Breeding woes a ‘real puzzle’

Why did so many grebes abandon their platform nests and eggs at the Lake Wanaka marina just before their chicks were due to hatch?

That is the question which has flummoxed retired zoologist and Lake Wanaka marina grebes project founder John Darby at the end of what he has described as a "very poor breeding season".

Over the last few months he has recorded 19 breeding attempts and 55 eggs laid.

Eighteen chicks hatched, but 31 eggs were abandoned which Mr Darby subsequently collected "muddied and stone cold".

He had pinned his hopes on this season’s last remaining breeding adult which was observed sitting continuously for nearly four weeks on its nest of four eggs.

According to Mr Darby’s calculations the eggs were due to hatch last Friday.

On Thursday, he saw the adult bird on the nest but on Friday it had gone.

"The real puzzle at the bottom of this is why we have had so many birds take their eggs to almost full term and then just walk away?"

He wondered whether the fluctuating levels of Lake Wanaka last year could be the reason.

"We started off with three to four months of very low lake levels over the winter and then suddenly they became very high to the extent that the lake flooded, receded and then came back again.

"I may be wrong but the most productive part of the ecosystem is on the lake shore edge and when it is bereft of nutrients for a long time then there is going to be a significant shortage of food for the grebes."

Mr Darby said he had observed two changes in the behaviour of some of the grebes breeding this year:

the absence of a mate to relieve the nest sitting bird at any time and a lack of nest-building activity a day or two before eggs were due to hatch.

"Maybe what we are saying is that one bird can incubate these eggs for a maximum of three weeks without getting any relief or help but after that it is a waste of time."

Over the range, Lake Hayes had had a very successful grebe breeding season despite its water being contaminated.

"A highly polluted or nutrient rich lake doesn’t seem to affect the birds, the factors determining the number of grebes will always be the amount of food you’ve got and there are huge numbers of very small fishes in Lake Hayes," Mr Darby said.

Not only did Lake Hayes have an abundance of food but the willow trees overhanging the edges provided a good habitat, he said.

He was confident the grebes would return to breed at the Lake Wanaka marina because they knew it was "a safe site" free from predators and vandalism.

This season’s abandoned eggs and embryos would be added to the Otago Museum collections, he said.

kerrie.waterworth@odt.co.nz