Absence of evidence and lethal control of corvid birds

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By Jonathan Reynolds, GWCT Head of Predation Control Studies

Having caused the existing system of General Licences to be dismantled, Wild Justice have published the arguments they made to Defra for implementing a revised licensing system. Wild Justice claim to have won their legal challenge of the old system, but I’m not sure it’s that clear yet. So let’s leave aside the legality of the General Licences as they were and concentrate instead on the problem of evidence for General Licences as they will be.

Wild Justice insist that the licensing authority, which has been Natural England since Defra delegated this role about ten years ago, must have a strong evidence base for authorising the lethal control of birds through General Licences. They believe that in most cases this is missing: there is no evidence, species by species, to say that magpies or jays or jackdaws have ever caused the decline of a native prey species. They don’t object to the licensing of lethal methods where these are justified, but there must be a clear reason and a proven benefit of lethal control. If the ‘problem’ is local and specific, they argue that individual licences are more appropriate.

So, essentially, they want to go back to the beginning and redesign the system from scratch. Unfortunately, we don’t have that luxury. We rarely do in real life, and when we do, things rarely work properly first time. Generally, we have to adapt what went before.

The now-dismantled General Licence system was put in place in 1995 to allow reasonable continuity of widespread pest and predator control practices, while also respecting the principles of the EU Birds Directive. It was a pragmatic solution that could be refined later, as and when better understanding came along. That’s essentially the dilemma: must we stop until evidence provides the qualification for starting, or do we carry on until there’s a reason to change what we do?

But why doesn’t the necessary evidence already exist? Perhaps because lethal control is a controversial topic, and only the highest grade of evidence will be persuasive enough to win the argument. Back in the 1980s, the Game Conservancy Trust (as it was then) was constantly being told that there was no sound evidence that predator control had any effect on population growth. Critics argued that it just created higher autumn densities and thus allowed more shooting, which served the interests of hunters but not conservation.

Well, as every GCSE pupil knows, if you want unambiguous evidence, you must set up a controlled experiment. At the necessary scale (several square miles at least), that’s an expensive and rarely repeated exercise. So when Dick Potts and Steve Tapper set up the Salisbury Plain Experiment (1985-91) they wisely chose to control the numbers of a suite of common predator species (fox, stoat, weasel, brown rat, grey squirrel, carrion crow, magpie, rook, jackdaw), as allowed by law.

The ‘suite’ was very deliberate. If only one significant predator species is removed, more nests and chicks are left for the remaining predators to find. The eventual outcome (number of birds fledging or reaching adulthood) may not be as bad as if you had done nothing, but it won’t fully measure the true impact of the one predator species you removed.

And after all, it’s the combined impact that matters to the prey species. So the top quality evidence that does exist (Salisbury Plain, Otterburn) shows how controlling a suite of predators benefits native wild gamebirds and waders. And it’s not surprising that we don’t have one of these massive experiments for every separate predator species and every prey species.

That’s a problem with knowledge. The better you know something, the less widely it applies. You put a heap of research effort into one set of circumstances, but it gives a poor basis for generalising. So high-grade experimental evidence should be accompanied by work that’s more widely representative (e.g. of different places, or gamekeepers, or years, or prey species) but can’t be done so thoroughly. Thus, we know from experiments that a predator control package including corvid control can turn local decline into local increase for both gamebirds and other ground-nesting birds.

We know from survey work on a broader scale that all these birds are more abundant on upland estates with predator control, amounting to a nationally significant effect. We also know there are circumstances where lethal predator control is less effective. This can be because culled predators are rapidly replaced through immigration, or because culling effort was inadequate, or because there wasn’t a predation problem in the first place.

To satisfy Wild Justice, there must be clear evidence that a particular predator species has caused the decline of a particular prey species. I think we can say, with confidence, that’s never going to happen, at least not with unarguable, high-grade evidence. By definition, a decline is only evident with hindsight, and you can’t do experiments with history. The best we can hope for are suggestive correlations, perhaps boosted with intensive field studies of declining species and computer models simulating alternative histories. That would get us to roughly where we were with grey partridges in 1985 after decades of work by the Game Conservancy Trust and its forerunners.

But in reality, there are always multiple factors involved in a species decline, and you can’t say that any one of them is the cause. Predation may often contribute to prey declines but is unlikely ever to be the sole cause. What we do know is that predator control is often transformative as a remedial measure. There are many examples where habitats have been lovingly restored but the response of the target species is underwhelming.

You worry that some unconsidered element of the habitat is missing. But add effective predator control and suddenly productivity booms. So clearly, at that late stage of decline (at least), predation is holding things back, and predator control is a way of getting them going again. If the motivation for doing that is hunting, so be it: that helps deliver conservation results across large swathes of land that are working landscapes rather than nature reserves.

I have spent my career as a scientist teasing out the complex interactions of man, predator and prey. I know how hard-won any evidence is, and how good it must be to carry the day. Wild Justice know this too. They are exactly the people who will test evidence to its limits before accepting it. Their suggestions of the starting evidence needed for General Licences sound philosophically principled, but they are flying a kite way up into the blue sky of unrealism. I hope Defra realises this.

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General licence revocation

at 23:25 on 21/05/2019 by Tony Rea

It is already evident to me that there has been a big seasonal increase in magpie numbers where I live in Essex. In my opinion, Mr Packham and crew have done untold damage to this season’s songbirds and lapwings, not to mention all the other affected species. His autism seems to be an excuse for his lack of judgement on this conservation catastrophe and he gets a CBE for his efforts on conservation! It’s a shame that his denial of an anti-shooting strategy is so obviously a lie but why does he not understand that corvid control is not an option but a necessity?

Wild Justice Foul Play

at 22:30 on 21/05/2019 by Ted Williams

Sir, surely Natural England should show some spine and demand evidence from Wild Justice or it’s cohort of hypocrites to explain why the existing arrangement should change. When have the huge resources of RSPB and other propaganda groups opposed to practical, deliverable result based control of pests ever been directed to produce such evidence instead of piggybacking on GWCT science, instead of criticism of successful practices?

Wild Justice

at 19:41 on 21/05/2019 by Paul White

Our dilemma in the shooting/conservation world is that no matter how much scientific and practical evidence is put forward for what we do, Wild Justice will never except it. Why? Because they are first and foremost anti- shooting or anti anything that involves the death of a bird or mammal. Chris Packham has even suggested that rats be allowed to breed....it is humans that cause the rat problems he says. He may have a point (witness waste food discarded everywhere), but he will not accept that they must be eradicated. Everything that is wrong with the environment he puts down to we humans and in particular those that shoot/ hunt or fish. Wild Justice are clever in their modus operandi, they use the media, especially Social media to get their arguments across. We need the same media savvy people to put our much better case across. Until that happens we are losing the battle.

general license

at 16:04 on 21/05/2019 by mark hirst

Jonathan What you say is perfect sense and in many ways you are preaching to the converts. But there is one point I should like to make, you say"to satisfy Wild Justice"? No amount of evidence will ever "satisfy" Wild Justice you would be preaching to the deaf and blind. It's the courts and general public we need to satisfy, with out them the likes of Mr Packham would have no audience.

There is evidence that the current level of shooting is NOT reducing numbers

at 13:47 on 21/05/2019 by Tom Charnock

The RSPB have an annual Birdwatch survey carried out in over 1/2-Million homes in UK. Trends here show either a high frequency or presence of Gen-Licence birds in gardens, or trending increase in their overall numbers. These surveys have been going for for some 20+years. A review of the frequency of Gen-Licence birds visiting gardens shows upward trends. The implication being there is more of them Also the BTO do reviews analysed by area, bird type and show long term trends, see https://www.bto.org/our-science/projects/bbs/latest-results/trend-graphs The BTO also produce population trend graphs, covering 10-years, by bird, see https://www.bto.org/our-science/projects/bbs/latest-results/population-trends I have seen in past Gamewise Annual report, studies that show the removal of corvids has improved the hatching rates of terns, waders etc. It is not beyond belief that the limited studies on corvid removal, that show improvements to certain birds as studied, would also apply to the myriad of other birds here in the UK. It is not unreasonable that corvids eating eggs, hatchlings, of one studied species would adopt similar feeding habits for any other eggs, or young. There is a pleasing element to using the data from RSPB to support the reinstatement of the Gen-Licence, and the BTO data on woodpigeon does prove there are increasing numbers in all regions of the UK, despite the millions being shot each year. Surely someone knows what a W-pigeon eats per-day, per-year. That times the millions in the UK gives an idea of what they consume, with perhaps 50% coming off farmland food crops. I recall a similar study-estimate being made in 2001 (Mammal Society -Look What the Cat Brought In) for the quantity of birds and small mammals being eaten by UK cats per year - 275-Million. With 55-Million being small birds

General licences

at 12:19 on 21/05/2019 by Dick Bartlett

From Jonathan's excellent explanations it seems that we are faced with providing an impossibly high level of evidence to get back the General Licences. If this depends on legal detail then surely we can deploy the concept of legal precedents which carry a lot of weight in UK law. These precedents cover many years and many predator/prey interactions and are widely accepted as valid. To overturn this weight of circumstantial AND scientific evidence Wild Justice must show examples of where their favoured methods have been successful which, of course, they can't. Our opponents will continue to attack us if we are seen to be always weak, fragmented and defensive. We need our supporting organisations to fight back by exposing the evidence of very poor biodiversity on "reserves" which don't have predator control and where endangered species are affected as well as gamebirds. We should not have to go to Judicial Review to get Buzzard control licences and we should not have a ban on heather burning on England's peatland in defiance of scientific evidence. There are plenty of other dreamed-up obstacles heading for us if we don't take a firm stand now.

Predator control

at 12:13 on 21/05/2019 by Lord Middleton

Whatever the science or research it is a well known fact that there is egg and chick predation by corvids and crop damage by corvids and pigeon. This can ONLY lead to fewer song birds etc and poorer crops. Unfortunately there are other winged predators that we were and are not allowed to control that are having a serious effect on the brown hare and plover populations. Nobody wishes to wipe out a species BUT lack of 'good gamekeeping' predator control must play the most important part in many species' decline. As I watched 2 carrion crows this morning I felt very downhearted.

Absence of evidence and lethal control of corvid birds

at 12:07 on 21/05/2019 by edward gallia

Dear Jonathan, Your blog demonstrates exactly my concern with the precautionary principle and how it can be applied to cause absolute inertia, especially when combined with scientists' desire for a degree of certainty which is only ever forthcoming after many years. I have said this for many years, and suffered many verbal bruises as a consequence. Regards edward

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