Are cats causing bird declines?
Estimates of how many creatures are killed by cats each year vary significantly.
The most recent figures of how many creatures are killed by cats are from the Mammal Society. They estimate that cats in the UK catch up to 100 million prey items over spring and summer, of which 27 million are birds.
This is the number of prey items which were known to have been caught. We don't know how many more the cats caught, but didn't bring home, or how many escaped but subsequently died.
The most frequently caught birds, according to the Mammal Society, are probably:
- house sparrows
- blue tits
No scientific evidence
Despite the large numbers of birds killed by cats in gardens, there is no clear scientific evidence that such mortality is causing bird populations to decline. This may be surprising, but many millions of birds die naturally every year, mainly through starvation, disease or other forms of predation. There is evidence that cats tend to take weak or sickly birds.
We also know that of the millions of baby birds hatched each year, most will die before they reach breeding age. This is also quite natural, and each pair needs only to rear two young that survive to breeding age to replace themselves and maintain the population.
It is likely that most of the birds killed by cats would have died anyway from other causes before the next breeding season, so cats are unlikely to have a major impact on populations. If their predation was additional to these other causes of mortality, this might have a serious impact on bird populations.
Those bird species which have undergone the most serious population declines in the UK (such as skylarks, tree sparrows and corn buntings) rarely encounter cats, so cats cannot be causing their declines. Research shows that these declines are usually caused by habitat change or loss, particularly on farmland.
Gardens - an important habitat
Populations of species that are most abundant in gardens tend to be increasing, despite the presence of cats. Blue tits, for example, the second most frequently caught birds, have increased by more than a quarter across the UK since 1966. Of the birds most frequently caught by cats in gardens, only two (house sparrow and starling) have shown declines in breeding population across a range of habitats during the last six years.
Gardens may provide a breeding habitat for at least 20 per cent of the UK populations of house sparrows, starlings, greenfinches, blackbirds and song thrushes four of which are declining across the UK. For this reason it would be prudent to try to reduce cat predation as, although it is not causing the declines, some of these species are already under pressure.
Cat predation can be a problem where housing is next to scarce habitats such as heathland. It could potentially be most damaging to species with a restricted range (such as cirl buntings) or species dependent on a fragmented habitat (such as Dartford warblers on heathland).