On their website, the American Bird Conservancy state that domestic cats that are allowed outside in the USA “kill approximately 2.4 billion birds every year. Although this number may seem unbelievable, it represents the combined impact of tens of millions of outdoor cats.”
It is common knowledge that there are more domestic cats allowed outside in the UK than in the USA. Therefore, in the UK one would expect that the predation rate on birds by domestic cats in the UK is higher than in the USA. Not so.
There are about 90 million domestic cats in the USA. At the date of a UK study which I’m going to quote, published May 27, 2003, there were an estimated 9 million domestic cats in the UK. So conveniently there are 10 times the number of domestic cats in the USA in this comparison.
This UK study concluded, by using a questionnaire sent to cat owners covering the period April 1-August 31, 1997, that an estimated 27 million birds were killed by outdoor domestic cats. That applies to 6 months. We can double that to 12 months which makes 54 million birds killed by outdoor cats in one year in the UK by 9 million domestic cats.
As there are 10 times the number of domestic cats in the USA (in this comparison) we can multiply 54,000,000 x 10 to make 540 million birds killed by domestic cats in the USA based upon the UK study’s conclusion.
This is one fifth of the American Bird Conservancy figure. Therefore, you might argue that the ornithologists of America have overestimated the number of birds killed by cats in the country by a factor of five! Okay the calculation is basic and simplistic but the difference in the result is startling.
Even if you do not agree that American ornithologists have overestimated as stated, you would have to agree that when experts estimate the number of birds killed by domestic cats in their country, they can get their numbers wrong. They can get them very wrong in fact. And the reason as I have stated before is that there’s never been a single nationwide study on the number of birds killed by cats in any country on the planet.
All the studies have concerned quite small areas, perhaps a village, or perhaps 500 cats or even less. If you work out the number of birds killed by 500 cats over a year can you magnify the resulting figure up to 90 million cats and expect the resulting number to be accurate? I would tell you that you can’t because there are many variables. But if you have an axe to grind and you have an ongoing campaign to encourage legislatures to impose cat confinement laws in the jurisdiction you are liable to produce a biased result. In my opinion, that is what is happening in America.
The UK study I have referred to; the full citation is: WOODS, M., MCDONALD, R.A. and HARRIS, S. (2003), Predation of wildlife by domestic cats Felis catus in Great Britain. Mammal Review, 33: 174-188. https://doi.org/10.1046/j.1365-2907.2003.00017.x