Category Archives: Population studies

At the end of May this year the BBC screened an episode of Countryfile in which John Craven interviewed a member of the scientific staff at the Game and Wildlife Conservation Trust (GWCT). Taking things literally I’d assumed the this was a trust who’s aim was the conservation of game and wildlife. However, as the interview progressed I began to question this assumption as the discussion centred around a study to be carried out by the GWCT in which magpies were to be eradicated from a specified test area to assess the local effect on songbird populations. I don’t think that removal of a native species such as the magpie by trapping and killing them is an ethical way to study predator/prey relationships. But is it scientific?

The magpie – villain of the piece – apparently

The interviewee from the GWCT, Dr Jeremy Stoate, justified the cull on scientific grounds suggesting that increased predator numbers result in significant reductions of prey populations and the species they identified as being largely responsible was the magpie.

Magpies are bold, brassy, ubiquitous, and are generally percieved as being a tad raffish. They undoubtedly predate songbird nests, but I struggled to understand how this could justify slaughtering them. Numerous other species of bird and mammal such as sparrowhawk, kestrel, owls, great spotted woodpeckers, rats, stoats and probably the biggest killer of small birds, domestic cats, all predate songbirds and some raid nests to poach eggs and chicks.

So why was it only magpies that were to be exterminated?

The action was being justified on a scientific basis, and whilst I’m no ecologist, I am a scientist, and the whole thing seemed unscientific, so I decided to do a little research of my own on the subject. Interestingly, since I did my initial research in June, it appears that references to the study on the GWCT website have been taken down. At least, I couldn’t find any.

However, what I did find out was that the study was commissioned and financed by another ‘conservation’ organisation called Songbird Survival. Whilst I believe that GWCT do have a genuine interest in the countryside as a whole, the same cannot be said for Songbird Survival. It is a registered UK charity whose statement of activities on UK Charity Commission website is this:

THE PRINCIPAL OBJECTIVE OF THE CHARITY IS TO IMPROVE, PROTECT AND PRESERVE FOR THE BENEFIT OF THE PUBLIC THE POPULATION OF SONG AND OTHER SMALL BIRDS BY THE CONSERVATION AND RECREATION OF HABITATS, BY SUITABLE EDUCATION OF THE PUBLIC AND LOBBYING MP’S AND THE HOUSE OF LORDS, TO EFFECT CHANGES IN THE LAW WHERE WE FEEL IT IS NECESSARY.

However, the details on the Charity Commission website also list the trustees, and that makes interesting reading too…

LORD COKE, MR CLIVE PATRICK SHERWOOD, MR FRED VALENTINE INGRAMS, MR JOHN RICHARD PUGH, LORD MICHAEL RICHARD RANKEILLOUR, MR ROBERT JOHN MIDDLEDITCH, MR DAVID GRIFFITH, MR NICK FORDE, LORD JOHN HADDINGTON, MR HUGH VAN CUTSEM, MR COLIN STRANG STEEL

All of these trustees are big landowners, or relatives of landowners, some with shooting estates, and they all support hunting and shooting. Lord Coke is the owner of Holkham Hall in Norfolk and he and several of his gamekeepers have been prosecuted for killing birds of prey by shooting and poisoning. So I’m struggling to believe that these people really have the best interests of songbirds at heart when they are funding ‘scientific’ destruction of a single predator species which has been implicated in taking gamebirds. And via the titled gentlemen listed as trustees they would be ideally placed to carry out their stated aim of ‘lobbying MP’s and the House of Lords‘.

Back to the science though. I recently attended a lecture in which Dr Mike Toms of the British Trust for Ornithology (BTO) talked about garden birds and how gardens are becoming an increasingly important sanctuary for increasing numbers of songbirds, and therefore predators, such as magpies, great spotted woodpeckers and sparrowhawks. In the course of his lecture Dr Toms talked about the decrease in songbird numbers and how magpies were being blamed but said that magpie predation was not the cause of the decline in numbers but habitat destruction and current agricultural practices were the primary reasons. I emailed Dr Toms and he was kind enough to send me several links to published work by the BTO into bird predation. This list included this publication: ‘Population changes of avian predators and grey squirrels in England: is there evidence for an impact on avian prey populations? Journal of Applied Ecology 47: 244- 252‘ describing a collaborative study between the BTO, GWCT and the University of St Andrews in which the impact of 2 groups of predators, those which predate adult birds and fledglings and those which predate nests (including the magpie) were analysed for 29 prey populations. Quoting directly from this paper:

‘For 22 avian prey species, there is no evidence that increases in common avian predators and grey squirrels are associated with large-scale depression of prey abundance or population declines‘.

Then:

‘Unexpected was a large number of positive (my italics) associations between predators and prey, particularly for native avian nest predators, which largely exonerates these predators as driving declines in passerine numbers‘

And:

‘Analyses of large-scale and extensive national monitoring data provide little underlying evidence for large-scale impacts of widespread avian predators and grey squirrels on avian prey populations…‘

The paper also describes negative correlations between the remaining seven prey species and predator numbers, and accepts that some of these may be causally related. Although e.g. the relationship between sparrowhawk and tree sparrow is probably significant, that between buzzard and goldfinch almost certainly isn’t. However, in an email from Dr Toms to me he indicates that the GWCT study showed negative causal relations on  a local level but, crucially, not at a population level.

This last point is key to the argument. In a study of population dynamics, if conclusions are to be drawn about overall depletion of numbers in response to a specific influence small local studies cannot be extrapolated to enable conclusions for the population as a whole, and to do so is erroneous at best, misleading at worst.

Incidentally, I also found a publication on the website of Songbird Survival entitled ‘Detecting an impact of predation on bird populations depends on the methods used to assess the predators’, (Methods in Ecology and Evolution (2010),1,300-310, Nicoll M., Norris K.). This is an interesting paper because rather than analysing actual population data it investigates the methods used to analyse the data. And in the summary the last point recommends: ‘…the findings for studies which use opportunistic data, for a limited number of predator species, should be treated with caution and that future studies employ bespoke census techniques to monitor predator abundance for an appropriate suite of predators.’

This is interesting because it argues that the methods used to analyse opportunistic, observational data must be carefully optimised to ensure that conclusions drawn from the data adequately reflect the observations.

None of the publications suggest that slaughtering the predators is a rational scientific way to assess the negative impact on prey species. And I’m inclined to think that culling is poor science, particularly as the latest evidence suggests predator numbers don’t reflect negatively on songbird populations and in some instances there is actually a positive correlation between predator and prey, presumably because the predators remove the sick and the slow before they can add their DNA to the genepool.

I think the notion that predators destroy prey populations is bizarre because I was taught, and importantly, I believe it to be the case, that prey abundance determines predator numbers, not vice versa. If prey numbers decline then shortly afterwards so do predator numbers. Taking the reverse of this theory to it’s logical conclusion, if predator numbers determine prey numbers, all prey species would eventually become extinct and then the predators would also become extinct and the balance of nature which has prevailed for the last few billion years would be turned on its head. Which is absurd.

So population studies which are centred around the slaughter of predators seem to me to fly in the face of all current theory and are therefore morally indefensible aswell as being scientifically unsound, especially when commissioned and financed by such obvious vested interests as Songbird Survival.