At number one is the rare and striking giant ibis (Thaumatibis gigantea) – the world’s largest ibis weighing in 4.2kg and reaching more than one metre in height. With only 230 pairs estimated to remain in the wild, it is a critically endangered species. Habitat loss, human disturbance and possible hunting have reduced its range to an extremely small, declining population concentrated in Cambodia. Photograph: FLPA/AlamyA drawing of the New Caledonian owlet-nightjar (Aegotheles savesi) by Joseph Smit for the Ibis Journal, 1881. This mysterious species, endemic to the island of New Caledonia, has not been seen since 1998. The bird has been classified as critically endangered as its population is unlikely to number more than 50 individuals. A California Condor (Gymnogyps californianus) in flight over the Grand Canyon national park. In 1981 the wild population numbered just 21 birds after a century of persecution (shooting, poisoning,), unintentional poisoning (lead shot) and loss of wildlands. Enormous efforts have been made to save the species from extinction. Photograph: Charles Melton/AlamyAt number four on the list is the kakapo (Strigops habroptila), a noctural parrot that has evolved as flightless due to the historic absence of mammalian predators in its New Zealand habitat. Hunting, the introduction of predators, forest clearance and habitat degradation have caused a catastrophic decline in numbers. It is now extinct throughout its natural range, and survives only on three small, intensively managed islands following a series of successful translocations. Dedicated conservation efforts have seen the population increase slowly to 125 individuals. Photograph: Stephen Belcher/CorbisThe kagu (Rhynochetos jubatus) is fifth on the list. This ash-white bird is mainly found in the dense, humid forests of New Caledonia and is known locally as the ‘ghost of the forest’. Photograph: Doug Cheeseman/Getty ImagesThe Bengal florican (Houbaropsis bengalensis) is one of the most threatened bustards in the world. There are two subspecies, one in India and Nepal, and the other thousands of kilometres away in Cambodia. Both populations are in decline and threatened by habitat loss for conversion to agriculture and illegal hunting. The total global population has been estimated at less than 1,000 adults. Photograph: FLPA/AlamyAt number seven is the Forest owlet (Heteroglaux blewitti), which has been reduced by habitat loss to an extremely small and fragmented population in central India. Photograph: PM Laad/ZSLThe Philippine eagle (Pithecophaga jefferyi) is one of the world’s largest, most powerful birds of prey. Endemic to the Philippines, the eagle’s small, rapidly declining population has been feared close to extinction for the past 40 years. In light of this, it recently acquired the status of the national bird of the Philippines, which has helped greatly to increase awareness of the bird and its plight. Photograph: Jef Maitem/AlamyThe Christmas Island frigatebird (Fregata andrewsi) is the rarest endemic seabird on Christmas Island, Australia. Its breeding habitat is affected by the invasive yellow crazy ant, while threats from habitat loss and pollution from phosphate mining are ongoing. Photograph: Franco Salmoiraghi/AlamyThe Sumatran ground cuckoo (Carpococcyx viridis), at number 10, is a forest-dwelling bird endemic to the Indonesian island of Sumatra. It is estimated that there are 70-400 individuals in the wild. While little is known about the species, it may well be in decline due to deforestation. Photograph: Nick Brickle/WCS• The headline and standfirst of this gallery were corrected on 11 April 2014 because they referred to the “most unique” birds. As our style guide points out, unique is “one of a kind, so cannot be qualified as ‘absolutely unique’, ‘very unique’, etc”.
