The Measure of a Penguin

April 16, 2019

By Caroline Cappello, Godfrey Merlen, Dee Boersma

When discussing our research in Galapagos, we often talk about the 120 nests that we built to provide high-quality breeding sites for Galapagos penguins. Each year, we visit the Islands in February and July to look for signs of active breeding and to check the status of both natural and constructed nests. Since our study began in 2010, penguins have used the constructed nests 28 times.

Galapagos penguins in a constructed nest

Galapagos penguins in a constructed nest. The male is in the back and the female is in the front.

Another very important aspect of our work is measuring and weighing penguins and marking them with small, numbered tags so that we can identify them on subsequent trips. By measuring and weighing individuals, we can determine their body condition, i.e. how skinny or fat they are for their skeletal size — which is a good proxy for food availability in the Islands. We can also use measurements to determine the sex of Galapagos penguins, even though the two sexes look very similar. Last year, we published a study in Endangered Species Research showing that adult males are larger than females. By measuring just the bills of Galapagos penguins, we were able to identify the sex of 95% of our study birds correctly.

Knowing the sex of individuals is important for studies of their ecology, behavior, and conservation. For example, several penguin and other seabird populations have skewed sex ratios, with more males than females in the population. Because penguins breed in monogamous pairs, a skewed sex ratio means that there are fewer potential pairs, more single males, and less breeding. Using measurements to determine the sex of Galapagos penguins, we know that we catch more males than females, the same trend that Dr. Boersma found during her study of Galapagos penguins in the 1970s. This suggests that this small, endangered population may consistently have a skewed sex ratio.

PhD student Caroline Cappello searches for Galapagos penguins

PhD student Caroline Cappello scans the coast for signs of Galapagos penguins.

How do we measure a penguin? First, we have to find them, which can be a challenge! Unlike many penguins that breed in dense colonies, Galapagos penguins nest in deep, dark, lava tunnels or crevices that protect their eggs and chicks from the hot equatorial sun. When a penguin is quietly incubating its eggs inside a lava tunnel, it can easily go unnoticed. Even when standing out in the open, their black backs provide camouflage against the dark lava shore, making them hard to see.

During our research trips, we spend hours each day slowly boating along the shores of Isabela, Fernandina, and Bartolomé Islands looking for penguins. We watch for movement and scan the coastline for white splashes of guano that indicate a penguin was standing on shore and may have a nest nearby.

Measurements of a penguin

Once we have a penguin in hand, we measure its weight, bill length, bill depth, flipper length, and foot length to determine its sex and body condition. We also record its wingspan, length of its middle toenail, and whether it is molting (replacing old feathers with new ones).

During our trip in February 2019, sea surface temperatures were high (79 to 82 degrees Fahrenheit), which usually means that there is less food available to the penguins than when the water is cool and nutrient-rich. We counted 165 adult and 27 juvenile penguins and measured 39 of them. Penguins were in good body condition and many had recently molted, showing that — despite the warm water temperature — the penguins were finding enough food. Though we saw no breeding adults in February, we found that penguins had recently visited several of our constructed nests, evidenced by the guano, feathers, and nesting material inside.

We counted fewer juveniles this trip (16% of penguins were juvenile compared with 31% in July 2018), suggesting that breeding was not as successful in the last six months as it was in early 2018. High sea surface temperatures are expected to continue through the spring, but if the temperature decreases over the summer, penguins may have an opportunity to breed. We will return to Galapagos in July to measure more penguins, check nests, and hopefully find pairs with chicks.

Adult penguins in good body condition

Adult penguins in good body condition during our February 2019 research trip. The penguin on the left just finishing molting its feathers.

To see student videos of Galapagos penguins using Dee’s footage, visit


Godfrey Merlen, Dee Boersma, Caroline Cappello

From left: Penguin research team Godfrey Merlen, Dee Boersma, Caroline Cappello

Our research team has traveled to Galapagos twice a year since 2010 to check natural and constructed nests and study the penguins there. Dr. Dee Boersma is a professor at the University of Washington, Seattle, where she holds the Wadsworth Endowed Chair in Conservation Science and directs the Center for Ecosystem Sentinels. She has studied Galapagos penguins since she began her dissertation research on Fernandina Island in 1970.

Caroline Cappello is a PhD student at the University of Washington studying the ecology and conservation of Galapagos and Magellanic penguins. She began her work with Galapagos penguins in 2015.

Godfrey Merlen is a conservationist, natural historian, boat captain, and Director of Sea Shepherd’s Galapagos office. He has lived in Galapagos since the 1960s and received the Disney Conservation Hero Award in 2015 for his conservation work. This project is a collaboration with Galapagos National Park (permit no. PC-47-10 through PC-65-18) and received funding from Galapagos Conservancy and the David and Lucile Packard Foundation.

Photos by Dee Boersma, Caroline Cappello, Godfrey Merlen

Galapagos Conservancy has been supporting Dr. Boersma’s project to increase the Galapagos penguin population since 2013.


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  1. This blog is amazing and informative. When considering the sex ratio disparity, the likelihood of fewer mating pairs and the more single males – can you distinguish mating males from single males? And if so, what makes a male Galapagos penguin more likely a mate versus single?

    Also, are there more males born than females or are the males just more likely to live?

  2. Hi Monica — We’re glad you like the post! We spoke with Dr. Boersma regarding your question and she said, “It is likely a number of factors such as nest quality, “personality” and body condition. As Galapagos penguins are rare it will be a hard question to answer for this species, but we are working on this problem with our data set on Magellanic penguins.”

    I know they want to publish a paper on that soon so keep an eye out for it and let us know if you need anything else.

  3. Thanks Kimber, can you post when the paper is published? I look forward to more penguin blog postings.

    • Yes, we can thank you!

  4. Ju st love your conservation efforts on the Galapagos Islands and the research done on the Galapagos Penguins. Can’t see there being too much of a shortage of fish as those types of tropical areas produce heavily because of the nutrients in the waters and pristine habitats to reproduce and flourish.

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