While a bird's plumage is often its crowning glory, feathers can become a scruffy mess by the end of the season, so, many species shed their plumage gradually, allowing them to remain aloft while the feathers regrow. However, the king penguin moult is truly dramatic. The ocean-going adults shed all of their feathers over a 4-week period while marooned on shore fasting until they have replaced all of their plumage. In the process, the birds lose half of their body mass, almost all of their subcutaneous fat and half of their pectoral muscle – which powers swimming and stores oxygen during diving – impairing their ability to forage as they build themselves back up again. But how king penguin youngsters deal with the privations of moulting was a mystery. So, Charles-André Bost and Henri Weimerskirch from Centre d'Etudes Biologiques de Chizé, France, and Yves Handrich, Yvon Le Maho and Celiné Le Bohec from Université de Strasbourg, France, collaborated to monitor the youngsters for up to 3 years – starting prior to their departure from their home colony on Possession Island, the Southern Indian Ocean – to find out how they deal with their annual feather loss.
‘One of the advantages of working with penguins is that they can't fly away’, smiles Enstipp, who describes how the team collected 30 youngsters from the colony before they embarked on their maiden voyage in late 2013. Next, Handrich and Caroline Bost skilfully implanted a minute data-logger – designed by Robin Laesser and colleagues – beneath each penguin's skin to record the pressure and temperature every 5 s for as long as the battery held out. Then, the team waited for up to 3 years for the birds to return home. Fortunately, each animal was equipped with a radio ID tag that triggered an alarm when the penguin returned; then it was a race against time for the scientists based on the island to intercept the new arrival before it vanished among its colony mates. Eventually, the team retrieved 19 of the data-loggers, 14 of which had recorded usable data.
Back in Strasbourg, Enstipp disentangled the millions of pressure and temperature readings for evidence of the voyagers’ moults. ‘From the pressure recording, you can see that a bird is not at sea’, says Enstipp. Then he scoured the recordings to reveal what the birds were up to before, during and after each moult. Prior to their first moult after departing from the colony, the youngsters went out foraging to stock up on energy in preparation for losing their feathers. They then remained on land for up to 4 weeks as their feathers fell out and were replaced; ‘the amount of feathers floating through the colony at this point is quite impressive’, chuckles Enstipp. Once their plumage was replenished, the youngsters returned briefly to the sea, this time to rebuild their body reserves. However, after analysing the dive durations, Enstipp realised that the effort of replacing their feathers had taken its toll. The dives of the newly moulted birds were between 20 and 42 s shorter (depending on the depth), ‘drastically reducing their bottom time, when they can capture prey’, he says. In addition, they took longer to recover after returning to the surface and they lost more heat from their bodies while submerged hunting for fish.
So the youngsters’ extreme moults definitely restrict their ability to forage at a time when they need to quickly replace their insulating fat and emaciated pectoral muscle. ‘This underlines how stressful this entire moult period is’, says Enstipp.