland. Large populations once existed on the South and North islands before the first mammalian predators arrived with humans around 1300 AD.