Like most of their stout-bodied, flippered kin, Magellanic penguins spend much of their lives in the ocean. From late autumn through winter and into spring in the Southern Hemisphere, these South American penguins swim off the coast of southern Brazil, Uruguay and northern Argentina in search of anchovies, sardines and squid.
But as spring turns to summer, they swim thousands of miles south and congregate in big coastal colonies. There, males and females pair off, breed and attempt to rear one or two newly hatched chicks. One of the largest breeding colonies for Magellanic penguins is at Punta Tombo in Argentina, where University of Washington biology professor P. Dee Boersma and her team at the Center for Ecosystem Sentinels have studied the penguins since 1982. They have documented a population decline at Punta Tombo of more than 40 percent since 1987, along with a rising male-to-female ratio, and have spent years trying to pinpoint its cause.
In a paper published Jan. 2 in the journal Ecological Applications, Boersma and UW postdoctoral researcher Natasha Gownaris report that juvenile females are more likely to die at sea, which has caused a skewed sex ratio of nearly three males to every female, as well as population decline. Their study incorporated more than 30 years of population data collected by UW researchers — including banding and studying individual penguins — into models of population dynamics. Boersma and Gownaris’ models show that juveniles have much lower survival rates than adults in all years, a common phenomenon in seabirds. In addition, among both juveniles and adults, females are less likely to survive than males, but this sex bias is much larger among juveniles. Adult females seem to fare worst in years when overall survival is low, suggesting they are more vulnerable than males to disruptions in the food supply during the nonbreeding season.
“From a conservation standpoint, this study shows us how important it is to try to protect the places where these penguins feed throughout the year, both in the breeding season and the nonbreeding season,” said Gownaris. “It all comes down to food for this species.”
Using population models, Gownaris showed that the higher mortality of females at Punta Tombo contributed to skewed sex ratios and consequent population declines at the site from 1990 to 2010. This contribution was much greater than that of variation in chick survival. The data also suggest that, due to the skewed sex ratio, the population may be declining faster than suggested by the population surveys that are typically used to calculate population trends in the colony.
“It makes sense that the worsening sex ratio is responsible for so much of the population decline at Punta Tombo based on what we know about penguin behavior,” said Gownaris. “Magellanic penguins are serially monogamous — with one male pairing with one female each breeding season — and both parents working together to rear the chicks. So, having fewer females means you have fewer pairings each year overall.”
Gownaris wants to survey other Magellanic penguin breeding colonies along the coast to see if they show similarly skewed sex ratios. She and Boersma hope that this information will help new conservation efforts.