Night-time tracking of island “ghost birds”

JUNE 13, 2017
Night-time tracking of island “ghost birds”

Plettenberg Bay zoology masters student Danielle van den Heever adopted a nocturnal routine to study tropical “ghost birds”, slightly bigger than doves yet tough enough to fly hundreds of kilometres out to sea. 

Van den Heever, 24, from Nelson Mandela Metropolitan University’s Marine Apex Predator Research Unit (Mapru), studied the wedge-tailed shearwaters known as “ghost birds” because of their eerie call, at Reunion and Seychelles islands over the last three breeding seasons.

She would conduct her fieldwork at night when they returned to their underground-burrow nests, after days at sea looking for food.  

“They would generally stay away for one or two days, but some spent up to 18 days at sea. Sometimes both parents would go out to sea, leaving their chick alone in the nest, sometimes for a very long time. But the chicks were safe from most predators in the underground burrows.”

The small size of the birds (they weigh between 250g and 350g) has in the past limited the technology scientists could use to track them, with most studies relying on nest or boat observation. But advances in technology have now led to light-weight tracking devices, weighing between eight and 12 grams, which can be attached to the shearwaters enabling scientists to track them out at sea – and this formed the crux of Van den Heever’s research.

“I used special waterproof tape to attach a GPS to the tail feathers of the birds.” She also attached time depth recorders, to find how deep and for how long they would dive to catch fish.

It was the first time these high-tech tracking devices had been fitted onto the shearwaters at Reunion, and only the second time at the Seychelles.

From the Reunion data, she found that some of the birds flew as far as Madagascar – some 800km west of Reunion. “These long trips were to a very productive area [rich with fish], that is also favoured by boobies, terns and other tropical sea-birds.

“At other times, they would visit a few spots just south-east of the island… These were upwelling areas [where nutrients are pushed up from the bottom of the sea] on the sea shelf, where there was lots of food available.

“One other area was a big sea mountain in the middle of the ocean, which pushed up a lot of sea nutrients. It was relatively shallower at this spot compared to the nothingness [of the vast ocean] all around it.”

She combined the tracking data she collected with her fieldwork, which included carrying out stable isotope analysis – a process which determines the carbon and nitrogen signature of the bird, through blood, feathers and diet samples.

“The nitrogen signature gives us an idea of the trophic level [food chain level] they feed in, while the carbon signature tells us how far inshore or offshore they feed.”

Van den Heever said her research in identifying the birds’ feeding hotspots would “hopefully contribute towards recommendations for the allocation of potential Marine Protected Areas in that area”.

Globally, there are over five million wedge-tailed shearwaters – however research has shown that their numbers are dropping for several reasons, among them invasive species and overfishing – hence the need to monitor their behaviour at sea and protect their feeding grounds.

Van den Heever said there were conflicts between the shearwaters and the commercial tuna fisheries at the two islands. “These birds don’t feed on the tuna, but the tuna drives up the fish they can eat. But with widespread tuna fishing, there is less food being driven to the surface for the shearwaters.”

She said the birds were also affected by rising sea levels as a result of climate change. “This influences where they can nest … What I found at the Seychelles is that during super moons [when tides are unusually high], the nests [which are burrows in the sand] near the high-water mark are washed away … Sometimes, you would see eggs floating away. Reunion was not as affected as the birds tend to nest in cliffs, burrowing in rock holes.”

She also conducted a census at the Seychelles, looking in over 700 burrows using an endoscope (a flexible tube with an attached camera) attached to a laptop.

If she couldn’t see a bird in the intricately-dug burrows – which sometimes went as deep as two metres – she would play recorded bird calls at the burrow entrance. “If it responded, I would know there was a bird in the burrow.”

Van den Heever, whose research was supervised by NMMU senior zoology lecture and Mapru head Dr Pierre Pistorius, worked with the Save our Seas Foundation-D’Arros Research Centre (SOSF-DRC) at the Seychelles, and with the University of Reunion.

Passionate about marine life since she was a young girl, she saved up her money at age 11 to pay for a scuba diving course, which sealed her decision to pursue ocean-focused work.