Adélie penguins often steal rocks from each other’s nests, but new research shows the crafty birds target some nests more often than others. Biologists observing Adélie penguins on Ross Island in Antarctica found that the birds tend to steal stones from nests on the outer edge of their colony rather than stones from nests in the middle.
Penguins and other birds that build nests in or near the center of their colonies tend to have better success raising their chicks. Central nests are less likely to be disturbed by predators and are better protected from the harsh Antarctic elements. The new results show that living in the center of the colony has another advantage: The birds don’t have to worry as much about others stealing their building materials.
Stone theft is more than just a nuisance – it can make the difference between keeping a chick or losing it. “It’s really important to have a well-shaped nest, and when you have a lot of birds trying to steal rocks, it’s not as easy to maintain it,” said Virginia Morandini, an ecologist at Oregon State University and lead author of a new study on the findings. “So they have to expend more energy to maintain their nests. If they don’t, they’re more likely to lose their offspring.”
Piece by piece
Nest building is an important activity for breeding Adélie penguins. The birds spend most of their time at sea, but they move to dry land in the southern spring – September and October – to build nests and raise their chicks. One by one, each pair of penguins collects small stones and pebbles and uses them to line a small depression in the ground that they have chosen as a nesting site. The stones keep the eggs dry by allowing snow and water to drain around them.
Well-built Adélie penguin nests can contain hundreds of stones. Collecting is tedious work and requires a lot of energy. According to David Ainley, an ornithologist who has studied Adélie penguins in Antarctica for decades, this is also one of the significant ways mated penguins interact with each other.
“The nest is a focal point and collecting rocks is not only practical but ceremonial,” Ainley said. Once the eggs are laid, the pair takes turns incubating the eggs and guarding the nest while the partner searches for food in the sea. When one of them goes into the water, they bring a few dozen pebbles to their companion before leaving to assure them that they will return. “It’s part of the pair-building process,” Ainley said.
Coveted building materials
Biologists have been studying Adélie penguins on Ross Island since the 1960s. They observed loners and monitored the colony. The slow pace of research means it can take years to get meaningful results.
Morandini was watching penguins at Cape Crozier during the southern summer of 2017-2018 when she noticed some birds trying to steal rocks from other nests. Cape Crozier is one of the largest Adélie penguin colonies in the world and is home to about 600,000 breeding individuals and about 300,000 individual penguins.
When Morandini observed the cunning birds stealing each other’s nesting material, she wanted to learn more about the fascinating behavior.
“When I saw them stealing from each other, I wondered what was going on,” she said.
Morandini decided to keep a closer eye on the stone thieves and find out where they lived in the colony. She marked which nests they tried to remove rocks from more frequently and how the owners of those nests reacted to the attempted theft.
She found that penguins tend to remove stones from nests at the edge of the colony more often than from nests near the center. And not surprisingly, nests near the center tended to be larger and better built.
“The peripheral nests had more attempts to lose stones and the thieves were likely to be successful if the nest was on the edge of the colony,” Morandini said. Nest owners did not always tolerate the theft – they often reacted aggressively by pecking or chasing the pebble poacher or hitting the offender with their flippers.
An open door may tempt a saint
It is more convenient for a penguin to steal stones from a neighbor’s nest than to venture outside the colony to collect a fresh stone. Morandini suspects that birds prefer to steal from peripheral nests rather than those in the middle because it’s easier to get away with it.
Birds usually walk around the perimeter of a colony, those with nests in the middle have to cross the outer nests to get there. Since nests outside the colony have more traffic, there are more opportunities for stone theft. Edge nesters also have fewer neighbors, so it’s easier to sneak away unscathed.
“We have found that a stone thief is not only targeted by the owner of the nest, but also by the neighbors because they see it as a potential problem for them as well,” Morandini said. “The nests on the edge of the colony have no neighbors on either side of the nest. We think penguins take advantage of that.”
Adélie penguins have become more numerous in recent decades, scientists are not sure why. The new findings can’t answer that question, but they do help scientists better understand why some penguin pairs are more successful at breeding and raising chicks than others.
The results suggest that penguins on the periphery get the proverbial short end of the stick – they face more rock attacks as well as more predation and exposure to the elements. But at the edge of the colony, they are closer to fresh stones in the area, so it is easier for them to replenish their supply.
Lauren Lipuma, Antarctic Sun