Stuart Benn, from RSPB Scotland, brings us this latest blog on golden eagle tagging, investigating how and if we can identify what has happened to birds of prey using technology. 


The recent publication of the report into the fate of satellite tagged golden eagles in Scotland makes shocking reading and highlights just how big a problem illegal persecution continues to be in some areas. 

In short, some tagged birds are still alive, some died of natural causes, some were illegally killed and the bodies retrieved, some tags were retrieved having become detached from the bird, a very small number were considered to have a tag malfunction but the lion’s share ‘went off the radar’ in suspicious circumstances.  Clearly, being able to assign tag fates to the correct category is key in determining what happened to them – but how can you tell?

To answer this, it’s worth a quick summary of the tagging process.   

Golden eagle chicks grow fast and go from being tiny balls of fluff to adult-sized and large enough to be fitted with satellite tags in eight weeks.  The tags are put on the chick’s back and attached by a harness with the straps being joined at the front but with a weak link put in. The reason for this is that even though the tag has a battery and solar panels to keep it charged it eventually runs out after a few years. All being well, the bird will live for much longer than this so rather than have the eagle carry a tag that is no longer working, the attachment is designed to eventually fail and fall off.

Two other points are worth noting. Firstly, fitting tags is a very skilled process and anyone doing this has to undergo rigorous training. Secondly, tagging clearly requires a supply of eagle chicks from nests where they are doing well and many landowners (including some grouse shooting estates) have been extremely helpful in facilitating tagging.

Two week old golden eaglets. Credit: Laurie Campbell (rspb-images.com).

So, once the chick has fledged and then left the area it was born in, it flies around Scotland for several years until it is old enough to set up territory. This part of a young eagle’s life used to be shrouded in mystery but now, with satellite tags, we can follow them every step of the way with their movements being automatically sent to your computer.

I’ve kept an eye on all the RSPB’s golden eagle tagging data since 2012 and you soon pick up what normal activity looks like – the information arrives at set times and includes data on the battery voltage, temperature and whether the tag is moving. From all of this, you can find out not only where the bird is but whether it’s still alive.

For much of the time, everything is fine but every so often the information arouses concern. On four occasions, birds stopped moving and I was able to direct the police and the RSPB Investigations section to the exact spot. Twice, the birds were found to have died from natural causes but on the other two they had been illegally killed.

However, more recently, a different pattern has emerged - the tag is working perfectly and then suddenly stops, never to be heard from again, and almost always from one of a restricted number of areas dominated by driven grouse shooting.  Searches of the last known position may show signs that an eagle has been there but there’s no bird and there’s no tag. 

So, when one of our tags suddenly stopped transmitting in one of those areas a couple of weeks ago, my suspicions were immediately raised and, after notifying the police, they agreed that I should visit the last known location.  However, the weather wasn’t suitable for me to go and search immediately and I had to wait a few days but, in that time and unusually, I did get some low quality info from the tag – nothing that was specific to location but it did make me think that the tag was still intact and trying to transmit, I was intrigued.

The last good position I’d received was in a remote area and it took me over two hours to climb up into the hills and traverse some very wet peat bogs before I got to the general location and could start the search.  It turned out to be a shallow gully cutting through open moorland but with a few wee rock outcrops – a classic place for an eagle to roost.

I homed in on the last known location – some rocks – and immediately spotted plenty of eagle feathers so a bird had definitely been here. Another wee scout about and there it was lying face down on a north-facing ledge – the tag, no bird just the tag!

Golden eagle satellite tag in situ. Credit: Stuart Benn.

I scrambled down to it and, mindful that it could be a crime scene (though I’d noted that there were no vehicle tracks leading to the site so this was probably unlikely), I checked the tag visually and I could see that the weak link had come apart naturally, exactly as planned. So, the bird had roosted there, the tag had transmitted this position but overnight the weak link had broken, the tag had fallen off the bird just as it is meant to do, the eagle had flown off, I’d gone to that location and there was the tag!

There are some interesting conclusions to draw from this experience. Firstly, the mechanism designed to stop a bird carrying a tag after it may have stopped transmitting works perfectly. Secondly, even a tag that is face down with very limited charge still tries to transmit and gives some indication that it is still intact (the tag isn’t entirely face down as the antenna keeps it propped up but the solar panels are not getting any direct light). Thirdly, satellite tags are astonishingly accurate – if you get a good last known location and there’s something there to find then you’ll find it.

The weak link of the tag had broken, and the tag had fallen off the bird just as it is meant to do. Credit: Stuart Benn. 

All this is in direct contrast to that other situation where tags are working perfectly well then suddenly stop transmitting never to be heard from again.  In these cases, the report concludes that ‘The final fixes of the ‘stopped no malfunction’ tags were significantly associated with persecution incidents. Their sudden demise was evidently due in large part to people killing the tagged birds (and the sudden disposal of the bird and its tag subsequently).’

Satellite tags are expensive so we’ll be able to get this one refurbished over the winter and put on another golden eagle in 2018 so, all told, this was a great result and it was a happy boy that made that two hour walk back to the car.

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