More questions than answers from Hen Harrier tracking data

New data published by Natural England on its hen harrier tracking programme does not offer any explanation for the lack of expansion of the English population of the bird, according to the Countryside Alliance. Of the 47 hen harriers that Natural England has tracked since 2007, the actual location of 37 birds is unknown. Of the remaining 10, four are known to still be alive, and six have been found dead. It has been determined that predation and disease were responsible for five of those deaths. The sixth bird may have been illegally shot.

It is not possible to draw any firm conclusions on the fate of the 37 missing hen harriers from the summary data published by Natural England. Exceptionally, all five of those tagged on the Isle of Man also went missing on the island. However, the majority of those tagged in Bowland, Lancs, the North of England, and at Langholm in Scotland went missing in areas other than that in which they were tagged, including Northern Ireland, Northern France, Lincolnshire, Shropshire and Dorset. Transmitter failure and damage is also a real possibility – the batteries only last for three years. Natural England states that it knows of three instances since 2007 where birds have been seen alive after their tags stopped transmitting.

Adrian Blackmore, director of shooting for the Countryside Alliance, said: "It is clear from Natural England's summary data that further research is required and we look forward to the publication of the full results next year.

"What can be said about the publication of this data is that it does not in any way substantiate unfounded allegations, made by the RSPB and others, of systematic persecution of hen harriers by grouse shooting interests.

"Any persecution of protected species is illegal and we absolutely condemn such killing, but the evidence of the tracking programme makes the claims made against grouse shooting unsustainable. Charities, in particular, have a duty to be factually accurate and we will hold them to that.

"This data also removes any barrier to the implementation of the hen harrier joint recovery plan (see notes below). Hen harriers in England clearly need help and any further delay in publishing and delivering the plan is unacceptable."

Notes

• The Natural England report can be found here: https://www.gov.uk/government/news/hen-harrier-satellite-tracking-programme-results-published#tracking-programme-results

• Satellite transmitters used on hen harriers are solar powered and transmit for around 10 hours before undergoing a recharging period lasting around 48 hours. Given that the grid reference of the last transmission for a bird's tag is only accurate to 100Km2, and birds can travel 350 miles in a day with the result that birds could theoretically travel up to 700 miles after their last signal, it is impossible to draw hard and fast conclusions about where the birds may be, and what may have happened to them since their signals ceased – unless their bodies have been recovered and examined. Three of the missing birds are known to have been in France at the time.

• Natural mortality rates will be highest for recently fledged and inexperienced hen harriers. Fledging at around one month, they remain dependent on their parents for a further month after that. Of the 47 hen harriers tracked, 15 went missing between the ages of two months or less, with a further eight missing at the age of three months. Almost half of the birds being tracked by Natural England therefore went missing when natural mortality rates can be expected to be at their highest. Interestingly three hen harriers went missing within 10 days of having tags fitted – one on the same day. This could suggest a faulty batch of transmitters.

• It is clear from Natural England's summary data that further research is required. The hen harrier is resident in some 87 countries across the northern hemisphere, and in a range of agricultural habitats. Although with a global population thought to be around 1,300,000, its numbers have never been high in Britain. As its name implies, it was in the past a predator of poultry, not many of which are now found on heather moorland in the uplands of the UK.

• On the continent hen harriers are found across a range of agricultural habitats, and given that 75% of the world's remaining heather is found in the UK, the remaining 1,299,000 are not reliant on that as its natural habitat.

• The Hen Harrier Joint Recovery plan - In August 2012 Defra asked moor owners, gamekeepers and conservation groups, including the RSPB, to work together and write a single plan to restore England's hen harrier. They reviewed the evidence and scientific literature to prepare their joint plan. Since January 2014 Defra could have published the plan; but has not yet.