be the bird’s habitat. This was obviously a species on its last legs, but that problem
was not recognized until 1969 after which the population was studied intensively.
The population was stable at between eight and 10 breeding pairs, although in one
year it went down to six pairs. Apparently no more than 10 territories could be fitted
into the 25 ha of space and so we can be confident that the population on Mt Gower,
and probably that of the entire species, did not exceed that size over the 60 years
between 1920 and 1980.
The obvious candidate for the contraction of population size and range was the
black rat, which has been implicated in the extinction of several species of birds on
islands. In this case, however, the rat does not appear to be implicated. The wood-
hen kills rats, and in any event rats are more common on the summit of Mt Gower
than on any other part of the island. The culprit instead appeared to be the feral pig,
which will kill and eat incubating birds and will destroy the nest and eggs. Pigs can-
not accomplish the minor feat of mountaineering needed to reach the summit of Mt
Gower. The pigs were shot out in the 1970s and the cats by 1980 with the consent
of the islanders, who now ban domestic cats.
In 1980 a captive breeding center was established on the island at sea level and
seeded with three pairs from Mt Gower. Thirteen chicks were reared in the first
season of captivity, 19 in the second, and 34 in the third. The birds were released
and the captive breeding terminated at the end of 1983. The population reached its
maximum at about 180 birds, 50–60 breeding pairs, and that number seems to
saturate all suitable habitat on the island, mainly endemic palm forest. A by-product
of the pig and cat control is the expansion of breeding colonies of petrels, shear-
waters, and terns.The Stephen Island wren
The Stephen Island wren (Xenicus lyalli), the only known completely flightless
passerine, was discovered in 1894. It lived on a 150 ha island in Cook Strait which
separates the North Island and South Island of New Zealand. Subfossil remains
indicate that it was previously widespread on both main islands but became extinct
there several centuries before European settlement, part of the extinction event that
followed the colonization of New Zealand by the Polynesians about AD900. The causal
agent of its extinction on the mainland was probably the Polynesian rat (Rattus
exulans) introduced by the Polynesians.
The wren was extinguished by a single domestic cat, the pet of the lighthouse keeper,
Mr Lyall. He was the only European to see the species alive and then on but two
occasions, both in the evening. He said it ran like a mouse and did not fly, a fact con-
firmed subsequently from the structure of the primary feathers. The first one he saw
was dead, having been brought in by the cat. Subsequently the cat delivered a further
21, 12 of which eventually found their way to museums. Then it brought in no more.
The species went extinct in the same year that it was discovered (Galbreath 1989).The effects of pest control often exceed the original intentions of the control exercise.The black-footed ferret
The sinuously elegant black-footed ferret (Mustela nigripes) provides an example of
a species paying the price for the control of another. This account of its narrow escape
from extinction is taken from Seal et al. (1989), Cohn (1991), and Biggins et al. (1999).316 Chapter 18
18.2.4The side
effects of pest control