Is satellite tagging reliable?

Matt Cross

Matt Cross

A freelance fieldsports journalist and Shooting Times contributor based in a tiny village in the South West of Scotland. His writing focusses on key countryside issues and he runs his own blog 'Countryside Controversial'. In his monthly guest blogs for BASC, Matt comments on Wild Justice and the current climate surrounding fieldsports in the UK.

In April this year, a satellite tag fitted to a golden eagle called Adam made its last broadcast from a vehicle turning circle on a grouse moor in Perthshire. Curiously, the bird seemed to spend its last six minutes on the ground before disappearing. To make matters worse, a few hours earlier the tag fitted to a different eagle, Charlie, had seemed to disappear from a hillside just 3.4 kilometres away. 

At first glance, that looks bad and certainly those who want to ban grouse shooting down were quick to jump on it. Chris Packham claimed: “The eagles’ satellite tags, which had been working perfectly for one and two years respectively, suddenly and inexplicably stopped working, (Charlie’s at 06.25hrs and Adam’s at 11.39hrs), just 3.4km apart.” This was enough concern for the police to begin an investigation, leading to a ‘fingertip touch’ search. What followed was the customary, video, blog and social media campaign. It was slick, effective and I was taken in.

Satellite tagging of birds of prey has delivered some very difficult messages for those of us who shoot. It has located birds that were illegally shot, trapped and poisoned. A large-scale study of the fates of satellite tagged hen harriers found that: “The likelihood of harriers dying or disappearing increased as their use of grouse moors increased.”

Peter Glenser QC, who at the time was BASC chairman, said:

Satellite tags are a tool in the fight against raptor persecution. We have to make sure there is no place left for criminals to hide.

Peter Glenser QC

Let’s go back to those two eagles on their Perthshire grouse moor. I don’t think the evidence shows what it is claimed to show. The eagles were tagged as part of tagging campaign organised by Dr Ruth Tingay of the blog Raptor Persecution Scotland and the TV presenter Chris Packham. The birds were fitted with the latest and best satellite tags; GSM GPS tags made by an American company called Satellite Telemetry. This technology is highly significant in understanding what happened to those eagles.

Those tags are solar powered and contain two systems. The first system fixes the bird’s position using GPS satellite; those fixes are then stored inside the tag. The GPS fixes are taken frequently, up to every minute. The second system passes stored ‘fixes’ back to the satellite tag manufacturer using the mobile phone network. The second system operates much less often; uploading the stored GPS fixes at either 12- or 24-hour intervals. The more light it gets, the more frequently it will fix its position. Up until the point of its last transmission Adam’s tag was fixing its position every minute.

What happened at that turning circle was that the tag contacted a mobile phone mast and uploaded its last 12 or 24 hours of stored fixes. In other words, it did exactly what it was designed to do. I can’t see anything that suggests the tag stopped working at that time or at that turning circle.

In all probability, the bird flew away from the turning circle and the tag on its back continued working perfectly for another 12-24 hours. The chance that a tag would broadcast all its stored positions then suddenly stop working is very slim. But if you want to provide ‘evidence’ for a tweet to the local MSP claiming that: “There is more eagle blood in your backyard – spilled by criminals from the grouse shooting industry” then it’s worth ignoring the difference between where the tag last broadcasts and where it actually stops functioning.

The most likely scenario to me seems to be that in the following 12 or 24 hours those eagles met and fought and, in the course of doing so, they were either killed or injured, damaging or dislodging their tags and subsequently preventing their tags from broadcasting the fixes stored within them. It could have occurred anywhere within a 12-hour flight of that point, which is a long long way. If the eagle flew out of mobile phone range, the event which stopped its tag transmitting may have occurred much more than 12 hours later. That is speculation, but it’s no more speculative than the claim that their blood was “spilled by criminals from the grouse shooting industry”.

Stored in that tag is the point at which that eagle died, or the tag was disabled, but because the tag is no longer functional, unless someone finds it and takes it to microwave telemetry we will never know what data is in it because it cannot talk to a phone mast and upload those fixes. The turning circle is a red herring, there is nothing to suggest anything unusual happened there.

Many people in shooting were in denial about raptor persecution and what satellite tags told us. Some still are. I regularly get messages, emails and phone calls from shooters who are angry with me for publicly stating the obvious; that raptors are illegally killed. They advise me to ‘shut up’ or do other less polite things.

But, most people in shooting have learnt to accept the uncomfortable truth and satellite tag data has been an important part of that. The danger now is that we become too credulous, too willing to believe every claim that is made about what a tag tells us.

There is a world of difference between what serious scientists can tell us based on large scale analysis of dozens of tags, and what campaigners concoct based on one or two tags. Serious science is to be treated with respect, the claims of media personalities should be viewed much more cynically.