Pest-free not the be-all and end-all for birds

On island sanctuaries, in native forests and backyards around the country, we wage a constant war on the mammalian pests that prey on our native birds. Whether we can ever be predator-free with the tools currently in our arsenal is a moot point. But let’s say we one day manage to get rid of all the possums, stoats and rats. Can we expect a harmonious and diverse chorus of bird song to emanate from our native forests the length of the country?

Not necessarily. The evidence suggests that once the predators are gone, some bird species will flourish while others will likely go into decline. There will be a reorganisation of the avian fauna in the absence of the threat from the eradicated mammals. It is already being observed on a small scale in sanctuaries and on islands where pests have been eliminated or suppressed.

A new study by Te Papa’s Dr Colin Miskelly and colleagues at the Department of Conservation looks at what has happened on the small islands of Fiordland, where efforts were successful in removing Norway rats and stoats and the islands were left to regenerate.

The scientists looked at surveys taken decades apart – from 1974 to 1986 and 2016 or 2019 – of bird life on islands in Breaksea Sound and around Anchor Island in nearby Dusky Sound.

South Island robins and ka¯ ka¯ seem to have been the big winners from the pest mammal eradications that took place in the intervening years. The species recorded on fewer islands include kereru¯ , tomtit, grey warbler, recent coloniser silvereye and introduced species dunnock and chaffinch. Bellbirds and the introduced blackbird, on the other hand, were widespread across the islands before and after the pests were taken care of.

The so-called ‘‘deep endemics’’, which have evolved here the longest, do better when the rats and stoats are gone than the ‘‘shallow endemics’’ who arrived here more recently. ‘‘Deep endemic birds exhibit a suite of behaviours and life history traits that make them especially vulnerable to mammalian predators,’’ the researchers note.

Some of them have reduced flying ability, are naive to predators, take longer to mature and don’t breed abundantly. But take their biggest predators out of the picture and they start to rule the roost once again.

The observations mirror what has been seen in Wellington’s Zealandia and Tiritiri Ma¯ tangi in the Hauraki Gulf.

We’ve been so focused on the impact of mammalian predators on bird life, we’ve dwelt less on the competitive tension created when we get rid of the pests. That needs more attention as we plan for a pest free world.

Take their biggest predators out of the picture and they start to rule the roost.