Revealing the fortunes of birds: what satellite transmitters tell us

Here Ian Thomson, RSPB Scotland's Head of Investigations, explains why fitting satellite transmitters to birds of prey is so important and what the data provided tells us about the these birds.

A golden eagle chick in Galloway who was fitted with a satellite transmitter before fledging in 2015 and disappeared in May 2016 (Dave Anderson, Scottish Raptor Study Group)

Ringing has been an established part of the scientific study of birds for over a hundred years. It has provided a fascinating insight into survival rates, important summer and wintering areas, and even the migration routes of a huge range of species. But, ringing has its limitations, with a bird having to be either found dead or re-trapped in order for the ring to be read.

Various developments in recent decades including the use of colour rings, neck collars and wing tags have allowed birds to be individually identified without having to catch them, but are still dependent on an observer being in the right place at the right time in order to link a bird with a location. Sometimes questions of science have to wait to be answered until technology has advanced enough to allow for that to happen.

Such is the case with the movement of birds: before the miniaturisation of GPS satellite tags, there were huge gaps in our knowledge on the daily movements of birds, important feeding areas and overnight roost sites. Satellite transmitters have changed all that. Now, the movements of a bird can be followed on a computer, allowing a fascinating insight into their daily travels, identification of roosting or nest sites and even allowing you to follow their migration across continents and oceans all with incredible accuracy.

A white-tailed eagle being fitted with a satellite transmitter in 2012 (Dean Bricknell, rspb-images.com)

It must be an amazing experience, fitting a satellite transmitter to a young bird of prey, with a feeling of great anticipation as you know you will be able to maintain a relationship with that bird for the following few years, knowing where it has gone and how it is faring.

I work closely with several people who have undergone the years of training, practice and supervision necessary to get them the unique skill-set that has allowed them to be licensed to fit satellite transmitters to birds like ospreys, red kites and hen harriers. Sadly, working for the RSPB Investigations team, my experience with satellite-tagged birds comes not in the early days of these young birds lives as they move away from their nest sites to explore the world. It happens when those experts, from a variety of agencies and organisations who are monitoring the birds, tell me they think there is a problem; a bird appears to have stopped moving, or seems to have disappeared.

One of our first experiences of this was back in 2009, when the signal given out by the transmitter fitted to a young golden eagle, named “Alma”, showed that the bird appeared to have stopped moving. Accompanied by police officers, we went to the location, and found her body, lying face-down in the heather on a grouse moor in the heart of the Angus glens in eastern Scotland. Her body was submitted to the laboratory of Science and Advice for Scottish Agriculture for post-mortem testing. Within a few days, the results came back – she had been poisoned with the banned pesticide, carbofuran.

Alma (courtesy Ewan Weston)

A follow-up operation, led by the police, searched the area, including estate buildings, to try to identify those responsible. But, as is often the case, there was not sufficient evidence found to enable a prosecution. However, the publicity surrounding this case showed very clearly that as well as giving very good information about where satellite-tagged birds were living, it could also show where they were dying.

In subsequent years, there have been further high profile cases where satellite-tagged birds have been illegally killed: the severed legs and wing tags of a red kite found stuffed in a hole in the moss on an Inverness-shire grouse moor in 2010; a golden eagle found poisoned on a grouse moor in the Strathdon area of Aberdeenshire in 2011; a golden eagle dumped under a tree after being illegally trapped on an Angus grouse moor in 2012; golden eagle “Fearnan” poisoned on another Angus grouse moor in 2013; hen harrier “Annie” found shot on a South Lanarkshire grouse moor in 2015.

Annie (John Wright)

There is a depressing pattern here – protected birds of prey found illegally killed on grouse moors. Of course, this is nothing new. In late 2015 we published a report documenting a total of 779 birds of prey confirmed as having been illegally killed in Scotland between 1994 & 2014. A significant majority of these victims were found on land where management for gamebirds, and particularly intensive driven grouse shooting, dominates the landscape.

One thing that has become apparent, however, is that there has been a decline in the number of detected illegal poisoning incidents in Scotland over the last few years. We don’t for one minute think this is coincidence, and is almost certainly linked to the increasing use of satellite transmitters. Publicity around incidents has very clearly shown that if you put poison baits out in our countryside, there is a chance that a satellite-tagged bird will become a victim, and those monitoring these birds will follow it up. The criminals killing our protected raptors don’t want to be caught, so now, perhaps, the risk of using poison is too high?

But that doesn’t mean that the killing of our birds of prey has declined. Despite claims by some to the contrary, there is no evidence to support this. What we have seen is merely a change in tactics, away from the use of poison. Contrast this with the situation south of the border, where there are no satellite-tagged eagles (indeed there are no eagles south of the border). Here the use of illegal poisons shows no signs of a reduction.

Map showing where eight golden eagles fitted with satellite transmitters were last recorded (RSPB Scotland)

In Scotland, what we are now seeing is satellite-tagged birds simply “disappearing”, as is the case of eight golden eagles in the northern Monadhliath mountains in under five years. This isn’t the first time we have seen this pattern – we have other clusters of locations where birds have simply vanished – upper Donside, for example, is an area where one satellite-tagged golden eagle was found poisoned in 2011, but four further satellite-tagged eagles, including a young white-tailed eagle, disappeared in the same area over the following three years.

Some will claim that these disappearances are the results of transmitter malfunctions, or birds dying naturally. Firstly, the experience of those who have fitted and monitored many of these transmitters, both in the UK and elsewhere, is that they are incredibly reliable pieces of kit, with malfunction exceptionally rare. Secondly, if a bird dies naturally, or if a transmitter becomes detached the transmitter will continue to give out location data, allowing it to be found. Indeed, we have recovered a number of these birds that have died as a result of starvation or tags that have come off the birds.

In short, if satellite-tagged birds die naturally, in the vast majority of cases we find them, because their transmitters keep sending out location data. In contrast, in the case of the “disappearing” birds, this is almost exclusively happening in areas of driven grouse shooting management. We don’t find the bodies of the bird, and we get no more data from the transmitter.

Coincidence? I think not.

As this year’s cohort of young raptors take to Scotland’s skies, fitted with satellite-transmitters, with the full support of some of our more progressive landowners, I’m afraid I can’t feel positive for their future. Of course, I hope they live long enough to breed and raise families themselves. But realistically, I know that a good many of them will “disappear” on grouse moors, never to be seen or heard from again.

Self-regulation by Scotland’s gamebird shooting industry has patently failed. If you want to give these young harriers, eagles and other birds of prey an improved chance of living long enough to set up their own nesting territory and fledge their own youngsters, you’ll join with RSPB Scotland in supporting this Scottish parliamentary petition, launched recently by the Scottish Raptor Study Group, and calling for licensing of gamebird shoots in Scotland.