How often has Sylvester killed Tweety?

From ANIMAL PEOPLE, July/August 2009:
How often has Sylvester killed Tweety?
by Judith Webster
The summer 2009 edition of B.C. Nature included an article
entitled, “Cat licensing: A conservation strategy that can work.”
As a cat and bird lover, I was inspired to investigate author
Sherril Guthrie’s claim that cat licensing and confinement bylaws
would “protect and restore our bird and small mammal populations, as
well as return cats to their rightful place as valued pets and
Guthrie relies on American Bird Conservancy “Cats Indoors!”
campaign literature, which includes too many disputable studies,
extrapolations, and anecdotal reports to delve into deeply here.
However, in her strong opening paragraph, Guthrie discusses a new
bird count analysis by the National Audubon Society revealing that 20
of North America’s most common birds have lost over half their
population since 1967.

But Guthrie was wrong to connect these troubling declines to
discussion of invasive species in BirdLife International’s 2008 State
of the World’s Birds report. That report considers invasive species
the third most important threat only to “globally threatened” birds.
These are birds who are at imminent risk of disappearing entirely
from their typically extremely limited habitats, most often on
remote islands. BirdLife International identifies cats as the second
most important invasive afflicting these species.
The 2008 State of the World’s Birds report attributes the
population declines of the 20 common North American birds to habitat
loss caused by altered human use of land and, in some cases,
climate change.
The 2008 State of the World’s Birds report does not
explicitly deal with the relevance of cats or other introduced
species to the population dynamics of “Least Concern” species.
However, BirdLife International fact sheets for the 20 common North
American birds in decline mention predation from native or invasive
alien species for three: the northern bobwhite (mammalian
predators); the common tern (mammals and gulls); and the northern
pintail (feral cats and rats on islands).
Data sheets for the same birds from the National Audubon
Society list cats not once. Global warming will increase the risk
from nest predators, who are not necessarily introduced species, in
the tundra breeding grounds of the greater scaup and snow bunting,
but feral cats do not live on the tundra. Neither could feral cats
invade the tundra, across vast expanses of thawed permafrost,
without evolving the ability to swim like otters.
The North American continental bird species who are menaced
by invasive threats are most often stressed by alien plants
transforming their habitat. Audubon explicitly names an invasive
animal threat to just two birds: fire ants plague the northern
bobwhite, and zebra mussels impact the greater scaup.
Although BirdLife International considers cats the invasive
species of second most importance to “globally threatened” birds,
the vast majority of these impacts take place on oceanic islands. Of
the 174 species negatively affected by cats, 27 occur on the North
American continent but only the Florida scrub-jay, found in six
fragmented Florida localities, is actually deemed to be threatened
by cats in North American continental habitat. BirdLife
International regards cats as a “low” impact threat to this bird,
causing a “negligible” rate of decline.
Among the 26 other “globally threatened” birds who land at
times in North America, 16 are endemic to Hawaii, nine are seabirds
threatened by cats in their island breeding colonies, and one is
threatened by cats at a Pacific island winter habitat.
Of the 134 confirmed bird extinctions occurring since 1500,
only 15 occurred on continents, only five involved introduced
species, and only the demise of the paradise parrot of Australia is
attributed in part to cat predation–but the primary causes of the
loss of paradise parrots were drought, overgrazing, altered fire
frequencies, and the prickly pear.
The other four continentally extinct birds whose loss is
attributed in part to introduced species are the passenger pigeon
(Newcastle disease), the Carolina parakeet of eastern U.S. (honey
bee), the Colombian grebe of Bogota (rainbow trout), and the
Atitlan grebe of Guatemala (largemouth bass).
Among 21 bird extinctions, probable extinctions, or
extinctions in the wild occurring from 1975 to the present, six
occurred on continents but none involved cats. Three of the lost
species were impacted by introduced species: the Colombian and
Atitlan grebes, and Spix’s macaw of Brazil (honey bee).
The other 15 were or are island birds. Six were hunted by
cats, among numerous other threats to their survival, both
introduced and native. Five of the six had very limited habitats in
Hawaii. Cats are said to have had only a “medium” role in the loss
of the Hawaiian Crow, a “low” impact on the Ou, and an “unknown”
impact on the Po’o-uli. Cats had an “unknown” effect on the sixth
species, the Guam rail, known to have been killed off chiefly by
brown tree snakes.
Clarification of cat and bird data is never an idle exercise.
Guthrie dismisses a “spay/neuter/release” program for feral cats as
“expensive” and “a limited approach that is not sustainable,”
despite evidence from around the world that neuter/return succeeds
quite well when a sterilization rate of at least 70% is quickly
achieved within a particular habitat.
Instead, Guthrie lauds Calgary’s “Responsible Pet Ownership”
bylaws as a “comprehensive approach that balances the needs of our
birds and wildlife with the needs of cats and their owners.” Yet
Calgary Animal Services is still receiving and killing approximately
the same numbers of cats since the introduction of mandatory
licensing and confinement as it did before cat licensing and
confinement were introduced. In addition, the MEOW Foundation of
Calgary continues to rescue homeless cats in numbers roughly
equivalent to those processed by Calgary Animal Services. The
Calgary Humane Society, which handles four times the cats as CAS and
MEOW combined, has adopted out fewer cats and has killed more.

[Judith Webster is a medical transcriptionist in Vancouver,
British Columbia, Canada.]
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