A summary of an article on cats and predation by George Johnson, biology professor at Washington University.

One of Dr. Johnson's four cats, Feisty, was a hunter. Dr. Johnson's knowledge of biology told him that "predators ensure that only the more-fit individuals of a population contribute to the next generation, by the simple expedient of removing the less-fit. By taking the birds that are least able to escape - the sick and the old - Feisty culls the local bird population of less fit individuals, leaving it on average a little better off."

Two French biologists put his hypothesis to the test. Drs. Andre Moller and Johannes Erritzoe of the Universite Cure in Paris devised a simple way to test the hypothesis.

They compared the health of birds killed by domestic cats with that of birds killed in accidents, such as flying into glass windows or moving cars.

Glass windows do not select for the weak or infirm - if cats are actually selecting the less - healthy birds, then their prey should include a larger proportion of sickly individuals that those felled by flying into glass windows.

The scientists examined the spleens of 500 birds. Sick birds harboring parasites have smaller spleens than healthy birds. They found that the spleens of birds killed by cats were significantly smaller than those killed accidentally.

Spleens were on average a third smaller in cat-killed birds. Another important fact was that 50 percent of the birds killed accidentally were young. Seventy percent of the cat-killed birds were young.

Again this proved the hypothesis that cats kill the old, sick and very young. Healthy, older birds are too experienced to be taken by cats.

This has led Dr. Johnson to state that: "bird-killing may be one of nature's ways of making better birds."

Hunting cats like Feisty actually help birds, in a Darwinian sort of way.

Cats and Birds

The Royal Society for the Protection of Birds has produced an excellent new leaflet on cats and garden birds and have allowed us to reprint it below. THE RSPB IS NOT an animal welfare organization but rather is a wildlife conservation organization concerned with populations of all wild birds and other wildlife. However, the RSPB is aware of how people feel about this issue, and does take their concerns seriously.

Cats may take millions of birds every year, but from a conservation viewpoint, we need to consider whether this is causing bird populations to decline.

This article provides some information on the impact cats might have on birds in gardens, and some ideas on what can be done to review our knowledge.

Some Facts and Figures

The best estimate of the number of pet cats in the UK is 8 million, according to the Pet Food Manufacturer's Association, plus an unknown number of feral cats (those living wild).

There is a species of wildcat, found in Scotland, which is native to the UK, but domestic cats are not part of our native fauna. They are maintained by humans and do not need to hunt to survive. Cats are individuals: their behaviour varies widely. Some will be prolific hunters; some may catch nothing at all. Town cats live at higher densities than country cats. Although each town cat will catch less prey overall than a country cat, their prey will include a higher proportion of birds. Most cats are opportunistic hunters - they will catch whatever they come across rather than actively hunting a particular species. This means whatever is most abundant or vulnerable is most likely to be caught. Cats will catch prey even if they are not hungry. According to a recent major survey by the Mammal Society, birds comprise a relatively small proportion (c 20 per cent) of all creatures caught by cats. Most of the rest of their catches will be mice or voles. Most of the birds are taken around dawn and dusk, during the breeding season and in mid-winter: It often seems that cats catch more birds than small mammals. This is because birds are mainly caught during the day, so you are more likely to see them, whilst mice and voles are mainly caught at night.

Estimates of how many creatures are killed by cats each year vary significantly. The most recent figures are from the Mammal Society, which estimates that the UK's cats catch up to 275 million prey items a year, of which 55 million are birds. This is the number of prey items that 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 (in order) house sparrows, blue tits, blackbirds and starlings.v

Despite the large numbers of birds killed, there is no scientific evidence that predation by cats in gardens is having any impact on bird populations UK wide. 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 need only rear two young that survive to breeding age to replace themselves and maintain their 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 or mortality, this might have a serious impact on bird populations.

Those bird species that have undergone the most serious population declines in the UK (such as skylarks, tree sparrows and corn buntings) rarely encounters cats, so cats cannot be causing their declines. Research shows that these declines are usually caused by habitat change or loss, particularly on farmland. Populations of species, which 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 over a quarter across the UK since 1966. Of the birds most frequently caught by cats in gardens, only two (house sparrows 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, and 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).

What can I do?

A range of relatively simple measures can be recommended to cat owners and non-cat owners which could help to reduce the risk of cats catching garden birds, especially where food is being put out for birds.

Put a bell on your cat's collar - a recent study suggests that this may reduce predation of birds, and may reduce predation of mice and voles too. The collar must be correctly fitted and should have a quick release mechanism to allow the cat to free itself should it become snagged. There are commercially available sonic collars that are designed to alert the potential prey to the cat's presence. We do not yet know, however, how effective these collars are or how they affect the cat.

Cats should always be well fed and cared for, but this may also encourage them to stay near home and be less likely to wander where they are not welcome (although it will not prevent them catching birds).

Keep your cat indoors when birds are most vulnerable: at lease an hour before sunset and an hour after sunrise, especially during March-July and December-January. Also after bad weather, such as rain or a cold spell, to allow birds to come out and feed.

Consider having cats neutered to prevent them 'wandering' or producing unwanted kittens.

Bird Friendly Gardens:

1. Avoid putting food on the ground, but use a bird table where cats cannot reach it.
2. Place feeders high off the ground but away from surfaces from which a cat could jump.
3. Place spiny plants (such as holly) or an uncomfortable surface around the base of the feeding station to prevent a cat sitting underneath.
4. Place an upturned tin or cone underneath the table to prevent cats from climbing the post (squirrel ‘baffles’ are already commercially available).
5. Make the table-slippery using a metal post, or plastic bottles around non-metal posts.
6. Plant wildlife-friendly vegetation, such as prickly bushes and thick climbers in the garden to provide secure cover for birds. These should be close enough to where birds feed to provide cover, but not so close that cats can use it to stalk birds. See RSPB leaflets on Feeding garden birds, Gardening with wildlife, and Planting gardens for birds for more information. This kind of planting may also provide food and nesting sites.
7. Position nest boxes where cats cannot reach them or sit close to them (preventing the parent birds from getting to the box).
8. Cat deterrents such as ultrasonic devices and strong-smelling repellents are available. The effectiveness of these is probably variable, and there is little scientific evidence of their effectiveness. However, many people tell us that they have achieved good results with some of these techniques, so it must remain a matter of individual choice whether to use them or not. Domestic cats are protected by law and it is an offence to trap, injure or kill them. The welfare of cats must not be ignored. Remember that cats, even if they are unwelcome in your garden, may be someone's much-loved pet, perhaps a child or elderly person living alone. Some people have called for legislation introduced to curb the freedom with which cats are allowed to roam. Whilst we understand why people feel this way, we are not able to urge the government to introduce such legislation, as we have no scientific evidence of the impact of cat predation on bird populations, which is strong enough to support such a call.

For more information on RSPB visit: http://www.rspb.org.uk.