Nature notes

The trolls of the Rum Cuillin

When it comes to sea birds, you’d think that there wouldn’t be any with a relevance to us mountain lovers. But there is one. Apart from the breeding season, it spends its whole year ceaselessly wandering. Its navigation skills are on another level, it nests on remote island mountain tops, and best of all it used to make Vikings fill their pants! How could you not want to know about the Manx shearwater? Sit back while I regurgitate some tasty facts for you to digest at your leisure.

The Manx shearwater has a wing span of around 80cm and a body length of just over 30cm. Both males and females are black on the upper parts and a greyish white on the lower, with wings that are long, narrow and pointed at the tips, enabling them to stay at sea for the greater part of the year. They feed on fish, their wings now helping them to swim underwater.

Britain and Ireland account for 90% (250,000-300,000) of the breeding Manx shearwaters in the whole world. And the majority of those nest on one of three islands; Skokholm and Skomer off the Pembrokeshire coast, or (and this is where it gets relevant for us) the Isle of Rum. There are other, smaller breeding colonies scattered around the north-west Atlantic coast, with only a few left on the Isle of Man, the place from where they get their name. Islands make ideal places to set up a breeding colony because they nest in burrows and clefts in rocks, making them vulnerable to ground predators such as rats. The introduction of those relentless rodents and our own predation of shearwater chicks are the real reason the once huge colony on the Isle of Man declined.

Up on the Rum Cuillin, near the tops, you’ll notice lots and lots of what appear to be rabbit burrows. These are in fact Manx shearwater nest chambers. If it’s the spring or very early summer then inside may well be an adult bird with one egg or chick. You won’t see them though. The other parent goes out to sea to collect food for up to two days, returning only after dark. Because the eyes of the shearwaters are small and not suited to nocturnal activities, they rely on their knowledge of the colony and crucially the birds call to one another continuously, helping them to identify the whereabouts of the burrow. I’ve not been lucky enough to hear it in person (it’s on my list), but it’s said to be one of the strangest, most blood-curdling cries in the natural world. Bear in mind it’s the middle of the night and the sound is coming from high in the mountains. It’s no wonder those Vikings soiled their clothes at the sound of the cries. One of the Rum Cuillin is called Trollaval – those trolls were surely the shearwaters?

The chicks are fed on regurgitated fish and squid, getting alarmingly fat before being abandoned by the parents once they’ve reached adult size. The chicks’ plumpness made them a desirable commodity – not only were they meant to be tasty, they also had a high fat content, enabling oil to be harvested. It appears that the nest chamber is just about an arm’s length into the burrow too. If only they’d have gone another six inches they’d have been safe from the long arm of our hunger. After the chick’s abandonment, it takes 8-10 days for it to get hungry enough to venture out of the burrow, taking flight for the first time as it does. They’re the longest living bird in Britain and have been known to live up to 50 years, travelling several million miles in that time.

Manx shearwaters migrate here from the south in the spring, returning that way in the autumn, so this is a good time to see them on the wing if you’re making a ferry journey to one of our west coast islands.

On most days, the sea’s surface is like an endless mountain range, its peaks forming and disappearing as the waves rise and fall. The sight of a Manx shearwater skimming low over the sea’s surface, endlessly wandering over its ‘mountains’, will fill you with awe and probably make you want to climb the Rum Cuillin.

MANX SHEARWATER IDENTIFY THE WHEREABOUTS OF THE BURROW WITH ONE OF THE STRANGEST, MOST BLOOD-CURDLING CRIES IN THE NATURAL WORLD...