Scientist studies what the cat brought in

5 January 2009 by Tom Marshall

Are domestic cats responsible for declining wild bird populations across Europe? To find out, a scientist at the University of Reading is collecting the hunting trophies of the town's cats after their nights on the tiles.

Domestic cat

Rebecca Dulieu, a PhD student in the department of Environmental Biology, has recruited 241 cats belonging to 148 owners located in nine one-square-kilometre areas around the town.

During each of four survey periods over the year, whenever a cat kills its prey and leaves it on the doorstep, Dulieu collects it for analysis.

The research into cat predation of birds is the starting-point for a wider investigation of the pets' impact on their prey. Cats often catch native birds like robins, blackbirds and great tits; the study should give a more detailed snapshot of which birds are at the greatest risk.

Europe's bird populations are plummeting, and cats may bear some responsibility. According to the State of Europe's Common Birds Report in 2007, 45 per cent of the continent's common bird species have declined since 1980.

"Cats are probably quite a small factor in this; the major causes of the decline in wild birds are likely to be habitat destruction, habitat fragmentation and changes in land use," says Dulieu. "But many bird species are in decline, and it's possible that cat predation could be contributing to this." A study in 2003 suggested that Britain's nine million cats kill 92 million animals a year, 27 million of them birds.

Part of the problem is that in urban areas, cats are found in far higher densities than would be the case for predators in the wild. By comparison, Britain has only around 462,000 stoats, 450,000 weasels, 240,000 foxes and just 3,500 wild cats.

Dulieu's research began in autumn 2007 and will carry on until the beginning of 2010. Studying cat predation rates is just one part of the project; Dulieu is also collaborating with colleagues at the University of Swansea to look at other factors, such as whether cats return all of their hunting trophies home.

It is thought that cats return only around 30 per cent of what they kill, but this figure comes from a study based on only 11 cats, so more research is needed. Inspection of cat faeces for the remains of prey and using data loggers to record the cats' hunting behaviour will contribute to this goal.

The research will attempt to measure the respective population densities of cats and of their avian prey, and the productivity of different species of birds - how many young a pair of birds raises successfully each year. Scientists think productivity is much lower in urban areas than in the countryside, but again this needs to be investigated in greater depth and quantified.

The project will also track cats' hunting behaviour using GPS technology. Putting all this information together "will let us understand whether cat predation rates are sustainable", she explains.

Finding out how many cats live in an area is fairly straightforward - just survey their owners. Birds are more challenging, requiring a trained surveyor to walk a set path and note all the birds they see and hear.

Rodents and other small mammals are harder to survey as they only come out at night, and don't announce their presence by singing like birds.

This is why Dulieu's research focuses largely on birds; she collects mammals that cats bring to their owners for analysis, but is not attempting to go further in assessing felines' impact on populations of small mammals. As well as this, many small mammals have healthy population numbers.

Another question that's being investigated is the prevalence of toxoplasmosis in birds that cats bring home. This is a parasite that can be passed on to humans through cat faeces; in rats, it causes activity to switch from night to day and natural fear of predators to fade. Effectively, the parasite causes rats to behave in a way that raises their risk of being killed by a cat.

Recent studies have even linked infection in humans with serious psychological problems. It's not known how common the parasite is in birds, or whether it affects their behaviour, making them easier prey; Dulieu hopes to find out.

Most urban cats are well-fed, but this does not stop them hunting birds and small mammals. Cats' hunting instincts are largely unconnected to how hungry they are; a well-known experiment established this by putting cats in a box with a large quantity of tasty food, before putting a rat into the box with them. Invariably the cats would find the rat, kill it, carry it back to near their bowl of food, and then continue eating the food.

The research is still at an early stage, but some insights are starting to emerge. Dulieu says ground-nesting and ground-feeding birds are very vulnerable to cats, as are young birds. Cats also seem to hunt more in spring and summer, though it's not yet known whether this is because there is more prey available at these times of year, or perhaps because the weather is better and there are more daylight hours.

If it does turn out that cats are a threat to bird populations, owners can take some steps to help. The old method of putting bells on cats' collars might not be as effective as has been thought as research suggests that some cats alter their hunting behaviour to minimise the noise made by the bell. A better method would be to keep cats indoors at dawn and dusk, their two best hunting opportunities.