Hawaiian birds
Avian malaria and avian pox have been suggested as contributing to the decline of
the Hawaiian birds (Warner 1968). Migratory waterfowl may have provided a reser-
voir for avian malaria on the Hawaiian islands, and the continuous reintroduction
by migration may have maintained a high level of infection in the face of a decline
in host numbers (Dobson and May 1986b). Alternatively, avian malaria may have
been carried by introduced birds such the common myna (Acridotheres tristis),
which may themselves have maintained the disease at a high level because they are
not greatly affected by it.
Originally there were no mosquitos on Hawaii capable of spreading malaria. The
accidental introduction of mosquitos in 1826 and their rapid spread throughout the
islands coincided with the decline of many species of birds. Six of 11 endemic passer-
ines died out by 1901 on Oahu before their habitats had been modified (Warner 1968).
Experiments showed that the Hawaiian passerines, especially the honeycreepers,
are highly susceptible to malaria, much more so than are the introduced species
(Warner 1968).
Avian malaria is a factor in restricting the present distribution of native birds on
Hawaii, lending credence to the suggestion that it is implicated in the extinction
of other species. Scott et al. (1986) noted that elevations above 1500 m that were
free of mosquitos hosted the highest densities of native birds, especially of the rarer
passerines.The heath hen
The history of the heath hen is related by Bent (1932). We use here the summary
and interpretation of that history presented by Simberloff (1988).Probably the best-studied extinction is that of the heath hen (Tympanuchus cupido
cupido). This bird was originally common in sandy scrub-oak plains throughout
much of the northeastern United States, but hunting and habitat destruction had
eliminated it everywhere but Martha’s Vineyard by 1870. By 1908 there were 50
individuals, for whom a 1600 acre refuge was established. Habitat was improved
and by 1915 the population was estimated to be 2000. However, a gale-driven
fire in 1916 killed many birds and destroyed habitat. The next winter was unusu-
ally harsh and was punctuated by a flight of goshawks; the population fell to
150, mostly males. In addition to the sex ratio imbalance, there was soon
evidence of inbreeding depression: declining sexual vigor. In 1920 a disease of
poultry killed many birds. By 1927 there were 13 heath hens (11 males); the
last one died in 1932. It is apparent that, even though hunting and habitat destruc-
tion were minimized, certainly by 1908 and perhaps even earlier, the species
was doomed. Catastrophes, inbreeding depression and /or social dysfunction,
demographic stochasticity, and environmental stochasticity all played a role in
the final demise.The previous sections summarized 12 examples of extinction or steep decline. The
decline of the heath hen and the Hawaiian birds may be attributable to several fac-
tors but research on those species has not adequately revealed the causes of the declines.
The extinction of several species of wallaby seems very likely to reflect habitat
modification. The extinction of the Stephen Island wren and the decline of the LordCONSERVATION IN PRACTICE 321
18.2.10The effect of
multiple causes