Population monitoring for the flightless rail Dryolimnas cuvieri aldabranus

Abstract

SummaryThe last flightless bird of the western Indian Ocean, Dryolimnas cuvieri aldabranus survives only on Aldabra. Its population numbered some 8,000 in 1973–1976. Surveys suggest numbers remained roughly constant between 1968 and 1988 (with a fluctuation of only 4% in responses to call playback between 1983 and 1988), but distribution continued to contract. Longevity can reach over 8.5 years (but is probably lower on average), and some birds remain within 100 m of the site of ringing for at least five years. Feral predators remain a threat, and captive populations are recommended. The monitoring procedure may have value for other Gruiformes.

Demographic resilience of territorial island birds to extinction: the flightless Aldabra Rail Dryolimnas (cuvieri) aldabranus as an example

“…Ostensible differences in territory sizes are probably due to the amount of dense mixed-scrub habitat intersected by transects on different islands. The congruence of the findings of this study with those of both Huxley (1982) and Hambler et al (1993) allows calculation of the Aldabra Rail population with more confidence on Malabar. Huxley (1982) calculated a density of 3 rails ha −1 in dense scrub and 1.5 rails ha −1 in open mixed-scrub.…”

“…Hambler et al (1993) found almost identical densities in 1988. It thus appears that there has been no significant change in breeding density since 1967 (Table 3; Penny and Diamond 1971, Huxley 1982, Hambler et al 1993). Ostensible differences in territory sizes are probably due to the amount of dense mixed-scrub habitat intersected by transects on different islands.…”

“…This model was robust towards decreases in annual survival and productivity. In the first sensitivity analysis, average annual chick production was reduced to 0.394 ± 0.021 (n = 50 Hambler et al (1993) Hockey, Wanless and von Brandis 6 simulations): the breeder population remained at 7000 and the average number (n = 50 simulations) of floaters dropped to 3 103 ± 112. For the second analysis we set very harsh environmental conditions, with average EV = 0.349 ± 0.014 (maximum = 0.67, n = 50 simulations).…”

Can the Aldabra White-throated Rail Dryolimnas cuvieri aldabranus fly?

“…Aldabra Atoll (9" 24' S, 46" 20' E) is home to the last flightless bird of the western Indian Ocean islands (Hambler et al, 1993): the Aldabra White-throated Rail (hereafter Aldabra Rail) Dryolimnas cuvieri aldabranus. It is currently classified a subspecies of the nominate form on Madagascar (Benson, 1967).…”

Behaviour, natural history, and annual cycle of the Henderson Island rail Porzana atra (Aves: Rallidae)

“…A further useful comparison can be made with the Aldabra rail Dryolimnas cuvieri, a flightless species some 2-3 times the size of the Henderson rail, living on the raised atoll of Aldabra in the Indian Ocean. In thick Pemphis acidula scrub Aldabra rails occurred at a density of c. 2 birds per hectare (Hambler, Newing & Hambler, 1994), similar to that of Henderson rails and spotless crakes.…”