HUGE props to you, D Dacatchair !!!@SeanS I am not sure which studies I mentioned you are asking about... As I posted this a couple years ago, not all of my notes and book marks or even devices where I saved this, are still easy for me to find. But I went through a while of being really curious about this and did a lot of digging around. So I will do my best to find and post links.... And also add some links to some other info I found in the past couple years, as my thinking on this has evolved.
As you will have gathered from my comments I am skeptical of the concerns that cats are causing declines in bird populations all over the world, but to be clear, I am NOT at all skeptical this is a very serious problem in some areas.
The information that 33% of the birds that cats kill are introduced species comes from a Nature article on the number of birds they estimate cats kill every year.
The impact of free-ranging domestic cats on wildlife of the United States - Nature Communications
“On average, only 33% of bird prey items identified to species were non-native species in 10 studies with 438 specimens of 58 species”
If you do some research on specific non native species you will see they usually have some impact on the ability of native species to survive. Sometimes this effect is serious, others not so much... But the fact is, birds introduced to North America, like house sparrows, pigeons, and starlings are often more comfortable in urban areas with large populations of cats, and they are often the birds caught by cats.
And this gets even more complicated because unless this estimate included an autopsy on every bird peoples cats brought in, according to the Royal Society for the Protection of Birds, in the UK, most of these were already dying from other causes.
How Many Birds do Cats Kill? UK Bird Declines - The RSPB
The count of number of birds killed every year needs to be considered in the context of the number of birds hatched every year... this number is usually much greater than the two parents, so for the population to be stable, all these extra birds, either the old or the young, are meant to be some predators lunch. In urban areas where we have squeezed out other predators, that predator is often a cat.
Then there is the whole question of how exactly we define native and non native species. There is recently found evidence in caves in Poland that shows Near Eastern cats had followed the agriculture and were living in Europe as far back as 6000 years ago, and analysis showed their main diet was rodents that ate human grown grain... but they were not yet eatting cooked food shared with them by humans. Near Eastern cats are the same species of cat that are the main ancestors of our so call ed domestic cats...
Ancestors of domestic cats in Neolithic Central Europe: Isotopic evidence of a synanthropic diet
And the fact is, these agricultural ecosystems that included cats and birds survived just fine for thousands of years.
These same agricultural ecosystems, along with the rodents and cats that found a niche within them, have been moved all over the world, and talking about what is or isn't a natural species in a landscape that in no way resembles the one that originally hosted the indigenous species, seems to be scapegoating cats when clearing land for sheep, cattle, and food crops has done far more damage.
Both rats and house mice originated in Asia. And they do frequently eat songbird eggs, nestlings, and even roosting birds.
Invasive rats strengthen predation pressure on bird eggs in a South Pacific island rainforest
I am having difficulty finding the study where nest failure in some areas was as high as 80%, but I did see a study that found this, and doesn't seem unlikely looking at the studies done using artificial eggs, and the other studies showing predation by rats on real nests usually occurs over several nights... (not likely to happen with artificial eggs) and real rat predation includes not just eggs but live chicks.
But thinking about this over the past couple years I am guessing the presence of cats probably does little to mitigate this, and cats real benefit is protecting human food stores from rodents pooping and peeing all over everything. Without the protection of free roaming cats, humans probably would never have developed agriculture... and as someone who grows food, I think this is still the case.
What If All the Cats in the World Suddenly Died?
Also here is a link to the aforementioned Small Mammal society counting what cats killed
https://www.mammal.org.uk/wp-content/uploads/2016/03/Domestic-Cat-Predation-on-Wildlife.pdf
That being said, I live with 2 active cats that have access to a large fenced yard and they do catch birds, both in their roofed enclosures and in the fenced area. And even though all wild animal die pretty horrific deaths, and they don't catch enough to put a dent in bird populations here, it breaks my heart when they get birds, and I even rescued an especially sweet and scared rat a couple months ago... Even though it is part of nature the suffering really bothers me... I used a 2X2 inch mesh for my enclosures and cat proof fencing, and birds get in, but the mesh prevents than from quickly getting out, and probably half the birds they catch would have gotten away if it wasn't for the fencing. I noticed their success seemed to go down when they were wearing bright colored walking jackets or a collar with a tracking device that flashed an LED light every minute. So there may be things we could invent that would warn birds while still allowing them to keep our food and homes free from rodents...
Cats place in our urban and agricultural ecosystems seems to be a really complicated topic, and the main point I was trying to make is it doesn't seem accurate to just count all the small critters killed by cats as unnatural or a bad thing that invariably damages ecosystems.
