Sources
Sources Excerpt here
Below are a variety of links to sources that explain how we know the scale of birds killed by cats in the UK.
Overall Cat Population in the UK
A report in 2010 found that 26 per cent of households owned cats, estimating the population at 10.3 million.
Similarly, a report in 2021 by Cats Protection found that "there are an incredible 10.8 million pet cats in the UK, with 26% of households owning at least one kitty" ('How many cats are there in the UK?')
This population figure has grown since the 2010 estimate broadly in line with the human population growth over the same period.
Most recently, a figure of 11 million was found in a YouGov survey commissioned by the veterinary charity PDSA in 2023. They estimated that 24% of the human population owns a cat, again in line with previous estimates.
Therefore, as of today (Winter 2024), we can be confident that there are roughly 11 million domestic cats owned by households in the UK.
The Cat Watch project run by Cats Protection suggests that there is an additional population of a quarter of a million more cats which are 'unowned', whether stray or feral. 11 million should therefore be viewed as an underestimate.
Almost every single one of these cats is allowed to roam freely across its local neighbourhood, with estimates that up to 97% of cats in the UK are 'outdoor cats', and are therefore a threat to their local wildlife.
Prey Brought Home By Cats
There have been many studies in this area, each with varying results due to the differing environments, but almost all paint a sobering picture of the effects of cats on the nearby wildlife.
Churcher and Lawton (1987) studied 70 cats in a Bedfordshire village for a year and found that they brought back 297 birds - about 4.2 birds per cat per year.
P. B. Churcher, J. H. Lawton (1987). Predation by domestic cats in an English village.
In 1997 the Mammal Society studied 986 cats over five months, who brought back over 14 thousand prey items (birds, mammels, amphibians, etc). They found that 73% of the cats studied brought back birds, meaning the average rate of birds per cat (including cats that were not recorded as returning any prey) was 3 birds each during that five month period, and would no doubt have been higher if measured across the whole year.
Woods, McDonald, Harris (2003). Predation of wildlife by domestic cats Felis catus in Great Britain.
In 2012 Thomas, Fellowes, and Baker studied 348 cats over several seasons, finding that they brought back 350 birds in total. The headline figure was that "the mean annual predation rate was 18.3 prey per cat per year" and that "birds contributed 30% of records", suggesting 5.9 birds per cat per year. (This also includes an assumption that only 30% of prey is returned to the home, which the authors admit "may be a significant over-estimate" and thus could suggest that "actual predation rates were substantially higher")
It also included some worrying conclusions about cats killing more adult birds of various species in survey areas than those areas normally support, implying inevitable population decline.
Prey brought back alive
Some may wonder - what about the birds that are brought back alive, and subsequently released? Sadly, this is not a significantly mitigating factor. Almost 90% of birds brought back are dead, and of the survivors, about 78% of those will typically not survive to be released.
Little can be known of the roughly 2.6% of the total - one in forty - that are lucky enough to get that far, such as whether they survived for long after their ordeal.
How many unseen kills are there?
To get a better picture of the prey that is killed but not brought back to the owner's home, and to understand how large of a problem that is, we need to understand what proportion of prey cats tend to leave behind or eat, so that we can count the 'hidden victims' in our figures. Thankfully, modern technology is allowing for closer monitoring of cats when they leave the home, meaning we can derive more accurate data.
Back in 2004, an early study into this using radio tracking and binoculars found a kill rate "3.3 times greater than the rate estimated from prey brought home", i.e. that only 30% of cat kills were typically returned to the cat's home.
That would be bad enough - but subsequent studies using more advanced technology have shown that the situation is likely to be even worse than that.
One study in the United States "monitored 55 cats during a 1-year period (November 2010–October 2011) using KittyCam video cameras" and found that only 23% of cat prey items were returned to households - 49% of items were left at the site of capture, and 28% were consumed.
Loyd et al. Quantifying free-roaming domestic cat predation using animal-borne video cameras (2013)
But most recently, Australian research performed a meta-study covering the two studies above plus five other examples of research tracking observed kills compared to returned kills, finding the percentage of kills brought home averaged at only 15.1%.
This would mean that mortality rates could be as much as than six times larger than the rate of prey brought home. For every dead bird brought back, there is likely to be at least three to five more left where they died, or eaten.
Non-lethal effects on population
Even with this worryingly high death count, it is likely to still be an underestimate of the actual effect on bird populations.
Quoting Pirie et al (see above), "disturbance and predator avoidance can decrease breeding success and fitness due to adults being flushed from nests, leaving nests exposed to predators [...] Predator presence may cause the avoidance of suitable habitat, or produce an increase in vigilance consequently reducing foraging rates".
Additionally, when an adult bird is killed during nesting season, there is a good chance that the bird was feeding nestlings who will now get no food, or less food, and are likely to perish as a result.
Some of this increased mortality will come at the hands of cats and will be included in the figures previously discussed, but those who die of the other causes will not, and sadly will currently go unmeasured.
Worse still, even the birds that are not directly predated are still affected, as predators also have indirect effects on their prey, causing birds to have to alter foraging patterns, use different habitats, or even to produce fewer offspring. Modelling done by the University of Sheffield suggests that "a small reduction in fecundity due to sub-lethal effects can result in marked decreases in bird abundances (up to 95%)"
A. P. Beckerman, M. Boots, K. J. Gaston. Urban bird declines and the fear of cats (2007)