The
foods of feral house cats in forest in the Orongorongo Valley, Wellington, were studied
over 3 years and related to the availability of prey. Some cats were trapped, tagged, and
released, and could be identified individually by coat colour and pattern. The number of cats,
estimated from live-trapping and sightings, was stable during the study. Examination of 677
scats revealed that mammals (rat, rabbit, opossum, mouse, and stoat, in descending order of
importance) formed the bulk of the diet by weight. Remains of birds occurred in 12% of
scats, but birds were estimated to form only 4.5% by weight of the diet. Insect fragments
were present in many scats; wetas (Orthoptera), cicadas (Hemiptera), and beetles (Coleoptera)
were important seasonally. Although eaten in large numbers, they contributed very little by
weight to the diet. Populations of rats, rabbits, and opossums were fairly stable during the
study; mice were abundant for most of the first 18 months, but were scarce in the last year.
The
literature on the food habits of feral house cats is reviewed; it emphasises that cats are
primarily predators of small mammals (rodents and lagomorphs). Predation by feral cats
can
be
important
in holding rat and rabbit populations at low densities and in reducing
seasonal fluctuations in their numbers. Cats can also exert heavy predation pressure on
low-density mouse populations. Although the cats now eat few birds, they may have been
responsible for reducing the numbers of some forest birds in the past.
Kiore were eradicated on Double Island using two different methods.
The larger islet was treated with one manual application of STORM 4 g
wax block rat baits containing 0.005% by weight Flocoumafen, in
October 1989. The smaller islet received one application of toxic
kibbled maize (0.005% bromodialone by weight) after 4 nights prefeeding
in November 1989. Kibbled maize was dispensed in silos. No
sign of kiore has since been found during four visits to both islands. The
cost per hectare for both methods were quite comparable. The nett
cost for eradication on the larger islet (19 ha) was $310/ha, the smaller
islet cost $432/ha. Both methods are considerably cheaper than the
technique used to eradicate Norway rats (R. norvegicus) from Hawea
Island. For future rodent eradication campaigns it is suggested that
aerial applications of poisons may be made on all islands greater than 50
ha, and that size should be no barrier to rodent eradication. However,
substantial non-target have been recorded when 1080 baits were
broadcast aerially to control possum. Non-target kills of a similar
magnitude may be experienced on larger and more sensitive islands.
Although Talon® baits containing brodifacoum have been used successfully in eradicating rats from some of New Zealands offshore islands, little is known about any environmental effects of this toxin. We sampled invertebrates, blackbirds, soil, and water at intervals of 2 days to 9 months to determine whether brodifacoum residues were present after aerial distribution of Talon® 20P cereal pellets on Red Mercury Island and after bait-station use of Talon® 50WB wax-coated cereal blocks on Coppermine Island. No brodifacoum residues were found in soil, water, or most (99%) invertebrate samples. Low to moderate residues were found in one sample of slugs collected 2 days after aerial sowing. Tissues from all birds (n=4) and rats (n=3) found dead and livers from all six birds collected alive 8 months after aerial baiting also contained low to moderate residues. These preliminary results suggest that few invertebrates are likely to be contaminated as a result of Talon® baiting. Tentatively, we suggest that although some invertebrates may eat Talon® baits, it appears that the brodifacoum is either metabolised and/or excreted within a few days. The dead blackbirds found, therefore, were more likely to have been killed by primary than by secondary poisoning. Further monitoring for brodifacoum residues after Talon® operations should be undertaken to confirm that contamination of invertebrates, soil, and water is unlikely. Some bird species may be at risk from Talon® baiting. Likely effects on population levels of such species should be considered to help assess the risk and benefits of Talon® use in rodent eradication.
Strangers in Paradise takes us on a discovery journey to identify and learn about various species living on the islands of Hawaii, Solomon Islands, Rarotonga, New Zealand, and Easter Island. In contrast, we also take a look at the impact of human development on such species. The introduction of rats and new predators had devastating effects on these, which prompted research into control measures to address this problem.
