When a hunter embarks on a chase, they skilfully balance the energy they consume against the energy gained devouring their quarry to make the most of each meal. Sum up each individual's balancing act and you begin to see how entire ecosystems function. Yet measuring the number of times a hunter successfully traps prey is easier said than done, especially when the predator is submerged beneath the waves. Nobuo Kokobun and colleagues from the National Institute of Polar Research, Japan, and the Korea Polar Research Institute, explain that resourceful scientists have used various ingenious approaches to measure penguin prey capture rates, although with varying degrees of success, so the team decided to take a different approach. Knowing that penguins open their beaks to trap prey, the researchers decided to find out whether they could measure the birds' head movements to identify successful prey encounters ().
Designing tiny custom-built accelerometers that could be strapped to the foreheads of gentoo and chinstrap penguins, the team headed south to Antarctica to test them on free-ranging birds. Fitting the penguins with the accelerometer headbands and camera backpacks to film the penguins' encounters with prey, Kokobun and his colleagues released the animals for a day's foraging before successfully retrieving the data loggers.
Back in the lab, the team scrutinised the 19,648 camera images collected by the diving birds for evidence of the penguins' favourite snack, krill, carefully analysed the bird's head movements in 3-dimensions and found a strong correlation between forward thrusting movements and their encounters with patches of krill. ‘The underwater head movement corresponded well with prey encounter detected by bird-borne cameras, suggesting that head movement (especially in the surge axis [forward thrust]) may be related to prey capture attempts in response to prey encounter opportunities,’ say Kokubun and his colleagues.
Having shown that a foraging penguin's success rate could be measured by keeping track of its head movements, the researchers suggest that this method could also be applied to other predators. However, they sound a note of caution – pointing out that birds move their head for reasons other than feeding – and add that this method could have powerful applications in aquatic environments where visibility is often limited. ‘Together with other information such as GPS locations, the method will help advance our understanding of the relationship between the environment and the foraging behaviour of small- or medium-sized predators,’ says Kokubun and his colleagues.
