"Why does it matter that krill became more abundant to a predator that for 8,000 years happily made eggs from fish?" he noted.
Further research, he added, should focus on the scarcity of fish rather than the abundance of krill.
Whatever caused the switch, the eggshells show a switch occurred, according to the analysis.
"And now with krill on a decline and fish harvested out in a lot of areas that's a concern," Emslie said.
"What do [the penguins] have left to switch to? They don't really have any options left."
Krill populations are dwindling, especially on the Antarctic Peninsula, where warming temperatures mean less sea ice. Sea ice holds algae that the krill feed on when it melts.
(Read a related story about melting glaciers on the Antarctic Peninsula.)
In addition, fishers are now actively taking krill, which are used as feed in fish farms. This "will further cause problems," he said.
According to Hobson, the "good news" from the research is how adaptable Adélie penguins are to environmental changes.
"Although I remain a pessimist when it comes to how they may cope with the onslaught of climate change," he added.
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