University of Otago researcher Dr. Yolanda van Heezik has collected data on about 200 domestic cats around Dunedin: what prey they bring back; how far they roam. The study started up last August and will run until this spring to cover the four seasons. She reports as follows:

SO FAR ABOUT 1000 prey items have been reported by the cat-owners involved in the study. Out of 208 cats in the study we have received records of prey brought home by 99—so apparently nearly half the domestic cats do go off hunting. Considering the prey items caught we get the following breakdown:

– 42 different prey items caught.
– mice most frequently caught   (237 items, 23%)
– rats only 8%
– blackbirds and sparrow most   commonly caught birds (10%)
– exotic birds (12 species, 20%)
– native birds (waxeyes, fantail,   tui, bellbird, 6%)
– lizard (common skink—11%)
– 19 types of invertebrate   reported (26%)
– remaining—stoats, rabbits, a frog.
 

Birds comprise 26% of the prey. Waxeyes are the most abundant native bird caught and they are also the most abundant in the environment. Although mice are a major item they are caught slightly less frequently than birds. Rats are infrequent in the diet of these Dunedin cats.

Considering the results from the point of view of the cats, rather than the prey, we discover that only 22 of the 99 cats are catching rats, and 19 are catching skinks. Insects are caught by 25 cats, and 75 cats have caught at least one bird.

Home range analysis (based on 6 days of tracking) gives a mean home range size of 2.6ha, and a range of 0.33—26.1ha. The longest distance one cat moved from its house was about 400m. Half of our tracked cats lived next to areas of native bush, and in all cases some locations were in the bush. Problems with receiving the satellite signal under a canopy will have resulted in an under-estimate of the number of locations in that habitat. Males had slightly larger home ranges than females.

Two radio-tracking studies have shown most domestic cats don’t roam very far: e.g. in Canberra, Australia home ranges were between 0.02—28ha. Size depended on how many cats there were in the area, social dominance of the cat, personality of the cat, locations of favoured hunting and sunning sites and barriers, such as busy roads.

In New York State average home range size was 0.24ha, with cats spending most of their time in their own or their neighbours’ back gardens, despite living next to a large nature reserve. Cats rarely entered the nature reserve. The New York cats mainly brought back small mammals.

It is important to note that usually only a few individuals bring back most of the prey. Cats seem to be opportunistic, catching the locally abundant species. Rodents, especially mice, are most frequently caught. However, a minority of cats seem to be specialists. A proportion of cats occasionally bring back native birds. To assess the impact of this predation on bird populations you would have to extrapolate up to estimate how many cats lived in the urban area, and what proportion of that total were catching native species. Then you’d have to have estimates of the bird populations to assess whether the estimated number of native birds caught was large enough to have a significant impact on them.

I have carried out counts of birds in various urban habitats, and the data collection was designed so that I should be able to estimate the densities of the more abundant species. I haven’t started to do that yet.

As you must be aware, the study has a number of biases. People with cats that regularly catch prey may have been more likely to join the study. People whose cats catch more are more likely to remain interested and keep recording and sending in data, while some people might be less likely to report the capture of a native species. We are assuming that what a cat brings back is representative of what it catches!

Bells on collars do not seem to prevent cats from catching prey - although my opinion is based on anecdotal evidence. Cats wearing the GPS collar, which is quite bulky and weighs 125g, were able to catch prey. Certainly, being fed doesn’t stop cats from catching prey.