As a scientist, I'd love to tell you that I carefully examined the data collection methods and statistics presented in the paper, but Nature Communications is one of those journals who publish manuscripts, usually for a fee, from authors (who are themselves required to publish or perish from their respective academic jobs) and then Nature Communications turns around and charges everyone else to read those articles, with no kick-back to the authors or the source of research funds for the study. I believe the for-profit-motivated proliferation of such firms is largely responsible for most of the hastily-completed and poorly-controlled bad science being published today. Although I am at the mercy of this Professor-prostituting racket myself, I refuse to pay good money for publishers to make profits off what should be globally-available information, so I have read only the original abstract and seen other data second-hand in news reports.
Setting aside that minor rant, Loss's paper estimates, not from their own research but by an analysis of other published studies measuring kill rates in urban and rural environments, and by using other various extrapolations and predictions of cat, bird, and small mammal populations, that "free-ranging domestic cats kill 1.4–3.7 billion birds and 6.9–20.7 billion mammals annually." In other words, these authors take a whole bunch of assumptions, apply specific data sets to broader populations, and come up with some numbers that could be off by orders of magnitude if their assumptions are in error. Not to mention any possibility of bias from authors who are all either employed by the Migratory Bird Center of the Smithsonian, or the Division of Migratory Birds of the U.S. Fish And Wildlife Service. Personally, I'd like to see a little more research about unanticipated impacts before we see a massive Federal program created from taxpayer money to trap, neuter, and relocate cats.
I'm willing, however to set those concerns aside and allow for the fact that domestic cats may kill around 3 billion birds and 20 billion small mammals annually. I don't believe it, but if I accept the premise, then my response is still, "so what?" And for the cats, "Good on ya!" Twenty billion dead mice means twenty billion less roses that have canes chewed away, twenty billion less rats eating seed from my bird feeders and corn from my garden, and twenty billion less snakes in my garden that would have proliferated to eat the mice if the cats didn't. I'm sorry about the birds, but folks, that's the nature of a Darwinist environment. There's a whole lot of killing going on out there in nature. If the majority of those 3 billion birds are starlings and urban pigeons, then I'm not really very alarmed. Millions of cats die annually as well, killed by cars and coyotes and domestic dogs and human psychopaths. Yes, I am aware that cats have been responsible for the extinction of specific island bird species. So have snakes, and both predators were introduced to those islands by Man, blundering around in our usual stupid fashion. Man, in fact, has been responsible for the extinction of many more species than the domestic cat, so perhaps we should talk about limiting our own numbers before we throw stones at the cats. Put a new predator in an environment where the prey don't have time to adapt before they are eliminated, and extinction happens. Ask just about any species group, including some native human populations.
Regardless, my personal experiences are directly opposed to the findings of the Loss study. I have a cat in my garden, a calico named "Patches" by my imaginative children, who is a most efficient mouser. I find almost daily presents of prairie mice remains on my doorstep, but I never once have seen that cat catch a bird nor have I found the organic remnants of such an attack. Even the fat little ground-dwelling quail endemic to this area seem to be able to escape the clutches of my supposedly super-lethal cat. I'm left, therefore, in a quandary, wondering where exactly the evidence of the slaughter is? And in the meantime, I'm searching for a couple of more cats to live in an under-construction barn. I would, personally, rather find more mouse parts strewn around the barn floor than find the snakes that would otherwise be hunting for the mice, so if it comes to a choice between having barn pigeons and having cats, the barn pigeons are just going to have to toughen up.