Every autumn in the Southern Hemisphere, Magellanic penguins leave their coastal nesting sites in South America. For adults, their summer task-breeding, or at least trying to-is complete. Newly fledged chicks and adults gradually head out to sea to spend the winter feeding. They will not return to land until spring.
Yet, these birds when they are a winter offshore are largely a mystery to study the Magellanic penguins and who advocate for their conservation in the population numbers.
"The winter period is something of a black box for us in terms of understanding Magellanic penguins," said Ginger Rebstock, a University of Washington research scientist. "We know the least amount of this part of their year."
But research by Rebstock and P. Dee Boersma, a UW professor of biology and the founder of the Center for Ecosystem Sentinels, is a black box and discover how Magellan penguins from one nesting site, Punta Tombo in Argentina, fare during the winter months. In a paper published Aug. 9 in the journal Marine Ecology Progress Series, they report that the Rio de la Plata-which drains South America's second largest river system after the Amazon-strongly influences oceanographic conditions in the Magellanic penguins' winter feeding waters. Those oceanographic features, they report, show up in the body conditions of the penguin females, but not the men, when the penguins return to their nesting grounds in the spring.
"Researchers only get to study the penguins up for their time in the year that they come to nesting sites like Punta Tombo to breed," said Rebstock. "Until now, we have not really know how the conditions out in the ocean, where they spend the whole winter, affect them."
Magellanic penguins are believed to swim hundreds of miles in winter to feed on fish such as anchovy and sardines. For penguins originating at Punta Tombo, this could mean more than 1,000 miles north of the coast up to southern Brazil. They generally stay along the continental shelf in waters usually no more than 650 feet deep. To understand the oceanographic dynamics in this region, Rebstock turned her attention to space. She analyzed 30-year-old weekly sea-surface temperature data, which was collected for those South American coastal waters from 1982 to 2012. Data show
For example, her analysis revealed that the Río de la Plata, which enters the ocean between Argentina and Uruguay, is the primary driver of oceanographic conditions in the penguins' winter feeding waters. The river discharges silt, microbes and nutrients into the ocean as a plume, which disperses in different directions based on prevailing winds. Strong winds from the southwest, for example, can spread the plume north along the hundreds of miles of Brazilian coastline. If winds are weaker, the plume stays near the mouth of the Río de la Plata.
Rebstock then analyzed whether variations in these oceanographic features-such as a strong, dispersed plume or a weak, localized plume-were associated with the body condition of the penguins at the time of their arrival at Punta Tombo. Boersma and her team have collected information on the health and state of the penguins when they arrive Punta Tombo for more than three decades. According to their data, the body conditions of male magellanic penguins were not correlated with the extent of the plume. But Rebstock found that female penguins came back at Punta Tombo early, and in healthier body condition, if the plaque plume was wintering in the winter. This may indicate that the plume affects how hard Magellanic penguins must work to find food.
"We believe that the de la Plata plume carries a great deal of nutrients into the coastal waters, making them very productive feeding grounds for the penguins," said Rebstock. "But winds will affect where the plume is distributed and how far penguins will have to go to reach it."
A weaker plume may keep the penguins' prey closer together and close to to breeding colonies, increasing an individual penguin's odds of catching fish. Magellanic penguins also are mainly visual hunters. A stronger plume that clings to the beach may block the visibility for the birds by making waters more turbid, said Rebstock.
The size and disposition of the plume may affect women more than men because of the magellanic penguins tend to be big, which allows them to dive deeper. This may give men a slight edge in catching food, especially in difficult conditions, said Rebstock.
"What we would like to do next is to test some of these hypotheses by the male and female Magellanic penguins during the winter months, to see if they are feeding in the same location and see how successful they are at receiving food in different conditions, said Rebstock.
For researchers like Rebstock, that may be the next black box to open. But it will also pose a logistical challenge. Researchers have tried to track Magellanic penguins during winter using satellite tags, but the penguins are very effective at taking them off.
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GA Rebstock et al, Oceanographic conditions in wintering affects the date and body condition in breeding female Magellanic penguins, Marine Ecology Progress Series (2018). DOI: 10.3354 / meps12668