Rats are known to have a significant impact on forest birds. They take eggs and nestlings of small perching birds like fantails, but are also large enough to kill adults of forest birds. Monitoring the success of nests is an effective way to determine the success of predator control. If rats are controlled to a low level, more birds are expected to successfully raise their chicks.
Tongariro Forest uses fantails as an indicator of what’s happening to other birds. Rangers carefully follow fantails they hear until they find their nest, and observe how many of them successfully raise chicks, and how many of them fail. Wherever possible, the cause of failure is also recorded. This can be difficult because so little evidence is left behind.
As expected, in the years where there was predator control (in this case, in the form of aerial 1080), nesting success more than doubled. As rats began to creep up in numbers again, the nests failed more often. An interesting finding was that long-tailed cuckoos frequently raid nests, completely cleaning them out of eggs and chicks. The impact of cuckoos as a natural predator had not previously been well understood.