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The large seabirds arrive en masse.
And since Otago shags were spotted congregating at the Waitaki River mouth in large numbers earlier this year, Dr Chris Lalas, who has monitored the birds since 1977, went for a look.
Dr Lalas has witnessed the population centre of the species move north for decades.
About 25km south of the Waitaki River mouth, at Oamaru Harbour — the northernmost colony of Otago’s only endemic seabird — the Otago shags began nesting on the 200m-long Sumpter Wharf in 2014. It had been closed to traffic in 2004.
But perhaps the birds had started to nest on one of the shingle bars at the river mouth.
A large patch of white droppings stained 20sq m to 30sq m of shingle on the northern bank.
There was evidence of witch, a non-commercial flatfish, in the bones left behind from a pellet — a compact mass of indigestible material covered in a gelatinous sack — a bird had cast.
But if there was a colony, there would have been birds flying in and out at the river mouth accompanying obvious signs on the ground.
Instead, only three juvenile birds perched on the rocks.
The evidence of the birds was not "fresh", he said, "so maybe they’ve abandoned it now that the breeding’s started".
At Oamaru Harbour, the Otago shag colony is a conspicuous point of harbour activity. And as Dr Lalas predicted a year ago, the colony has grown this year to be the largest colony of the birds anywhere.
Maukiekie Island at Moeraki held the largest number of Otago shag nests from the 1980s to 2017, but this year the number of nests at Maukiekie Island dropped 24%, or about 91 nests, from 383 to 292.
At Oamaru, this year Dr Lalas recorded a 19% increase in nests, a rise of about 65 nests from about 346 to about 411.
The birds typically nest on "quite steep" to flat rock faces or wharves overlooking the water.
With a "huge" turning circle, the shags, which land into the wind, have more or less unimpeded access to their nests at Oamaru. Dr Lalas will again check the river mouth next year to see if the birds start nesting there.
"A critical odd thing is that they’re a cold-water species, they’re just designed for cold conditions."
Nevertheless, the birds appear to be moving north. Otago shags have not started colonies further south than the Catlins since about 1990.
"People don’t believe what you say unless you can give an explanation, but for me, facts are facts. How does one explain a bird ... their physiology is such that they have to pant at 15degC, because it’s hot for them, and yet they’re moving north?
"There is not logic to it, but animals don’t use logic. It’s humans that use logic. And not many of them do it, either."