The impact of predation by domestic cats (Felis catus) on British wildlife populations

Non-native predators can cause great harm to natural ecosystems, through competition for resources and by directly predating on native species. Despite decades of research into predation by domestic cats (Felis catus) globally, the impacts on British wildlife populations are less clear than, for example, Australasian species. Since predation by cats is dependent on prey availability and local faunal composition, capture rates of different taxa can vary throughout the world. In order to assess impact, reliable and representative prey return data must be gathered, and total predation estimated (since it is unlikely that cats return all prey caught).

Here, prey return rates were established using a ‘cat diary’ where cat owners (n=553 cats) recorded all prey items returned home each month, yielding a total of 4,674 data months. This dataset was also used to investigate the key influencers of predation, finding that return rates were linked to the sex and age of the cats, with males returning larger numbers than females, and younger cats returning the most prey. Overall, most cats returned fewer than 0.5 prey per month. However, despite a monthly median of 0.3 prey, the mean was 1.57 prey per month owing to a small number of ‘super predator’ cats returning up to 61 prey in a single month.

Collar-mounted video cameras were used on a subset of these ‘diary’ cats, observing their behaviour outdoors and how they treated prey after capture (whether returned, eaten, or discarded). These video data were then used to calculate a coefficient by which return rates can be multiplied in order to estimate total predation (between 1.22 and 1.57 for vertebrate prey). This study is the first piece of video research in the UK and gives an important insight into cats’ behaviour after prey capture.

It was apparent that shrews and other insectivores may be unpalatable to cats, as just under 97% of shrews returned were ‘whole’, suggesting that there had been no attempt by the cat to consume them. It was concluded that, due to this difference between taxa, insectivore capture rates should be extrapolated separately. Similarly, the potential differences in detectability by cat owners of prey remains may also vary dependent on taxon (for example, with bird remains being more readily detected than small mammals). Therefore, using a single multiplier to estimate all prey species was found to be inappropriate, and the most reliable approach is to apply taxon-specific multipliers.

Mapping techniques were used to assess the overall impact of pet cats in the UK on the five most frequently returned bird species. This found that the relationships between cats and their prey are complex and dependent on environmental variables, and potentially other anthropogenic factors.

Overall, prey populations are currently stable or increasing, although data are lacking for some mammalian species. This research suggests that while cats do kill millions of prey per year in the UK, there is no measurable effect on prey at a population level, likely because British prey species have evolved alongside similar terrestrial predators.