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."
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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