But the intruders were not predators, Penguin Rescue scientist Dr Hiltrun Ratz said, rather they were "lonely males".
At Moeraki — mainland New Zealand’s most successful breeding site — males outnumber females two to one and Dr Ratz observed concerning behaviour resulting from the gender imbalance in the population of the "very social" birds.
In October, cameras set up at the reserve captured "fights" in nests as single males entered and disturbed the incubation process.
In some cases, chicks went missing overnight, and she held concerns the single males might be to blame.
"What happens is you have three adults fighting with a chick underneath them and the chick dies," she said.
"Just three birds in the nesting box, and then you see a glimpse of an egg underneath.
Not all the nests were monitored by camera but other nests lost their eggs and "we don’t know why — we’re speculating".
"We’re expecting something similar happened there, but we don’t know, we only have so many cameras.
"You get a pair fighting and it’s just hell for leather."
