Penguins, poo and the surprising 'junk food' discovery in their diet

Phillip Island’s little penguins are the rock stars of Victoria’s beaches. When they walk home late after a long day at sea, visitors from around the world gather for the show. The penguins earn more money for the state than all its commercial fisheries.

Monash PhD student Catherine Cavallo spent two years analysing what the little penguins ate during their daily workouts in Bass Strait. And her fellow PhD student, Sonia Sanchez, tracked members from two adjacent sub-colonies to see how far they swam and where they went. Their findings contained some surprises.

Earlier research showed that penguins mostly consumed small fish – anchovies, sardines and immature reef fish. 

But  Cavallo and her supervisors, Dr Richard Reina (Monash University) and Dr Andre Chiaradia (Phillip Island Nature Parks) suspected that previous methods weren’t capturing the whole diet. Previously, penguins were made to vomit up their dinner when they came back to shore. The food that's hardest to digest is most likely to appear in vomit – fish bones or squid beaks, for instance. 

Cavallo instead used DNA barcoding to analyse penguin faeces. She placed cardboard trays outside about 100 penguin burrows at two separate sub-colonies and analysed the contents in the morning – altogether, this allowed her to monitor the diet of about 400 penguins. 

It was important to collect the faeces as soon as possible, she says, so it wouldn't collect parasites and other contaminants. She called it “fairy dust” because it was grey and sparkly (it also had a slightly fishy smell).

Cavallo’s research found that penguins had been eating jellyfish and salps – tiny tube-like animals. Altogether, the jellies and salps made up about a quarter of the DNA found in the penguin poo. The finding was unexpected – it hadn't been known that penguins ate salps at all, or that jellyfish formed such a significant part of the little penguin diet. 

What does the finding mean? 

Because this is the first research of its kind, it's impossible to know whether penguins eat jellies and salps because there are fewer fish in Bass Strait, or whether it's always been part of their diet. 

“We know that penguins are opportunistic feeders,” she says. “They eat what's available, and can snack on more than one kind of food.” 

It's also known that jellyfish proliferate when oceans become more acidic – one of the effects of climate change. Jellies and salps are less nutritious than small fish, containing less protein, for instance. In addition, the waters of southeastern Australia are a hot spot, warming at four times the global average. 

Are penguins filling up on junk food out of necessity? 

Sanchez in the meantime was tracking where the penguins swam by means of a GPS and another tiny device, similar to a Fitbit, that she attached to their feathers with waterproof sticky tape. Information from the devices had to be downloaded when the penguins returned, which involved Sanchez visiting the penguins daily (she used data from 112 penguins altogether) to retrieve or reapply them.  

She discovered that the two sub-colonies, which were only two kilometres apart, kept to separate and discrete fishing grounds while at sea. This allows the two groups to avoid competition when hunting – Sanchez didn't research how the arrangement between the penguins was determined or policed.

She tracked the penguins while they were incubating, guarding and feeding young chicks. The penguins are free to range further during incubation – one member of her study travelled 400 kilometres from the colony in seven days, not including vertical dives. 

The devices also showed that most penguins swam 40 to 50 kilometres a day,  a respectable distance for a creature that weighs about a kilogram and is only 30 centimetres tall. They tended not to range further than 15 to 20 kilometres from their colony, suggesting that food was plentiful close to home. 

At the start and end of their fishing trips the penguins walk over an automatic weighbridge, allowing researchers to check their body weight (the birds can be identified by microchip). 

“By subtracting their return weight from their departure weight we can measure the mass of what they ate that day,” Cavallo explains. “We also know how long they spent at sea, which is a measure of how much effort it took to catch that prey.”

Cavallo says her research has established a more comprehensive baseline for the little penguin diet. 

Because the penguins are significant predators in the sea around Phillip Island, analysing their diet is a good way of monitoring how marine life is affected by climate change, pollution and other environmental changes.

Because the penguins are significant predators in the sea around Phillip Island, analysing their diet is a good way of monitoring how marine life is affected by climate change, pollution and other environmental changes, as well as tracking the health of the penguins themselves. 

Conservation measures have removed the houses on the Summerland estate, where the penguins perform their nightly show, and also controlled the dogs and foxes that ravaged the colonies. The population is rebounding steadily, and is now more than 30,000-strong. 

The penguins share their hunting grounds with Australian fur seals, crested terns, several fish and shark species, and short-tailed shearwaters – or mutton birds – that fly 16,000 kilometres from Alaska to spend the summer fishing in Bass Strait. 

Cavallo and Sanchez stayed on Phillip Island over the spring and summer of two consecutive years. Their research was funded by an ARC Linkage Grant between Monash, Phillip Island Nature Parks, the Australian Antarctic Division, Deakin University and the National Centre for Scientific Research (France).