A which mammals were previously absent, has led to the reduction of most of the indigenous flora and fauna of the island to very small relict populations.
Indigenous birds have fared particularly badly. It is known that St Helena supported at least six endemic land bird species and three endemic seabirds in the past. It is likely that at least seven of these were present at the time of first human colonisation of the island. Today only one species, the St Helena Plover or Wirebird Charadrius sanctaehelenae, survives.
The Wirebird is featured on St Helenas coat of arms and is held in great affection by island residents. First mentioned in 1638, the species was not fors a result of its extreme isolation, the island of St Helena has given rise to a surprising number of unique endemic animals and plants. This biodiversity has been dramatically reduced, directly and indirectly, by human agency since the islands discovery in 1502. St Helenas natural vegetation has been almost entirely removed by over-grazing, deliberate removal and inappropriate agricultural practices. This has resulted in extensive erosion and land degradation. Such large-scale habitat disruption, and the introduction of such exotic predators as cats, dogs, rats and pigs to an environment frommally described until 1873. Its numbers are, however, small. The total Wirebird population was estimated at around 450 individuals in 1988/89 but has declined since. A census during November 1998 recorded only 340 adult birds. The reasons for this decline are not yet understood but are currently being investigated as part of a research project on the species ecology carried out by the University of Reading (UK). The Wirebird is currently classified by IUCN and BirdLife International as “endangered”. This category includes species which are “facing a high risk of extinction in the wild in the near future”. The Wirebird is included on the basis of its small population size, extremely restricted range and recent rate of decline.
The Wirebird is most closely related to Kittlitzs Sand Plover C. pecuarius of Africa with which it undoubtedly shares common ancestry. Some behavioural traits suggest that the species has diverged from ancestral stock relatively recently, though some sub-fossil bones have been tentatively dated to the early Pleistocene.
Nothing is known of the biology of the Wirebird prior to the enormous ecological changes wrought on St Helena as a consequence of human colonisation. It seems likely, however, that the species, in common with all other members of its genus, has always been a bird of open habitats. If this is so then the Wirebirds population must always have been small, since early records suggest that the island was originally extensively wooded. It may well be that the Wirebird has in fact benefited to some degree from deforestation. It has been suggested that the Wirebird may have inhabited the open floor of native Gumwood Commidendrum robustum woodland. This habitat once covered much of the island at middle altitudes, including many of todays major Wirebird sites. The behaviour of the present day Wirebird population lends little support to this theory, however.
The majority of Wirebirds today inhabit dry, middle altitude pastureland and the semi-desert environment of the Crown Wastes. During the 1988/89 survey Wirebird density on pasture was almost three times higher than in semi-desert. The largest population (around 80 individuals) is found on Deadwood Plain.
The areas most favoured by Wirebirds can be categorised as having grass swards less than 10cm tall (typically dominated by Kikuyu Grass Pennisetum clandestinum), of relatively low stem density and mixed with broad-leaved weeds and patches of bare earth. Good Wirebird sites generally have shallow gradients and annual rainfall within the range 300-500mm.
The Wirebirds diet consists entirely of invertebrates. Beetles and caterpillars appear to be particularly important components of the species diet. Foraging typically accounts for around 60% of daytime activity and is most intensive in the early morning and late afternoon. Wirebirds will occasionally continue to feed after dark, at least on bright moonlit nights.
Wirebirds appear to be monogamous. Adults are territorial and all evidence suggests that they are highly sedentary. In common with many other island endemic species world-wide, Wirebirds show a strong disinclination to fly. They are, however, fully proficient flyers when the necessity arises. Most flights tend to be of short duration, normally reaching a height of less than 50m. A large proportion of Wirebird flight activity appears to be associated with display, though some individuals regularly fly several kilometres to favoured drinking and bathing sites.
Wirebirds nest throughout the year but there is a distinct dry season peak in breeding activity from October to February. The nest is a simple scrape in the soil with a thin lining of dry grass stems and rootlets. This lining is used to cover the eggs when an incubating adult leaves the nest in response to disturbance, thus making the nest extremely difficult to find. The clutch is, almost invariably, of two eggs and both sexes share incubation. The incubation period is approximately four weeks. Chicks normally leave the nest within 36 hours of hatching and are led to feeding areas by the parents. Young Wirebirds fledge when 5-6 weeks old, but may stay within their natal territory for some time afterwards. Wirebirds in their first year tend to range much farther than adults.
Little is known about rates of breeding success in the Wirebird. In 1988/89 less than half the clutches of known outcome produced chicks. Chick mortality has been estimated to be at least 65%, with survival from egg to fledging probably less than 20%. These are very rough estimates, however. It is known that Wirebirds may nest more than once in a year but there is no information on what proportion of the population do so, neither is it known how frequently lost clutches are replaced. Feral cats are probably the only potentially significant predators of full-grown birds. No evidence of serious predation was found during the 1988/89 survey, however. In the absence of a major predation problem mortality could be expected to be low and adult Wirebirds to be relatively long-lived. Hopefully the current ringing programme will provide more information on survival rates and longevity.