Henderson crake  (Porzana atra)

The Henderson crake is the last surviving species of flightless rail (member of the Rallidae family) in Polynesia (3), and occurs on only one remote island in the Pacific Ocean. Its former generic name, Nesophylax, translates as ‘the black guardian of the island' and arises from its deep, velvety black plumage with a slight greyish gloss (2) (4). It has a contrasting red iris and eye-ring, and the bill is blackish with a yellowish-green base. The Henderson crake compensates for its inability to fly with long, well-developed legs for running. Males have red legs, mottled with black, females have plain red to orange legs, and juvenile birds have black legs. Juveniles also differ by being greyer than the adults on the throat and underparts, and their irises are brown (2) (5).

Endemic to Henderson Island in the Pitcairn group in the southern Pacific Ocean (2).

The Henderson crake's preferred habitat is the shady understorey of dense forest carpeted with a thick layer of leaf-litter (3) (5). It also occurs in open forest, thickets, and coconut groves (2) (5).

The Henderson crake, known as ‘chicken-bird' to the Pitcairners (4), is an opportunistic feeder that scratches in the leaf litter with its feet and tosses away leaves with its bill, consuming any insects, spiders, skink eggs, roundworms, and terrestrial molluscs it encounters. It also picks prey from the undersides of leaves (2) (5). Pairs of Henderson crakes forage together, remaining within ten metres of each other and maintaining contact with frequent frog-like ‘kwa' calls (3). Bold and curious, Henderson crakes are known to approach stationary observers (3).

The monogamous Henderson crake forms pair-bonds, and is thought to be territorial. It breeds from July to February, laying two to three eggs in generally spherical nests. The nests are constructed primarily by the male, from shredded palm leaves, and are placed up to 30 centimetres off the ground in a leaf clump or at the base of a trunk. Incubation of the eggs lasts for 21 days and is carried out by both sexes. The velvety black chicks leave the nest soon after hatching and are fed and cared for by both parents, with the assistance of young of the previous brood and by other adults in the family group. These helpers also assist with protecting the eggs and chicks from predatory crabs and rats (2) (5).

Henderson Island is a raised reef island, uninhabited by people, yet its animal inhabitants may still be threatened by human activities. The Polynesian, or Pacific, rat (Rattus exulans) was introduced by man centuries ago, and preys on the eggs and chicks of the Henderson crake (2). Yet, as the crake and rat have coexisted for some 800 years, it appears that the crake has adapted to the rat's presence (3) (4), and is not significantly threatened by this alien species. However, the accidental introduction of a more aggressive predator to the island, such as domestic cats or a second species of rat, could have a devastating effect on the crake population and result in its rapid decline and extinction (2). The introduction of further predators and diseases is sadly thought to be inevitable because of unauthorised landings of yachts on the island (3).

To ensure the Henderson crake's survival it is essential to implement measures to prevent the accidental introduction of any further alien species to the island (1). In addition, establishing a viable population on another bird-less island would protect this species against extinction in the event of a catastrophe on Henderson Island. Translocations have a much higher chance of success when larger numbers of individuals are moved, so it has been recommended that a translocation occurs before the Henderson crake declines to numbers so small that any translocation has a low probability of success (3).

For further information on the Henderson crake see:

• BirdLife International:
  www.birdlife.org/datazone/species/index.html?action=SpcHTMDetails.asp&sid=2913&m=0

Authenticated (08/05/08) by Dr Michael Brooke, Department of Zoology, University of Cambridge.