The Impact of Feral Cats on Island Bird Populations
Anthony Dunne, Third year (1442/E).
The domestication of the cat (Felis catus) has been documented as occurring
some 4000 years ago. The wild ancestor of the domesticated form is attributed
to be the African wildcat (Felis silvestris libyca). However, one may
claim that unlike the dog, the cat has never really been domesticated
and such evidence is readily identifiable by the common occurrence of
feral cat populations. These feral groups can have detrimental effects
of island species, most notably on island bird populations.
It has been well documented that feral cats on small islands can eradicate
complete species from islands and sometimes confine that species to extinction.
One notable example is that of a single cat belonging to a lighthouse
keeper on Stephen’s Island, New Zealand, which single handedly caused
a species of wren to become extinct (Rothschild, 1905). In other cases
human intervention has slowed or even halted the island cat populations.
Tory Island, off the west coast of Ireland is home to the endangered Corncrake
(Crex crex). Many studies of the population have cited the feral cat population
there as the primary source of the birds decline as it spends much of
its time in long grass thus increasing its susceptibility to predation.
An ongoing program of removing the cat population from the island is ongoing
and is funded in part by the Irish Government (Jones, 1998).
One cannot claim that humans have played a major role in the decline of
these island species as in the majority of cases they are responsible
for the introduction of cats. In past times many a ship had a small complement
of cats in order to control the mice populations but which subsequently
left the ship of arrival at the port. These cats could then predate on
the native fauna which, in many cases, were ignorant to the threat and
whose population succumbed quite easily as a result. In other cases the
cats were introduced intentionally as pest control of lagomorphs and rodents.
Therefore, an obligation emerges for man to attempt to rectify his mistake.
Recently, a review of controlling procedures adopted across a number of
islands has been reviewed (Nogales et al., 2004). A total of 48 islands
were assessed on the methods of cat removal which included islands off
South America, in the Pacific and Indian Oceans. The main method was hunting
the cats using dogs along with direct poisoning via fish baited with poison,
secondary poisoning by ingestion of drugged rats and, somewhat disturbingly,
by the introduction of viral disease (feline panleukopenia). The success
rate varied and seemed to decline with increasing island size. It is noteworthy
that the largest island documented as a success was some 290km2 (Marion
Obviously, removal methods will inevitably draw attention from animal
welfare groups which cite procedures such as poisoning and shooting as
unfair and unjust to the cats. Many groups advocate neutering the animals
but in reality this will not reduce the predation rate of an animal that
can only possibly survive by predation. A number of studies have looked
at the effect of neutering on predation rates of the domestic house cat
and the results are variable to say the least (see Woods et al., 2003).
What does become evident, however, from such studies is that animal behaviour
is extremely difficult to understand and predict, even on isolated islands.
Furthermore, one should consider whether the time and expenditure invested
into the removal is beneficial in the long term. Many would argue that
the failure of a particular species to respond to a novel threat that
becomes established is in fact a species waiting to be confined to the
annals of distinction. One could argue that removal of the primary predator
(the cat) would simply leave a vacuum in the ecological food web which
will inevitably be filled in by another predator. In fact, many theorists
devote their academical life to the pure study of food web patterns and
designs. Nonetheless, the fact still remains that cats are having serious
implications on island fauna and their removal, regardless of the debate,
will prove itself only by long term studies.
• Jones, M. (1998): Corncrake Fieldwork in North Donegal, 1998. Unpublished
report. BirdWatch Ireland, Dublin.
• Nogales, M., Martin, A., Tershey, B.R., Donlon, C., Veitch, D., Puerta,
N., Wood, B., and Alonso, J. (2004). A Review of Feral Cat Eradication
on Islands. Conservation Biology 18 (2): 310-319.
• Rothschild, W. (1905): On extinct and vanishing birds. Proceedings of
the 4th International Ornithological Congress, London: 191-217.
• Woods, M., McDonald, R.A. and Harris, S. (2003). Predation of wildlife
by domestic cats Felis catus in Great Britain. Mammal Review. 33 (2):