General Lifehistory Characteristics

Seabirds are highly visible, charismatic predators in marine ecosystems that feed primarily or exclusively at sea. They have a range of relatively unique life-history characteristics associated with this predominantly marine lifestyle. Many of these characteristics are directly linked with having to forage over large distances to obtain sufficient food to breed.

In general, seabirds are long-lived species that have deferred sexual maturity, small clutch sizes, slow chick growth rates and extended fledgling periods. For example, crested terns (Sterna bergii, Fig. 31.1A) may live 18-20 years, sooty terns (S. fuscata, Fig. 31.1B) up to 32 years and larger species such as boobies and frigate-birds (Fig. 31.1C) even longer. Most seabird species do not become sexually mature or return to breed for between 5 to 12 years after fledging and the nestling period for some species, such as frigatebirds, can be up to six months long.

When nesting, care by both parents is usually required to successfully rear chicks. Following hatching, adults take turns brooding chicks until they are able to regulate their own body temperature. After this, chicks are left alone at the colony while both parents forage for food. At this stage the chicks of some ground-nesting tern species are mobile and gather together in cr├Ęches for protection from predators (Fig. 31.1D). Chicks of other species usually remain at the nest site.

Chicks of many seabirds also accumulate extensive bodyfat reserves to buffer themselves against long periods between adult feeding visits. Greatest mortality occurs during early life-history stages, such as eggs, nestlings and in the postfledging and prereproductive periods. Predation levels in seabird colonies are often significant and any disturbance by humans usually leads to increased levels of predation on exposed eggs or chicks.

Most seabirds breed on relatively remote islands, different species tending to nest in specific habitat types. Seabirds of the Great Barrier Reef (GBR) prefer coral cays to other island types. Most nest in dense colonies on the ground using either bare scrape nests or nests built among low lying vegetation. Other species use rocky crevices or burrows dug among vegetation dense enough to stabilise the soil structure and still others nest in trees on platform nests built of leaves or sticks (Table 31.1).

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