North
Island Robins through two breeding seasons I can see why they
disappeared over a hundred years ago from Great Barrier. The rats
certainly love (or hate!) them – both eggs and juvenile chicks. This
season 13 eggs and 2 banded chicks were predated by rats despite
intensive protection – nests surrounded by traps, poison baits, and
often the tree banded to stop rats climbing up to the nest. Rats usually
eat all the egg leaving shell remnants and sometimes only eat the brain
of a juvenile bird. If this is the level of impact on birds that are
being watched over and ‘protected’, then it can only be full scale
slaughter that occurs every night over the breeding season on fantails,
silvereyes, tuis, and greywarblers. In the recent GBIs Trust summer
lecture on the impact of rats in the bush, Landcare research scientist
John Innes’ analysis of bird predation laid 58% of the bird mortality on
rats over all other predators. They are highly successful hunters and
very thorough in covering all eating opportunities in their territories.
There is not a single place on our Island that a rat cannot get to.
The photo the
GBIs Trust has currently on display as a poster around the Island was
taken by David Mudge and shows a ship rat eating juvenile fantails.
David was undertaking a study on fantails and had set up a night vision
camera to record nesting. The rat visited that particular nest some four
or five times over the night before consuming both young. While fantails
still appear to be fairly abundant they have to lay up to five eggs per
clutch in 4-5 nests per season to keep their numbers up under the
onslaught of rats. On occasion rats will even devour the
nesting adult.
Over the last
two years we have had mild winters and summers with enough rain to
ensure high fruit and seed abundance in the bush. The seventy monitoring
tunnels at Windy Hill showed an unusually high number of rats present
last winter. I would say that this autumn we will all experience serious
rat problems as their numbers have built up with the warmer winters and
food abundance. While birds and insects also benefit from these
conditions the rat numbers generally swell to such densities with all
that food abundance that the small bird populations can be hugely
impacted. Rats are highly adaptive and will breed more frequently and
have larger litters as food availability increases. One of the main
problems with rats is that not only do they predate eggs, birds,
invertebrates, worms, and lizards but they also out-compete other
species for the same food. I have heard several scientists say that
in the presence of high rat numbers both kiwi and feral pig weights are
lower – a result of reduced food avail-ability. Of the all the
monitoring undertaken here on the pest projects at Windy Hill and
Benthorn Farm the most significant difference shown between bush managed
for rats and bush not managed for rats is in the seedlings. In rat
reduced bush, seedlings are more plentiful and there is more species
variety – in unmanaged bush there are less seedlings and some species
are eaten out completely. While this is probably the least obvious
effect of rampaging rats it has a profound effect on the future
regeneration of the forest and the subsequent food and habitat
availability for all bush dwellers.
