Are birds of prey being unfairly persecuted?
Government proposals to trap and displace buzzards to protect captive-reared pheasants have been dropped after a public outcry. So can birds of prey live alongside shooting interests?
Diving, looping and barrel-rolling, the sky dance of a male hen harrier is one of the most spectacular sights in the sky.
But if you want to see it in England, you should go soon. The species is vanishing.
Radio-tracked hen harriers have been flying into mysterious black holes in the north of England, disappearing in areas principally managed for shooting, according to a Natural England report.
The nation could be down to one nesting pair of hen harriers this year, a result of illegal killings committed with the intention of protecting grouse, according to the Royal Society for the Protection of Birds (RSPB).
"If you create an area where there's lots of prey available... you're going to get predators homing in on that if they are able to," says John Calladine from the British Trust for Ornithology (BTO).
But do harriers, buzzards and other raptors really have a devastating effect on local game bird populations?
Many studies suggest not, but experts say that some on both sides of the argument are guilty of oversimplification.
Different raptors will take different prey from different places, says Mr Calladine.
And some species are better understood than others.
"Merlin will be too small to take a healthy adult pheasant, though a larger peregrine would have that capability... [and a] hen harrier will be unlikely to forage within a dense woodland, but is a capable hunter on open moorland."
The impact of raptors is a political hot potato because shooting game birds is an economic lynchpin in rural communities, and apparent attempts by some to control raptors have conservationists worried.
Killing birds of prey is illegal in the UK and is repeatedly condemned by the shooting industry and gamekeepers association.
But the use of poisoned bait as a predator control technique is indiscriminate and its intended target is not always clear.
Whether deliberately targeted or not, four Scottish golden eagles were poisoned in 2010 and one in 2011 according to the Partnership Against Wildlife Crime in Scotland (Paws). In all there were 16 Scottish raptors killed with poison in 2011 and 28 in 2010.
Meanwhile, scientists have been studying the interaction between raptors and game birds.
The most straightforward attempts to study this have been done on captive-reared game birds such as pheasants, which are released from their pens for the shooting season, says Dr Kirsty Park, a University of Stirling ecologist.
"You know how many should be in there, and you know how many have disappeared.
"As long as you can demonstrate that they were actually taken by a raptor then you've got your number of pheasants that have been taken," Dr Park says.
From a review of studies, Dr Park says the average predation rate of buzzards on a captive-reared game bird population appears to be under 5%, although that figure varies considerably and depending on local conditions, could be far higher.
But the heart of this row is not really how many game birds raptors take, it is how many game birds are available to shoot. That is the economic impact of the predation rate.
This is much harder to work out, explains Dr Park.
When a predator eats a game bird, competition may be reduced among the remaining birds and their survival chances can be improved as a result, she says.
"Released game birds will die for all sorts of other reasons and not all released game birds are shot, so you can't use number of game birds being taken by a buzzard and then calculate [the economic impact] by the price of what you would get for a pheasant being shot.
"That's just not valid but it's often done because it's all the information that's available."
Wild game bird populations tend to move in cycles and research suggests that the effect of predation will vary depending on where the population is in that cycle.
A study completed in 2004 by Mark Watson showed that the impact of sparrowhawk predation on grey partridges was at its greatest when the game bird population had already been reduced to low levels.
The finding is supported by conservation scientist Steve Redpath's work, who has conducted the longest-running study of the impact of birds of prey on game bird populations, and now works to reconcile the interests of conservation and shooting.
His work at Langholm moor on the predator-prey relationship between hen harriers and red grouse appeared to show that the effect of predation by hen harriers was a downward pressure on numbers when the grouse population was already at a low.
The study found that the effect of raptor predation was to suppress the grouse population cycle.
But for all the nuances in the debate, Prof Redpath says that there remain some simple principles that apply.
"You can have low densities of hen harriers and have plenty of grouse-driven shooting.
"There were places that we worked where there were one two or three pairs of hen harriers breeding and they happily had plenty of grouse to shoot."
But if there are a lot of hen harriers around for a long time, the grouse population will suffer, he says.
Hen harriers are not territorial and in some cases "you get colonies forming and they can then have quite a big impact".
Prof Redpath's work about what brings hen harriers into contact with grouse shows that the number of harriers in an area is related to the "density of the small prey that they eat in the spring, so things like meadow pipits and voles", rather than the number of grouse.
He did an experiment in which food such as rats and poultry chicks were left at hen harrier nests.
This diversionary feeding resulted in "an 86% reduction in how many grouse chicks they ate during the breeding season".
Perhaps there is some hope of reconciling the competing interests of conservationists and gamekeepers after all.
Source: BBC Nature