Scottish Deer Are Culprits in Bird Killings

James Owen in England
for National Geographic News
August 25, 2003
Red deer on the Scottish island of Rum may be eating the heads and legs of live seabird chicks as a way to get minerals they need to grow their antlers.

Scientists believe this surprising addition to the red deer's diet stems from mineral deficiencies in the vegetation they eat. By munching on Manx shearwater chicks, the deer are able to get the extra calcium they need.

This annual health drive by the red deer of Rum has drawn a National Geographic documentary team to the island. The team hopes to capture the deer's behavior on film for the very first time.

Situated 16 miles (26 kilometers) off the west coast of northern Scotland, Rum has one of the world's largest colonies of Manx shearwaters (Puffinus puffinus). In late August thousands of youngsters start to venture from the safety of their hillside burrows. Still unable to fly, their emergence provides a windfall for predators. And chief among them is that picture of doe-eyed innocence, the red deer (Cervus elaphus).

For years bird-watchers visiting the island nature reserve puzzled over the appearance of decapitated Manx shearwater chicks at the end of each summer. Leg and wing bones were also missing. Everything else, including feathers, flesh and skin, remained intact.

"We find their carcasses up on the hills near the nesting sites," said nature reserve warden Mick Blunt. "The main period of predation occurs when the young ones finally emerge from their burrows. They come out mainly at night for a mosey around. This is when there are rich pickings to be had, especially if there's a full moon and the chicks are clearly visible."

Prime Suspects

While crows, ravens, and eagles sometimes take live birds, Blunt says red deer became the prime suspects a few years ago when a deer hunter saw one chewing a chick.

Blunt added: "I've never witnessed this myself and there's still a lot we don't know about the phenomenon."

Bob Furness, a seabird ecologist at the University of Glasgow in Scotland, is among a handful of scientists worldwide to have researched bird predation by deer. He has also studied bird-eating sheep, on Foula in the Shetland Islands.

His observations revealed that Foula's sheep target unfledged Arctic terns. In years when the island's tern colony was at its largest, more than 200 chicks were found with amputations characteristic of sheep attacks. Ewes were also seen turning over chicks in the nest before biting off their legs.

As deer and sheep are both ruminants that usually feed exclusively on plants, their digestive systems aren't equipped for a carnivorous diet. This, plus the fact they eat only the crunchiest bits of the birds, leads Furness to suggest they take seabird chicks as a dietary supplement.

He said: "It occurs in deer and sheep populations that live on blanket bog over mineral-deficient rock, so the nutrient status of the vegetation is very poor. They only eat the bones and are very careful to avoid swallowing the feathers, meat, and skin."

Furness says bird bones could be a source for two important minerals, phosphorous and calcium.

He added: "Folk in Australia studied mineral-deficient cattle and showed they search for and chew bones. In that case they considered phosphorous to be the limiting nutrient, but on acid soils a calcium shortage may be as likely."

Antler Growth

In the case of deer, the need for topping up calcium levels is obvious when there's a new set of antlers to grow each year. Reproductive success in stags has been found to correlate with antler size, which in turn is influenced by the quality of the vegetation they eat.

Furness says calcium levels on Rum are low by comparison with mineral-rich rocks elsewhere in Scotland. He found the deer there consume, on average, 36 grams (1.27 ounces) of shearwater bone each season. This represents three chicks per animal. But as over 60,000 Manx shearwater pairs nest on Rum, Furness says deer predation doesn't represent a significant threat to the colony. At most they account for just 4 percent of chicks each year.

The attacks last from late August to late September. Before the shearwater youngsters can attempt the long migration to their winter quarters in the seas off Brazil, they must get in shape by exercising outside their burrows.

They usually do this under the cover of darkness, and some are unable to find their way home. These birds are particularly vulnerable to being eaten by deer, which forage both by day and night.

National Geographic film makers hope to catch the deer in the act next month. As most chicks are taken during darkness, the team will use an infrared night-vision camera borrowed from the French military.

Wildlife film director Jacqueline Farmer said, "This behavior has never been filmed before. Hopefully the footage will form part of a documentary we're making about the way animals use natural medicines."

Seabirds may seem an odd food for a ruminant, but for Rum's red deer they could be the key to healthy living.

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