Draft:Cat predation on islands

Three-quarters of threatened bird species on oceanic islands are in danger from introduced species and a much publicized claim is that cats are responsible for the extinction of at least 33 bird species on such islands. Recent IUCN Red List data about the conservation status of bird species shows the claim is wrong (Table 1).

The claim is often repeated in science and wildlife publications and retold in popular articles as though it is an unquestionable fact; for example Nogales et al. 2004 say “Feral cats are directly responsible for a large percentage of global extinctions, particularly on islands” and that “Feral cats are responsible for the extinction of at least 33 bird species”; and the 2020 issue statement by the Wildlife Society states “Domestic cats have tremendous impacts on wildlife and are responsible for the extinction of numerous mammals, reptiles, and at least 33 bird species globally.”

The problem
Claims that cats have exterminated at least 33 bird species have prompted some wildlife researchers, in particular Dauphiné and wildlife co-researcher to advocate for “removal” of feral cats to save birds and wildlife should become “a permanent, regular feature of wildlife management” and draw attention to removing cats on continents because cat predation might contribute to declining bird populations in “habitat islands” like open spaces within developed environments.

Some experts argue that in particular circumstances cats can harm wildlife, but accusing cats of laying waste across biodiversity rests on irrational prejudice, poor scientific reasoning and selective picking out of data to support biased arguments. Some researchers detect ailurophobe attitudes and scapegoating of cats in some published science papers. Indeed, Dauphiné was convicted in a Washington D.C. courtroom of attempted animal cruelty by poisoning cats in her city neighborhood.

Chinese whispers
Chinese whispers is “a situation in which a piece of information is passed from one person to the next and is changed slightly each time it is told.” The initial information that started the story of cats exterminating 33 bird species is from Extinct and Vanishing Birds of the World, a book written by Greenway and published in the 1950’s. The sketchy documentation he gathered is from testimonies he heard and from his own deductions at a time when the extinction of species was beginning to attract serious attention.

A decade later Ziswiler added information from Greenway’s book to his own book, Extinct and vanishing animals: a biology of extinction and survival, in which he lists in an appendix the apparent causes of the extinction of many species.

Then in the late 1970’s Jackson gave a talk at a symposium about endangered birds. During the talk he presented a bar graph showing cats had exterminated 33 species of birds. His talk was published a year later in which he states he condensed the bird data from Ziswiler.

Liver in 1994 used Jackson’s data for his book Naturalized Animals: the ecology of successfully introduced species. In the book Lever states that “…naturalised predators have collectively been responsible for the extermination throughout the world of no fewer than 61 avian taxa, the principal culprits being feral domestic cats which have caused 33 extinctions…”

Finally, as stated above, Nogales et al. attribute Lever as the source for their claim that cats have exterminated 33 bird species.

Ziswiler’s appendix
Ziswiler’s appendix lists wide ranging basic information on many species presumed exterminated, including species exterminated by cats. But he lists 18 bird species, not 33 bird species as claimed by Jackson. Some of the bird species he lists are actually sub-species and do not count as extinctions since the claim that cats have exterminated 33 bird species is about exterminating species, not sub-species. All 18 bird species inhabited small oceanic islands, not continents, except the Eyrean Grass-wren of central Australia (whicperh is actually extant, number 16 in Table 1). A recording difficulty is that Ziswiler lumps the Hawaiian Honey-creeper (number 17 in Table 1) as “16 forms”. By this he might mean they are species or sub-species. IUCN records that some Hawaiian Honey-creeper species are extant and some are extinct.

IUCN records
Of the 18 bird species listed by Ziswiler, IUCN records that just nine of them are extinct (Table 1). Adding all the “16  forms” of Hawaiian Honey-creeper to these nine would mean that cats may have been involved in the extinction of from nine up to perhaps 25 bird species. This does not add up to the 33 bird species that cats are commonly claimed to have exterminated.

Ziswiler also lists a further eight mammal species (Table 2) presumed exterminated by cats (Table 2). Of these, he does not describe three sufficiently to identify them (numbers 6 to 8). For the remaining five, IUCN record that one species is extinct and four species are extant.

Cats on islands
Cats are one of the players in the ecology of bird extinctions on some oceanic islands where native wildlife has not evolved to defend itself against introduced predators. No one making the claim that cats have exterminated at least 33 bird species cites an accurate first hand source: the mistaken claim is based on Chinese whispers.

Tables
Table 1. Conservation status according to recent IUCN data of the 18 bird species listed in Ziswiler. The names for some species have changed since the 1950’s and are recorded in the right-hand column.

Table 2. Conservation status according to recent IUCN data of the eight mammal species listed in Ziswiler.