Today sees the publication of two interesting and important studies of the impacts of predator species on their prey's populations.  I'll discuss them both here in this blog.

This post is about the analysis carried out by the BTO (with the University of St Andrews and the Game and Wildlife Conservation Trust) on the huge datasets of the Common Birds Census and the Breeding Bird Survey.

Magpie - not the cause of farmland bird declinesThe study looked to see whether predators such as sparrowhawk, magpie, kestrel and buzzard have any discernible impacts on the populations of potential prey such as songbirds.  If this study is the jury deliberating on the question of whether predators are guilty for songbird declines then the verdict is not guilty!

This doesn’t come as a great surprise – it repeats previous findings and is biologically realistic.

Is it quite as clear as ‘Not guilty’?  Almost, but not quite. There might be three reasons for treating the findings with some caution: bad dataset, bad analysis and weak approach.  Let me explain.

Is the dataset a good dataset?  The dataset can’t be bettered in the UK or Europe, and hardly in the world.  It is a long term set of data stretching back for decades with good geographic coverage, good species coverage, good standardisation of methods, high sample size etc etc etc.  No dataset is perfect, but there is hardly a better dataset on long term national bird trends than this.

Is it a good piece of analysis?  It pains me to say that I can’t really comment on this because, despite being a scientist by training, things have moved on sufficiently that I struggle fully to understand the details of this analysis!  However, the authors are very clever and this paper is published in a high quality journal and has been peer-reviewed so we can trust that there are no big errors in it.

Is the approach strong?  By this I mean if there really were an impact of predators on songbird populations would this dataset and analysis pick it up? The answer is yes but since it is a study of correlations it is not as strong an approach as an experiment.  But few would want to start slaughtering predators on a massive and widespread scale to investigate this question more rigorously – there are some though!

So it’s a good study, which will allow a bit of quibbling to continue, but which absolutely knocks on the head the idea, proposed by some, that predators are the main reason for the decline in songbird populations.

Let me try to summarise a complicated story.  To do this I am going to ignore the results from the analysis of the Breeding Bird Survey and from including the great spotted woodpecker as a predator.  I do this to make the story simpler but it doesn’t mean I have missed out the juicy or inconvenient bits – the paper is published, you can go and see for yourselves!

There are actually more positive correlations than negative ones – which suggests that the more prey there are, the more of their predators too, which would make a lot of sense.  Magpies, jays, carrion crows and buzzards look pretty innocent of any impacts on songbird populations.  If one were to imagine any predator having an impact at all from these results then it would be kestrel and sparrowhawk. Let’s look at sparrowhawk in more detail.

Sparrowhawk - not the cause of farmland bird declinesOf the 29 prey species, there is some evidence of sparrowhawks having a negative impact on the populations of 18 of them.  However, of the 29 species, the results for only seven prey species are statistically significantly correlated with sparrowhawk numbers – some positively and some negatively.  The seven prey species are:  lapwing, yellow wagtail, robin, nuthatch, tree sparrow, bullfinch and reed bunting.  The two most statistically significant results are that sparrowhawks depress tree sparrow numbers and increase yellow wagtail numbers.  I find it easier to believe the former than I do the latter – but who knows?  And although I can imagine that sparrowhawks might depress both reed bunting and bullfinch numbers I find it more difficult to believe that they increase robin numbers or decrease nuthatch numbers.  And a whole host of regular sparrowhawk prey species (blue tit, great tit, blackbird, song thrush, starling and house sparrow) are unaffected, it seems, by sparrowhawk numbers.

This analysis could have come out very differently – it could have shown clear across-the-board negative correlations between predators and their prey populations.  Or it could have shown just a small number of clear negative correlations for some predators and some prey which were biologically realistic.  Neither happened.  So let’s stop fingering predators as the cause of widespread songbird declines and look to the real causes – unsustainable agriculture, pollution, climate change and the overall pressure of our species on the natural world.

But let’s also wonder what we would have done if this analysis had shown that predators reduce the populations of their prey.  I think the answer is not much!  If that’s what happens then let’s get the habitats right – as we have at Hope Farm – so that predators and their prey can coexist and let’s have many more of both!

  • From my experience (limited to mostly conventional, southern lowland, medium-large arable, dairy and livestock farms) there is a bit too much ‘woe is me’ and stubborn attitude in farming (and The Farmers Weekly often is a frustrating read!)  Those in agriculture have done well to adapt and keep up with the times but it’s not just farming that has changed, it’s the whole of the UK – that’s life.  I’m not suggesting that we should go back to a nostalgic olde time system of farming (the RSPB and GWCT seem to have good example of productive modern conventional farms that also benefit wildlife).  Agriculture is a business and a science with a lot at stake (high price of land, livestock, fertilisers/pesticides and having to investing in hundreds of thousands of pounds into machinery and infrastructure) it needs to attract younger, educated people with a business/science head and the ability to adapt.  Currently the average age of a British farmer is what?  Late 50s, early 60s?  I know I was the youngest by at least 30 years when I went to a cattle sale last year!  No wonder farmers kick back at the system – what with the ever changing goals posts of various governments and the negative stance on farming often seen in the press (something every group has to put up with though I’m afraid).

    Everyone needs to recognise that the environment is important for the long term health of agriculture and our food supply.  For example, some crops benefit from or require wild insect pollination but we wage a war against them all with pesticides without realising not all of them are ‘bad’.  Lots of animals above them in the food chain suffer as a result (such as our lovely native gamebird the grey partridge).  Soil with a good population of earthworms will have better drainage and improved nutrient content…and let’s not forget all the useful soil microbes too.  

    It’s not all about the birds but they are a good indicator of the health of our rural environment.  It’s why we all need to work together…but a bit of healthy debate can be good too.

    I’m not an expert either - just an interested, rambling young whippersnapper!

  • Hi Harebelly - interesting life you have! I agree entirely, some people are far too ready to blame this predator or that, sometimes to cover up their own inability to get up in the mornings! I don't think we've seen a decline across the board, more a shift in the balance of populations in recent years. And I for one don't think it's all about sparrowhawks and magpies (don't tell my mum I said that - she hates them both!). I agree with you, it's more to do with the way we've changed the countryside. Where are all the little family dairy farms, for instance? All gone to the wall. How many sparrows, starlings etc lived on those? Farming gets more industrialised every day as small farmers drown under a sea of forms and regulations. It would be a miracle if the balance of species didn't shift. To get to the root of the problem, we'd have to reinvent British agriculture. It couldn't be done, and we wouldn't like the result anyway. Nostalgia isn't what it used to be.

  • Hi James,

    As a farmer’s daughter, shoot supporter and former tern warden (a ruddy hard job!) I agree that on a local level predators can affect populations of say, rare bird species but predator control for economic and conservation reasons should be a last resort and preferably be backed by science to make sure we are not causing long term negative effects.  In the farming and field sport communities we need to change unfounded or exaggerated perceptions about predators and I welcome this report.  Pointing the finger at predators detracts from some of the bigger issues we need to address: urbanisation, habitat loss/change/degradation, intensification of agriculture etc.  Do sparrowhawks, crows and foxes prey on rare birds like grey partridge and little tern? Undoubtedly and measured predator control may be necessary in Britain’s heavily managed, semi-natural environment, but it wasn’t predators that caused and continue to cause their dramatic decline in recent decades!  We need to go to the root of the problem.

  • A specialist predator wouldn't last long if it wiped out its prey. There's a natural brake on it; as prey is thinned out it becomes harder to find, and each meal requires more energy to be expended. That doesn't mean the same thing as 'natural balance' - predator/prey populations can go through cycles of boom and bust and are vulnerable to other factors at the low point of the cycle.

    Generalists like foxes and magpies are another thing entirely. They can happily wipe out one source of food and move on to another. At a local level, this can be unacceptable (to us humans, for one reason or another) as wardens looking after breeding terns, for instance, can attest to.

  • Yes Mark I have to go along with the results but emotionally still see Magpies and Crows as killing anything they can as opposed to a Hawk just taking weak or sick bird if it can catch it.The only thing I disagree with in your blog is where you say unsustainable agriculture as for sure our whole way of life as it is now is unsustainable,perhaps more so than agriculture.For sure it would be a massive improvement for wildlife if you could get the message of Hope Farm across to farmers and get them to contribute,that is probably a big job in itself but the rewards over a large area would be huge.