A remarkable bird that flies up to 1 million miles over a lifetime? Athayde Tonhasca, SNH Species Advisory Officer, tells its story.
From September to October, thousands of Manx shearwaters (Puffinus puffinus) leave their breeding grounds located on a few islands off Britain’s west coast – Rum alone hosts around a third of the world’s population – and set out to South America. These trans-Atlantic journeys involve a careful flight plan. Travelling at about 55 km/h (35 mph) for up to 139 hours nonstop and making a few stopovers to replenish their reserves, Manx shearwaters may take 5-7 weeks to reach Brazil, Uruguay or Argentina, where they spend the northern winter. When they return in March, they will have covered some 11,000 miles. On top of that, Manx shearwaters travel anywhere between 40 and 400 miles round trip every day in search of food – mostly shoaling fish, but also squid, crustaceans and offal.
Like other pelagic birds (of coastal waters and open ocean), Manx shearwaters live for a long time, usually more than 20 years; some ringed birds were over 50 years old at the time of recapture. Adding a conservative 20-mile daily feeding trip to their annual migration, a 50 year-old individual would have clocked around 1 million miles – not bad for a relatively small bird (about 400 g).
With such a wandering life, how do these birds find their way from South America to a small Scottish island across vast expanses of featureless seas? For years it was believed that Manx shearwaters and other birds navigate by visual landmarks and by interpreting Earth’s magnetic field. More recently however, evidence has accumulated to support another hypothesis: birds have the remarkable ability to orientate by odours distributed in the atmosphere. They seem to be able to pinpoint their targets by assessing gradients of scents from numerous sources such as plankton distribution in the ocean, and from areas where prey concentrates.
While impressive in the air, a Manx shearwater doesn’t do so well on land. It can barely walk, relying instead on shuffling or sledging on its belly. This handicap of course makes it an easy prey for predators (in fact, rats are a serious threat to a whole colony if they make their way to a breeding ground). To reduce the risk of predation, Manx shearwaters usually don’t come to land during daylight. On returning from fishing trips, they form ‘rafts’ of floating birds offshore, waiting until dark before making their way to their burrows. Then vocalization becomes important for communication between partners and orientation among similar burrows. So colonies are very noisy after dark, the air filled with a cacophony of thousands of birds growling, cackling and screeching to each other.
You can hear some of these sounds in this recording from the British Library Sounds
In the past, these peculiar, otherworldly nocturnal cries were considered manifestations of the supernatural, and 11th-century Vikings believed that Rum was inhabited by noisy trolls. They couldn’t have been more wrong; Manx shearwaters are not trolls, but rather tireless itinerant ambassadors of Britain’s natural heritage.
A final biodiversity note: the flea Ceratophyllus fionnus is found only in Manx shearwaters’ nests on the island of Rum, nowhere else in the world: it is one of the few Scottish endemic species.
Visit the Manx shearwater breeding grounds on the Isle of Rum by night – find out more here.
Check out the Rum NNR website.