Manx shearwaters
Every year, Skomer’s ‘Manxies’ travel 7,000km to Argentina – without a map. So how do they know where to go?
Every year, more than 18,000 people set an early alarm to spend a day with superstar seabirds at The Wildlife Trusts’ Skomer Island, Pembrokeshire. After a short boat journey on the Dale Princess from Martins Haven – looking out for harbour porpoise while Atlantic puffins whizz overhead – the tourists disembark to start their island adventure.
Visitors take hundreds of photographs and while away the hours watching the frenetic activity of an array of breeding birds on a cliff-face known as The Wick. But what they don’t see, while the sun is out, is arguably Wales’ best-kept secret – hidden beneath the island’s grassy top and only making an appearance after dark.
Joe Wynn’s arm disappears into the earth, as he lies on the ground to conduct a daily check of a study burrow on Skomer. There are 100 burrows marked for research on the island and the doctoral student knows exactly where to place himself to avoid damaging any of them.
He removes a single white egg from the hole and puts it carefully into a container beside research assistant Daryl Mcleod. It is April and the start of the breeding season
for this particular feathered resident. “An egg weighs up to 15 per cent of the body weight of an adult, which is kind of nuts really,” he says. “It’s a huge investment, as they only lay one per year, but they do have a high breeding success rate.” Joe’s limb disappears again but this time he removes something much larger, a Manx shearwater or ‘Manxie’. “This one is very chilled – it’s been involved in experiments numerous times,” he says, holding the ringed, adult male. He places it in a bag to weigh it and Daryl jots 485g in a notepad. “That’s a good weight,” comments Skomer warden Sylwia Zbijewska as she watches the researchers at work. “You can see how relaxed the bird is – normally it’ll bite you and try to get away but because they handle the chicks when they hatch, it’s normal for them.” Joe is studying this species for Oxford Navigation (OxNav), an Oxford University Department of Zoology group that focuses on the behavioural and ecological aspects of animal navigation. OxNav has been researching Manx shearwaters on Skomer for 15 years – Joe has been involved since 2016.
Throughout spring and summer, he’ll weigh any eggs, chicks and adults in the study burrows. The research is looking at the relationship between a male and female raising their young.
Perfect partners
Manx shearwater pairs take it in turn to incubate their egg, which hatches after about 51 days. The chick remains in the burrow for a further 70 days, during which time feeding visits become more frequent. Male and female Manx shearwaters seem to take equal amounts of incubation stints, with the male usually taking the first.
“In chick-rearing, adults adhere to one of two strategies – they either visit the chick daily, or they provision for themselves and visit every three to five days,” says Joe. “Parents swap between these two duties, so generally the chick is being fed every one to two nights by at least one adult, and they only visit once per day, always at night.”
After dark, guests on the island get to experience the cacophonous sound of the Manxies.
A previous study using radio frequency identification (RFID) readers revealed parents co-ordinated when feeding their chick but this co-ordination became less pronounced as the season went on. The work OxNav is currently doing is building on this original finding by using the same technology to research over 50 tagged birds.
“The readers use an electromagnetic pulse to cause a radio wave to be emitted by the tag, which is picked up by the detector circuit,” he explains. “Each radio wave is specific to a bird, and consequently we can identify individuals as they enter and leave a burrow.”
Skomer visitor officer Sarah Parmor elaborates on the birds’ behaviour: “Their dual foraging strategy means one adult at a time can take ‘breaks’ from chickfeeding foraging trips (closer to the colony) and spend up to a week on long-distance foraging trips to food-rich areas.
“The long-distance trips are mainly for self-reconditioning when they don’t have to keep flying back to the colony. During this time, the other partner will make regular visits to feed their offspring.”
Joe is also intrigued by the ability of the Manx shearwater to migrate over 7,000km from the Welsh coast to their
feeding grounds in southern Brazil and Argentina at the end of the summer, often refuelling in West Africa and the Caribbean on their way: “Manx shearwaters are one of the greatest navigators of the bird world,” he declares.
Flight of the navigator
How do they do it? “By magic,” he jokes. “In terms of genetic inheritance, it’s a bit of a can of worms!” The researcher is yet to publish his own theory but refers to a 1958 study on migrating starlings to explain what he means.
Songbirds are thought to be born with an inherited, predetermined migration distance and a given direction in which to travel. This is navigated using a ‘clock’ and compass rather than innate map information. “A 2017 study on streaked shearwaters in Japan suggests the mechanism may be similar,” Joe reveals.
“There is currently no evidence that shearwaters use a magnetic sense. However, we know that Manxies use a time-compensated sun compass because if you make them think it’s earlier/later in the day than it is, then you get a predictable deflection in their homeward orientation.
“This is because the compass functions based on knowing where the sun is at a given time of day, and so by changing a bird’s understanding of the time of day we can change the direction they head in. This is the only known compass in Manx shearwaters, but it’s likely they have more, and we just haven’t discovered them,” Joe explains.
It is reported that the island is home to half of the world’s Manx shearwater population.
“In terms of map sense, it’s possible that they rely heavily on their ability to smell, because in other shearwater species, when you remove this ability, they struggle to navigate,” he says. “However, there are no Manxie studies on this, so we can’t be sure.”
Remarkably, young Manx shearwaters make the same journey as their parents in late August or early September and some even manage to complete it in less than a fortnight: “Fledglings are thought to migrate asynchronously with respect to their parents.”
Healthy population
Once weighed, the Skomer Manxie and egg are placed back underground, and Joe and Daryl move on to the next burrow. Throughout the morning, they zigzag across a landscape that is covered in so many holes, it looks like the top of a pepper pot.
“Manx shearwaters like to nest in burrows on a slope. It makes it easier for them to take off,” says Sarah. The birds are beautifully adapted to life at sea, with long, narrow wings and feet placed far back on their bodies for efficient swimming, but this makes it tricky for them to walk. Due to their biology, they are easy prey at breeding colonies for great black-backed gulls, which is why they wait until night-time to leave the safety of their burrows, heading to deep ocean in search of prey.
After dark, a lucky group of guests staying on the island get to experience the cacophonous sound of the Manxies leaving or returning. The Seafarers author Stephen Rutt describes the bird’s call: “It is reminiscent of a stifled cackle… a throaty, repetitive, gurgled sound: delivered like the braying jeers of a football crowd.”
Now imagine 349,663 breeding pairs on Skomer making that noise. This is the estimate population figure, according to the most recent Seabirds Count census in June 2018, managed by Natural Resources Wales. It reported that the island is home to half of the world’s Manx shearwater population.
To obtain this figure, the social call of the seabird (pre-recorded on an audio device) was played into a sample of burrows. If a bird responded, the burrow was noted as active. This method encouraged a high proportion of nesting birds to make themselves known.
When compared to the 316,000 pairs estimated in the last census in 2011, it suggests the Manxie population on the island has risen. “Skomer is a safe environment for shearwaters,” says Sylwia.
As we stick to the path and wander past a grazing black rabbit – a descendent of the lagomorphs introduced here by the Normans in the 13th century as a source of food and fur – Sarah explains that the herbivores provide readymade burrows for Manxies to nest in.
Surprisingly, despite humans inhabiting Skomer from the Middle Ages, the island has avoided a rat invasion. This, coupled with the fact the island is surrounded by a Marine Protected Area, makes it easy to see the appeal of this isle for Manx shearwaters.
The pelagic species may not hold the record for the longest migration in the avian world but it tops the list of long-living birds in Britain and, during its lifespan, it could travel over 7 million kilometres. “If a Manx shearwater lives to 50 years of age,” says Sarah, “it would cover the distance to the moon and back 10 times.”