Domestic cats: Guilty of the purrfect crime
Your soft, cuddly, purring companion beside you on the sofa may never have brought home the remains of one of our declining songbird population. But Felis catus is part of a worldwide problem that is destroying billions of wild birds every year, which could, in turn, have a serious impact on global biodiversity. The trouble is that few people want to address this catastrophe apart from a bit of hand-wringing. The billion-pound conservation industry, the Government and even the media look the other way.
We are talking about colossal numbers. For instance, in the US it is estimated that domestic and feral cats kill 2.4billion birds annually. The number in the UK is also staggering. According to the Pet Food Manufacturers? Association, cat numbers have doubled since 1975 to more than 10million, which probably kill about 100million songbirds every year.
So, how many songbirds are there in the UK? According to the British Trust for Ornithology, the total breeding population of the 32 species killed by cats is fewer than 100million birds ? the same number as the annual toll taken by cats. Of course, many songbirds lay large clutches of eggs and attempt to raise large broods to compensate, but this strategy breaks down if the predation rate is too high. Predation is natural and a certain attrition of eggs, nestlings and adult birds is to be expected. However, what level is sustainable and will not lead to declines in prey species?
The annual productivity of the songbird population is about 200million, so the non-native cat may be killing half the birds? productivity. The effect of cats can also be likened to ?hyperpredation?, where large numbers of introduced predators have been sustained by introduced prey species, such as rats or rabbits, but pose a real threat to native species. Cats, of course, are being sustained by their owners.
In recent years, the populations of all other predators, such as crows, magpies, sparrowhawks, grey squirrels, foxes and badgers, have doubled. They could also be adding to an unsustainable pressure on our diminishing dawn chorus. Many species of songbird, such as house sparrows, dunnocks, bullfinches and songthrushes, have declined steeply. Others, such as the robin and wren, are holding their own, possibly because they are better at concealing their nests.
We are seeing increasing evidence that cats and other predators kill and eat vast numbers of songbirds, nestlings and eggs and lower their productivity. But other factors can play a part. For instance, a major cause of rural songbird decline has been modern farming intensification. However, the Cockney house sparrow, which thrived in the smog and slums of old east London, has also declined where it is far removed from any farming influences.
There is also a wide variation in some factors. For example, some cats do not kill any birds at all and some may be too old or unfit to do so. But they can live at a density in our towns and villages of more than 500 cats per sq km, far higher than other urban mammalian predators. Meanwhile, feral cats in the countryside are even more deadly for wildlife.
Before we can draw up possible strategies, we need evidence. Cats are relatively easy to study compared with other wild and generalist predators ? but only up to a certain point. This is because cats are domestic pets and most research can be summed up as ?look at what the cat brought home?. The key question, though, is how much prey cats actually do bring home ? most studies use a correction factor of about x 3.3. This implies that about a third of prey items are brought home, but a recent study in the US arrived at a factor of four, which means predation rates could be even higher. But only about a third of prey items are birds. In the UK, this suggests that if 10million cats (including one million ferals) only bring home three birds each year, the total is 100million birds (using a factor of 3.3).
But nobody is prepared to say if this is reducing bird populations and whether cat predation mortality is compensatory (birds would have died anyway) or additive (impacting populations). The only type of study that would answer this would be a conventional predator removal experiment. The problem is that the domestic cat population cannot be manipulated.
The RSPB claims ?there is no scientific evidence that predation by cats in gardens is having any impact on bird populations UK-wide? but ?there is evidence that cats tend to take weak or sickly birds and it is likely they would have died of other causes?. This is potentially misleading because it could imply that research has been carried out that found no evidence. In fact, no research has even been attempted to ascertain whether cat predation is having a negative effect on bird populations. This research is long overdue.
Cats certainly tend to take weak or sickly birds. However, recent studies have highlighted the importance of the sublethal and indirect effects of predators where the mere presence of cats and their disturbance can reduce parental provisioning rates by a third. This, in turn, reduces nestling growth by about 40 per cent and leads to increasing predation by magpies and grey squirrels which are alerted by the youngsters? persistent begging calls for food. The weaker birds taken by cats may well be those which are flying greater distances to provision their young so their mortality could be additive, reducing populations.
We cannot expect much help from DEFRA. A weakened coalition government wouldn?t dream of considering any form of cat control ? it would be an instant vote loser. However, that does not rule out supporting much-needed research.
Some will point out that cat owners are stakeholders and their interests need to be taken into account. Pet ownership provides health and therapeutic benefits in an ageing population in which increasing numbers live alone. Cat licensing and curfews are probably a nonstarter here, though they are being used successfully in Australia and New Zealand.
What can be done?
Any attempts to reduce the impact of cats on bird populations could only be achieved on a voluntary basis. Some of this already takes place with neutering and collar-mounted anti-predation devices. However, 70 per cent of cats are still allowed to roam free at night, with most predation occurring at dawn and dusk. Voluntary curfews near ecologically sensitive areas could also be encouraged.
Public awareness needs to be raised on the back of the quality research which doesn?t yet exist. There are two priorities: first, to determine a more accurate ?correction factor? ? the amount of prey that is not brought home by cats. This is becoming more achievable with modern technology such as ?cat cams?, activity data loggers and stable isotope analysis. Second, it cannot be beyond the wit of man to devise a non-lethal predator removal experiment to examine the impact of cats on prey species. This would require finding one or two areas where the maximum number of cat owners could be persuaded to keep their pets indoors at night and the prey counts compared with other areas. In the US, between 50 per cent and 60 per cent of cat owners keep their pets indoors. The figure in the UK is only three per cent.
With 100million songbirds killed annually we are suffering from a Big Garden holocaust ? and a state of denial. The price is likely to be extinctions. Cats are the most wonderful, easy pets for a family but if we are going to enjoy owning millions of them we should at least address the unforgiveable absence of quality research on their impact.