Setting the cat among the pigeons
I often read and enjoy Tom O’Connor’s column, but I think he was outside his otherwise broad knowledge zone with the moggy issue (June 20).
About half of NZ households have at least one cat, and here’s some of what is actually known about them.
First the bad stuff. They can annoy neighbours by pooing in gardens. They can carry a zoonotic disease risk especially with toxoplasmosis, which can be quite serious in humans (uncovered sandpits are risky for children). They can prey on lizards and some ground-dwelling birds. Wandering un-neutered toms in particular can beat up other people’s cats, keep people awake at night with caterwauling fights, and scent-spray with pungent urine. In conservation areas they can be serious killers of brown teal, kakapo, some threatened lizards, and ground-nesting coastal and riverbed birds like NZ dotterel and wrybill, but there is little evidence of them affecting kiwi much.
Cats have been in NZ since Captain Cook’s time, and were probably major contributors to the extinction of more than a dozen native bird species – but the most cat-vulnerable species are now long-gone from wherever cats have established. However, some vulnerable species like saddleback are now being returned to the mainland from refuge islands, and cats need to be excluded from these sites.
Now the good stuff. They can be great companion animals with well- documented health benefits to owners – eg, petting a cat can lower blood pressure, help counter some mental health disorders, and exposing young children to cats can reduce their likelihood of developing allergies and perhaps antisocial behaviours. They can kill rats and mice (readily killing animals under 100g, but they can baulk at a 300g+ Norway rat). The main predators of bird nests in trees are possums (rare in towns), harrier hawks (rare-ish in towns), and ship rats (the worst predator of bird nests in trees). Individuals can vary, but the average moggy doesn’t normally raid bird nests up in trees very much.
So what does that mean? There is normally a high density of cats in suburbia, so the nuisance factor can be high. But there are few ground-nesting birds in suburbia, certainly almost no native ones; the normal grounddwelling suburban birds are treenesting exotics like sparrows, finches, blackbirds, thrushes, mynas and starlings – which have well-developed anti-cat behaviours so they remain common in towns. Large Norway rats are relatively terrestrial and tend not to bother bird nests in trees too much, so perhaps it doesn’t matter if the cats tend to leave them alone. But the smaller ship rat is a very agile climber, and it’s possible that high densities of cats in towns might reduce their population significantly – ie, if a high cat density reduces ship rats to a low density, then perhaps the nett impact on birds’ nests in trees might be less than with a low cat density and a higher ship rat density. So could it really be the case that more cats = fewer ship rats = more successful tree nests? One study has shown that removing cats from a forest can allow ship rats to increase fourfold. Another has shown that the mere presence of cats can repel stoats (another serious tree nest predator). There was a classic case in NZ where feral cats were removed from Little Barrier Island in 1980 because of perceived damage to the bird populations, especially petrels. The kiore (Polynesian rat) population consequently increased, and had an even worse impact on the petrels. Ecologists call this ‘‘mesopredator release’’. Kiore have now also been eradicated there (in 2004) and the petrels have increased quite spectacularly.
There has been research on the numbers of different prey items that owners report their urban cats catching, but (a) this may not reflect the real numbers caught, and (b) it is the numbers remaining uncaught that are more important (and more difficult) to know.
Another thing we do know is that humans are subject to ‘‘confirmation bias’’, they tend to cherry-pick evidence that supports their existing opinions/ prejudices/ideologies/vested interests – and that will perpetuate this cat controversy, along with 1080/vaccination/fluoridation/climate change etc. For those who are interested in actual evidence, there is a glaring need for research into cat/rat/bird relationships in towns – but it will unfortunately be very hard to do, as you’d probably need to totally remove cats from some square kilometres of suburbia and measure any difference that might make to rat and bird populations.
A common recommendation for cat owners is that they should keep their cats inside at night. That might reduce human nuisance effects but, at night, most birds are safely roosting in trees, and that is when the rodents come out. Do you want to reduce potential annoyance to neighbours, or control rodents? If you are lucky enough to have lizards, cats can catch them at night, but they can be protected somewhat by providing safe refuges like loose rocks or roof tiles and dense vegetation. But . . . rats will also prey on lizards.
Obviously this stuff isn’t easy, but it’s now particularly topical with the increasing expansion over the last decade of native birds into Waikato towns from bush conservation areas like Maungatautari, Pirongia and the Hamilton Halo sites. Hamilton is restoring some of its gullies to encourage them in, but the effect could be considerably enhanced with more strategic planting, and if more attention is given to these pest management issues. Other towns like Cambridge are certainly not taking full advantage of this new opportunity to have even more tui, bellbirds, kereru and kaka in town.