Australasian Science: Australia's authority on science since 1938

Predator in a Penguin Suit

Little penguins

The aim of the study was to find out if group or solitary hunting strategies were influenced by prey type, and how this affected how much prey an individual caught and how many calories an individual gained.

By Grace Sutton

Miniature video cameras and GPS have given an underwater bird’s-eye view of the hunting behaviours of the world’s smallest penguin.

When we think of little penguins, most of us imagine cute birds waddling up the beach in their tuxedos. What we overlook is that penguins are skilled and tactical hunters in the ocean, foraging sometimes for days at sea to provide food for their offspring.

Penguins are perfectly adapted for life in an aquatic environment. Spending most of their time at sea, their compact torpedo-like bodies and strong flippers allow them to travel effortlessly through the water column in a movement that has often been described as “underwater flying”. However, these physical adaptations are not enough on their own, so these animals must make decisions and develop behavioural strategies to enable them to successfully find and catch prey at sea.

Little penguins are the smallest of all the penguin species. With a height of approximately 30 cm and weighing in at about 1.2 kg, these feisty little birds are residents of the southern coastline of Australia and New Zealand.

During the breeding season, little penguins are restricted in how far they can travel, so they must hunt at sea close to the colony in order to return most days to feed their chicks. As a result of this limitation, these small, flightless birds develop hunting strategies so that they can successfully find food during each foraging trip.

Recently it was discovered that little penguins, like many other penguin species, hunt in groups when foraging at sea. Birds tracked using GPS and depth recorders were observed travelling and diving in synchrony with other penguins. They spent considerable time, and occasionally the whole time, with the same individual at sea.

This research provided new insights into how individuals hunt in groups at sea. However, it was unknown how prey type and its abundance influenced group behaviour in little penguins.

The aim of our study was to find out if group or solitary hunting strategies were influenced by prey type, and how this affected foraging success (how much prey an individual caught) and energy gain (how many calories an individual gained). To do this, I attached miniature video cameras and a GPS to the backs of adult breeding little penguins using special waterproof tape that allowed the attachment of these devices without injuring the birds.

I collected data from 21 little penguins from two colonies at London Bridge and Gabo Island Lighthouse Reserve in south-eastern Australia. Each study bird wore the tracking devices for just one day at sea. After they returned, I recaptured them in their nests and removed the devices.

The GPS allowed me to track where the birds were foraging, and the “penguin-cams” gave me information about what prey they came across and if they were alone or with other penguins. I identified what the penguins were doing during each second of the video (e.g. travelling along the surface, or diving and catching prey), and categorised these behaviours depending on whether they were with other penguins or foraging by themselves.

I then focused on what prey the birds came across when hunting, and found that there were distinct differences in success relating to the types of prey they came across.

I measured the success rate of hunting on a dive-by-dive basis by counting the number of prey items caught and estimating the number of calories that were consumed during each dive. For the latter, I obtained the mean calorie value for each prey species from previous research. These measures are often used in order to establish how efficient an animal is and how beneficial certain prey items are in its diet.

Little penguins were more likely to hunt in groups when they came across schooling prey such as anchovy, sandy sprat, white bait and small crustaceans or “krill”. While these are commonly known prey items for little penguin, they were also seen eating whole small jellyfish or breaking down larger jellyfish using their bill.

Jellyfish have not previously been recorded as a prey item in previous studies of the little penguin diet. Their gelatinous structures most likely results in rapid digestion, and their lack of hard remains like bones and teeth mean that it may have been overlooked in previous studies of the little penguin diet.

Regardless of prey type, little penguins did not seem to gain any benefits from hunting in groups. In fact, little penguins not only caught more prey when on their own than when other penguins were present but they also gained more energy when foraging alone. On average, when a solitary penguin was hunting schooling fish it gained 15 kJ more than when hunting with other penguins.

Penguins gained the most energy when hunting alone among groups of small schooling bait fish (between 30–40 kJ per dive compared with 1 kJ for krill). This is because bait fish are energy-rich, and the lack of other predators would reduce competition and thus make bait fish easier to catch.

So why do little penguins hunt in groups if they don’t receive any hunting benefits? The findings of this study suggest a potential trade-off between an increased chance of finding prey and hunting success.

Team work usually occurs in species that live in family groups and assist each other as a consequence of genetic relatedness. Working together provides many benefits to an individual, both when hunting small and large prey. For example, lions work together to bring down larger prey while marine animals, such as dolphins, may coordinate their movements to herd many fish into a small area. Animals have developed these strategies in order to make prey easier to catch.

From 295 prey encounters, 45 were in the presence of other penguins. Penguins were seen more often in groups when they were travelling than when hunting. This may provide better protection to penguins at sea.

Animals that do not live in family coalitions may also hunt in groups, although the benefits are less clear. For example, seabirds use cues in their environment, such as the presence of other individuals, as an indicator of the presence of food.

In the case of the little penguins, this may increase their success of finding prey but, once prey is found, they didn’t seem to work together to concentrate their prey or assist each other with their captures.

In addition to group hunting with other penguins, little penguins used the presence of other seabirds as a cue to find a meal. Individuals were observed on some occasions at the surface following other seabirds, such as short-tailed shearwaters, and subsequently foraging in the same region. Penguins and short-tailed shearwaters were also seen together underwater feeding on the same prey patches of bait fish.

Many penguin species spend time in groups at sea, although it is clear that individual little penguins trade prey capture success for reduced risk of predation and an increased chance of finding prey in their environment.

Studying the behaviour of little penguins is important in light of the current changing climate. If we can better understand their current predator–predator and predator–prey interactions we might be able to predict their behaviour under changing environmental conditions, such as loss of certain prey types, and be able to provide informed management strategies in the face of a changing climate.

Grace Sutton is a PhD student from Deakin University’s School of Life and Environmental Sciences.