Nonmarine Birds

Many other terrestrial, coastal and migratory bird species also use the continental islands and cays of the GBR. The numbers and types are too extensive to detail here, but these bird communities often resemble those occurring in similar habitats on the mainland. Some of the more common non-migratory species include the buff-banded rail (Gallirallus philippensis), Australian pelican (Pelecanus conspicillatus), eastern reef egret (Egretta sacra), pied (Torresian) imperial pigeon (Ducula bicolor), sooty oyster catcher (Haematopus fuliginosus) and beach stone-curlew (Esacus neglectus). Three species of fish-eating raptors also inhabit islands in the GBR, the osprey (Pandion haliaetus), brahminy kite (Haliastur indus) and white-bellied sea eagle (Haliaeetus leucogaster).

BOX 31.2 SEABIRDS AND CLIMATE CHANGE ON THE GBR

Climate change is predicted to raise sea-surface temperatures (SST) and increase the frequency and intensity of El Niño events. Recently it has been shown that for many seabird species breeding on the GBR the amount of food adults can gather during a single foraging trip is directly related to the SST over the same period. During periods of increased SST food availability to these species declines dramatically, decreasing meal sizes, feeding frequencies and rates of chick growth.

Total food availability to these same species decreases even further during intense El Niño events. For example, throughout the 2002 El Niño/coral bleaching episode in the southern GBR (see Chapter 10), food availability to shearwaters breeding on Heron Island dropped to one third normal levels. Adults were required to forage three to four times longer for equivalent meal sizes. This lack of food resulted in the complete reproductive failure of the Heron Island colony with almost 100% mortality of chicks and the loss of at least 2000 adults. It is likely that similar reproductive failures occurred throughout the Capricorn-Bunker island group.

Such findings demonstrate that predicted increases in both SST and the intensity or frequency of El Niño events will have serious detrimental impacts on tropical seabird populations throughout the GBR and in adjacent areas. Importantly, current evidence also suggests that at least some species, and possibly the majority of species, at all significant GBR breeding colonies are already in decline due to climate change related phenomena and/or show no recovery from recent ENSO impacts.

The eastern reef egret or eastern reef heron is common throughout the GBR and adjacent mainland. The species has an unusual, non-sexual colour dimorphism, with different individuals having either entirely white or slate grey plumage. The dark form is more common in temperate areas while the white form is more abundant in the tropics. The reason for the colour dimorphism is unknown. The two colour forms randomly interbreed and both white and dark offspring can be found in the same brood. When nesting on coral cays, reef egrets hunt both day and night at low tide using small territories they establish and maintain on the reef flat.

The buff-banded rail is widespread and secretive on the Australian mainland, but is also a highly dispersive and an obvious reef island coloniser, reaching many of the heavily vegetated cays. It is an omnivorous scavenger that feeds on small vertebrates and invertebrates as well as seeds, fallen fruit and other vegetable matter that it collects from within a defended territory. The species is highly susceptible to the impacts of introduced predators or competitors and often disappears from islands where these threats occur.

The pied imperial pigeon is one of the most numerous terrestrial birds on the GBR. It is a migratory species that visits coastal reef islands and the adjacent mainland south to about Mackay during summer. It then returns to Papua New Guinea during the winter. Greatest numbers of pigeons occur between Cooktown and Cape York with some colonies in this area reaching greater than ten thousand pairs.

Other less numerous but important terrestrial species on the GBR includes the capricorn white-eye (Zosterops lateralis chlorocephalus) and the yellow chat (Epthianura crocea macgregori). The Capricorn white-eye is the only endemic bird found within the GBR region and is restricted to the wooded cays of the southern Capricorn-Bunker region. Recent research suggests that it may be a distinct species. The yellow chat is the most vulnerable terrestrial bird species

Sea surface temperature (°C)

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