 On islands , birds can contribute as much as 60 % of a cat 's diet . In nearly all cases , however , the cat cannot be identified as the sole cause for reducing the numbers of island birds , and in some instances , eradication of cats has caused a ' mesopredator release ' effect ; where the suppression of top carnivores creates an abundance of smaller predators that cause a severe decline in their shared prey . Domestic cats are , however , known to be a contributing factor to the decline of many species , a factor that has ultimately led , in some cases , to extinction . The South Island piopio , Chatham Islands rail , the Auckland Islands merganser , and the common diving petrel are a few from a long list , with the most extreme case being the flightless Stephens Island wren , which was driven to extinction only a few years after its discovery . 
