It sounds bad, but not everyone is convinced that cats are actually putting bird populations under pressure. “The kneejerk reaction is that they must be having some sort of impact – they’re killing millions and millions,” says Baker. However, numbers can be deceptive. According to Baker, the birds most hunted by cats have so many young that they can afford to lose a lot of them. In the UK, he says: “I just categorically say there is no evidence of an impact.”
The UK’s largest bird charity, the Royal Society for the Protection of Birds (RSPB), is not particularly concerned about the impact of cats on the British mainland. Instead it focuses on what it says is driving UK bird declines:
global warming,
intensive agriculture and expanding towns and cities leading to habitat and food loss. “While we know that cats do kill large numbers of birds in UK gardens, there’s no evidence this is affecting decline in the same way that these other issues are,” said a spokesperson.
A big reason why they are less worried is the evidence that cats primarily take “the doomed surplus”: weak or injured birds likely to die anyway. In 2008, Baker led a
study in Bristol showing that birds killed by cats on average had less fat and muscle than birds killed by collisions with windows. While there could be other explanations – such as birds having less fat in the morning when cats tend to pounce – Baker says that the fat and muscle scores were so low that the birds were “in dire trouble before they got killed”.
Another study from 2000 found that cat-killed birds in Denmark had smaller spleens, indicative of a weaker immune system.