The finding that chicks are leaving the nest earlier is from data collected between 1983 and 2017. Boersma and research students regularly checked the nests of banded Magellanic penguins for chicks. Like hospital midwives, they marked the newborns with temporary identifying bands, and then with stainless-steel tags on the outer webbing of the penguins’ feet. They measured the chicks’ wing and bill length with a ruler and calipers.

Boersma and her colleague Caroline Cappello found that over those decades, hatch dates shifted 10 days later. Yet the chicks still typically went into the ocean in late January to early February, despite being generally younger and smaller. They also found that over this period of 34 years, only 46 of 542 banded fledglings returned as juveniles the following year — that is, 8.5 percent.

A surprising finding was that in more recent years, younger chicks were heavier and in better condition when they first left the nest. The study offered two possible reasons for this. With the shortened period in the nest, it is possible that only the best quality chicks are surviving. Also, with the significantly reduced population, each individual penguin chick may now be slightly better fed. The study calls for further research into this phenomenon. The overall picture remains bleak.

“The main reason the colony is declining is that it’s no longer as ideally situated for foraging as it was when it first got established,” said Gordon Orians, a biologist emeritus at the University of Washington who was not involved in the study. “The fact that they have to go farther for food now and are having more trouble is almost certainly related to some degree to climate change.”

Previous research has linked climate change to some — but not all — shifts in key life-history events of animals and plants. By observing Magellanic penguins for so many decades, Boersma and her collaborators have been able to observe changes occurring in various life events in a single species.