Murderous – but not so bad after all?

Carrion Crow. By AnemoneProjectors (Flickr: Carrion Crow (Corvus corone)) [CC BY-SA 2.0 (http://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons
Carrion Crow. By AnemoneProjectors (Flickr: Carrion Crow (Corvus corone)) [CC BY-SA 2.0 (http://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons
A new scientific study has shown that crows, including Magpies and Ravens, do not have as big an impact on bird populations as has previously been thought.  This has implications for the legality of some aspects of gamekeeping in the UK.

It is one of the most obvious things in the world to imagine that if a predator eats lots of a particular species, then it must have an impact on the population level of that species. This can happen, but, for example, there is little evidence that predators like Sparrowhawks have much impact on their common prey such as Blue Tits and Great Tits.  If lots of tits weren’t eaten by Sparrowhawks they would simply die of starvation later in the winter. And if they didn’t die in the winter than we would soon be drowning in a sea of tits!

Predation certainly can drive populations lower, eg introduced species or high levels of illegal persecution on raptor populations, but it doesn’t always happen like that.  It depends on the biology of the prey species and the level of predation. It’s far more complicated than ‘they kill them, so they must reduce the population levels’. If that weren’t so, then all hunting would, of course, be unsustainable and should be banned.

But we have all thought that generalist predators such as Red Foxes and Carrion Crows can reduce the population levels of their prey under certain circumstances.

This new study suggests those impacts of corvids are rather smaller than has been thought by the average biologist (and miles less than you would be told by the average gamekeeper).

The study found that Carrion Crows – along with Magpies and Ravens – have surprisingly little impact on the abundance of other bird species.  The authors state that these birds are in fact being menaced by mankind in the mistaken belief that removing them is good for conservation.

Ravens Photo: Tim Melling
Ravens Photo: Tim Melling

Of course corvids take eggs and chicks, the question is, do they take enough to affect bird population levels.

The study was led by researchers at the University of Cape Town and published this week in the leading ornithological journal Ibis. It found that in the vast majority of cases (82%), corvids had no impact at all on their potential prey species.

Many nature lovers have been distressed to witness a crow or magpie raiding the nests of their beloved garden songbirds, stealing their eggs or eating their defenceless chicks,” said study co-author Dr Arjun Amar from the Percy FitzPatrick Institute for Ornithology, “Although this predation is entirely natural, these observations can be upsetting to witness and often leave people wondering if these predators might be reducing bird numbers.

However, our global review suggests that we should be cautious before jumping to conclusions over the impacts these species may have. Just because a predator eats something occasionally does not always mean that they have an impact,” Dr Amar said.

The study reviewed all published evidence on whether predation by corvids actually reduces the overall breeding performance of birds or, more importantly from a conservation perspective, reduces their numbers. Data were collated from 42 studies of corvid predation conducted across the globe over the last sixty years.

Not only were corvids unlikely to have any impact on their potential prey species, if there was an impact it most often affected the breeding success of the prey species rather than their subsequent numbers. Half of cases found that corvids reduced breeding success whereas less than 10% of cases found that they reduced prey numbers in the long term.

The review analysed the impact of six corvid species on a variety of prey species including gamebirds, songbirds, waders, herons, cranes, sea birds, waterfowl and raptors.  The 42 studies incorporated into the review included 326 cases of corvid –  bird prey interaction Most of the data stemmed from field research in the UK, France and the United States.  The impacts were determined partly by comparing bird counts before and after corvids were either removed or their numbers reduced.

The full paper is freely available via open access.

The lead author, Chris Madden said she hoped that the review would challenge the perception that all corvids were bad, thereby preventing needless killing: “Our results suggest that this is a mistaken belief and that generally speaking people would be wasting their time killing corvids to increase bird numbers.  Overall therefore, our study points to the fact that we are often too quick to jump to the conclusion that crows and magpies may be the cause of bird population declines,” she said.

In the UK, most killing of corvids is done by gamekeepers under the ‘General Licence’. The General Licence allows people to kill a variety of birds which some, but not I, would call ‘vermin’.  However, it is not legal, as I understand it (and maybe I am wrong) to kill things simply to increase the number of gamebirds available at the end of the breeding season to enter the shooting season.

The General Licence allows you and me to kill Carrion Crows, for example:

 

  • to prevent damage to agriculture, livestock, fisheries, property, or archaeology
  • to protect public health and safety (including air safety) (see the quite awful www.GOV.uk site)

Rather better than the government site is the RSPB site (click here). You see there is no mention of allowing killing for game preservation. Gamebirds are not livestock (nor property) – some of them are ‘wild’ birds and released gamebirds would have to be cossetted rather more than they are if classed as livestock. Your average gamekeeper isn’t killing crows etc to protect you on your flight to Malaga, nor to protect agriculture, nor fisheries, nor to stop the spread of disease.  The only ‘excuse’ for killing crows would be, and maybe it should be tested in law, to ‘conserve wild birds’.

This paper suggests that it is a rather thinner excuse than before this study was published.

Now, I am not an expert on the law, even wildlife law, but this is how I have always understood things. If I’m wrong then please correct me.

As far as the biology is concerned, would we want all gamekeeping of corvids to be eliminated in the countryside?  That’s a question for you, but my answer would be ‘Maybe not’. It might be though, that in any licensing of shooting estates we should expect them to send in returns on how many crows etc they have killed, for what reason and by what means. And, perhaps, if this paper is right, to exercise rather more restraint in the numbers of corvids killed.

 

Magpie Photo: Tim Melling
Magpie Photo: Tim Melling

 

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28 Replies to “Murderous – but not so bad after all?”

  1. As I have suggested before the other impact of corvides is the mobbing of birds of prey preventing successful predation of prey. I can watch that out of my window and when numbers of non breeding crows gather and attack even kestrels the chances of success fall considerably. Even had crows mobbing a fox on an RSPB reserve recently. So it is not just birds of prey! Keep up the good work Charlie!!!

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  2. Two points: (1) I like corvids for their cleverness in tool use etc. (avian chimps is one description) so I don't think we should be wantonly killing such intelligent birds. (2) People who rear pheasants seem able to adopt Humpty Dumpty's use of English ("when I use a word it means what I choose it to mean") as pheasants can be either livestock or wild birds depending on which is more advantageous at the time, see George Mobiot: http://www.monbiot.com/2014/04/28/the-shooting-party/.

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  3. The definition of ‘livestock’ is a moot point. Legal advice taken by the UK Raptor Working Group (JNCC 2000) suggested that game birds within release pens, or otherwise reliant on man for food, could be classed as ‘livestock’ in the context of the Wildlife & Countryside Act 1981. So that would probably include pheasants but exclude red grouse. However, I’m not aware that any of this has ever been tested in a court.

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  4. I can't see gamekeepers changing, as they enjoy killing things too much and don't do science, but hopefully some of the conservation organisations will take note of this paper and stop culling corvids (and foxes too) on their reserves.

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  5. They do seem to publish proportional (not absolute) numbers of corvids killed annually at:

    http://www.gwct.org.uk/research/long-term-monitoring/national-gamebag-census/bird-bags-summary-trends/carrion-hooded-crow/

    http://www.gwct.org.uk/research/long-term-monitoring/national-gamebag-census/bird-bags-summary-trends/magpie/

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  6. One thing that fascinates me about all this is just how much time, effort and money over the years has gone into killing wildlife with absolutely no impact whatsoever on the supposed protected species (usually Pheasants). I stopped squirrel shooting in the New Forest: FC keepers were shooting 5,000 grey squirrels at a booked cost of £70,000. Yes, Greys are an introduced pest - but what was the population of the New Forest - 50,000 ? and how fast could breeding replace that 5,000 ? If management had sent the Keepers to bale the Solent with thimbles there'd have been an outcry - but shooting those squirrels was having pretty much the same impact on the overall population - the Keepers time was rather better spent on activities where there could be a beneficial impact on the environment, like deer control where shooting quite clearly was keeping the population (and therefore grazing impact) lower.

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    1. It's similar to the effect of trying to control potato late blight by eliminating cull piles. A diminishing returns activity, which only delays the onset of an epidemic, not necessarily the extent, provided that the other components of the disease triangle are present. At least, I think that's what J. E. van der Plank was trying to say. But - like a Brief History of Time, I never actually finished reading the book. Too many sums and that ...

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  7. Rooks mob herons too. When I lived in West Huntspill my bedroom window was about 20m from a holm oak marked on OS maps as a heronry. Rooks nested all around in tall trees, but not on the holm, which had been pruned to a flat-topped shape by many years of scorching by heron guano. When the herons returned from the rhynes in the evening the rooks would mob them and occasionally drive them away. However, I was very impressed by the aerobatic ability of the herons. Little egrets also roosted and bred successfully on the holm despite also suffering mobbing by rooks. At a guess the main cause of chick mortality was falling out of the tree. The young of both herons and egrets kept up a day-long racket of melodious song - similar to the sound of retching in a drain-pipe - before they fledged and offed to the Levels to decimate the GCN population.

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    1. "Death knell for the Larsen trap?"

      A tad optimistic perhaps to expect scientific analysis to overcome the ingrained "common sense" view of (too) many that corvids are villainous killers and 'maintaining the natural balance' requires blasting them out of the sky at every opportunity.

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      1. Common sense to me is that corvids also take out sick or diseased birds helping to reduce spread of some of the diseases that kill wild birds. They're like the UKs vultures. Killing the corvids I feel actually increases the loss of other wild birds as they have no predators to take the sick to stop the spread of disease.

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  8. This great news has put me in such a good mood. I'm a big fan of corvids and happily welcome any information that dispels negativity towards them. It's also another big blow to gamekeepers, yet more evidence against their unsound and dated traditions. They must wake up each morning worried about what's going to hit them next!!

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  9. The simple fact is both predator and prey species have evolved to co-exist over millions of years. The misguided perception that some species are a pest or have detrimental effects on the population levels of others is purely human and often so deeply ingrained in the psyche of gamekeepers and farmers that any kind of scientific evidence to the contrary will be ignored in the favour of unsubstantiated tales handed down through the generations.

    Our wildlife is under threat like never before. We need to reverse this trend while we still have some left that isn't a species intensively reared to be blasted from the sky by a select few.

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    1. I couldn't agree more. I wish the term 'pest' were not used - and especially 'vermin'. They are disrespectful. All creatures have their place in the environment and we humans, damaging as we are, should learn humility and accept that we are but one part of the whole.

      End of sermon!

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  10. I certainly have a soft spot for corvids.
    Jays especially.
    Here are some of my jay photos (sorry I don't know how to do links on your blog) when I started "training" jays when my wife and I lived in Reading a few years ago.
    https://www.flickr.com/search/?w=10226699@N00&q=jay
    Took a year or so, but eventually they even came to my whistle, and fed from our kitchen windowsill, where I could prop up a clothes peg, thus obtaining my desired "J-Peg" of a "jay peg"....
    https://www.flickr.com/photos/the_black_rabbit/3260230392/in/photolist-7fP9oG-5zeMHv-5Edp68-5Y6wUQ-5y4Zy8-5JRVQE-5y8uPG-787fR8-5Y2fQP-5EGX3w-5ECR7R-5Y6wD1-5ECDwz-5y47PP-5ZDZ5P-7EjTE8-7863Aa-5BeV7H-5znfZG-59Ckvq-5yxXfR-5LJzcL-5ZWemW-5xtbzo-5JSX97-8g2Pps-5CBMhp-8oAhLL-5wymxo-7soPAa-62ccnj-6gonmz-7hrsDk-92VRbP-62FL2J-5WDVbJ-93UkQY-66NsHb-5ZDyT6-5zteQt-63Kyz2-6vE1j1-5Pxwuz-6c4r1q-6yFjBQ-6Ug7xT-8dD7MR-7npacg-7EoKhm-5BQq2e/
    I think they're quite stunning birds to look at - and great fun to feed also.
    This year's "operation glandarius" in the back garden has resulted in 5 jays feeding from my squirl-proof jay feeder each day - they're such fun to watch, so clever and make a beautifully-wide array of calls to each other, many very soft and barely audible to an onlooker if that onlooker is a distance away.
    Can I please take this opportunity to ask readers of your blog to take a little time to feed the jays this winter - especially this winter as the acorn crop this year has been dire. It is very rewarding to do so, and set the jays puzzles to solve - it doesn't often take them too long!

    I am getting 'round to the idea of "liking "magpies too.
    Bully boys for sure, but anywhere else in the world and their plumage alone would make them like birds of paradise - absolutely stunning birds.

    Then there are the carrion crows... impressive birds generally apart from the individuals that CAW like old broken telephones - they seem to be getting a little more common these days, the telephone crows! They do make me laugh.
    I've yet to hear a telephone "hoody" in Scotland though. Maybe its an English, Carrion crow thing?

    I'll never forget the first time I saw my first chough, nor my first raven - both on the rugged Pembrokeshire coasts in howling gales.

    Finally the "humble" jackdaws and rooks of course - at this time of year, it's one of my few winter pleasures, watching flocks of these big black birds fly noisily overhead at dusk and dawn.

    I'm hardly surprised they have less effect (than some thought) on long term populations of other bird spp. Short term local effects maybe, but no... this paper doesn't come as a suprise.

    Crows are super birds that's for sure.
    Beautiful and so clever. You can almost SEE them think.

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  11. Grouse: the general licence (in Scotland) is being applied erroneously. The only possible 'purpose' for culling corvids is for the conservation of wild birds, however the 'purpose' that the licence is currently employed for is the production of a shootable surplus. (Rules historically created and implemented by shooting interests?)

    Contrast with the SNH guidance for a specific licence for killing specific predator species to conserve wild birds at
    http://www.snh.gov.uk/protecting-scotlands-nature/species-licensing/bird-licensing/wild-birds/
    which states:
    "Therefore, in order to issue such a licence, SNH would need to be satisfied that a cause and effect link exists between, for instance, predation and significant declines in the population or distribution of other species."

    Which I believe to mean that unless you can show that your corvids are without doubt creating gaps in the distribution of grouse on a moor by reducing its population, then you can't have licence to control them.

    So for specific licences for say ravens cause and effect needs to be proved, however for other covids under the general licence, crows can be killed, well, because "that's what we've always done".

    Then of course there's the research (Calladine et al 2014) showing that moorland management support delivers little in terms of species conservation against broader forces - however 2 species do benefit in terms of numbers from predator control; stonechat and carrion crow (the latter being culled!).

    http://dx.doi.org/10.1080/00063657.2013.876615
    Its all rather Kafkaesque.

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  12. The speculation when I was involved in upland management was that changes to how sheep were kept on the hills longer and later meant more carrion, which supported higher levels of foxes and crows. Hence some conservation problems, especially for nesting waders already suffering fragmentation of breeding areas. I was also aware of a similar link between pig rearing, crows feeding on their feedstock and stone curlew.
    Presumably the conservation issues would still need to be addressed on a local basis, especially given the relatively small areas under direct control of RSPB and the like?

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    1. And conversely I heard of a farmer who began having problems with itinerant ravens only after the sporting tenant started maintaining gamebirds over the winter.

      Its not simple is it. Don't want to go too far off subject, but anyone know what the relationship is between fox control and pine marten abundance?

      Is there a too simplistic approach to predator prey relationships in countryside management?

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      1. There is a 'simplistic approach' to predator control. This is because of the massive complexity of predator/prey relationships and our current lack of knowledge thereof.

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      2. Bimbling, there is a study from Scandinavia that showed an increase in pine marten numbers when the fox population was reduced by an outbreak of sarcoptic mange. This is often quoted as an example of mesopredator release and as evidence that foxes can reduce pine marten populations either through predation or competition.

        I have some anecdotal evidence of a similar effect from baited camera trapping that I have done in two forests about 5 miles apart. There is no predator control in either forest, but one is surrounded by land where there is also little predator control and foxes were the most common predator, appearing at baits approximately every 2-3 days. I only once (in about 3 months) recorded a pine marten in this forest. The other forest was surrounded by land that is heavily keepered. There were no foxes in this second forest, but I was recording pine martens at the bait approximately every 2-3 days. This is obviously not proof, but is consistent with some sort of exclusion effect.

        You can then take your pick as to whether it would be better for ground-nesting birds to have foxes or pine martens in the forest, but you are right that predator-prey relationships are not simple.

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  13. As just one of the species on this planet we need to recognise that we are part of the web of life. As we evolve so do other species and it is essential that we do not interfere with the development of nature. The human species has already created enough problems without the help of any other species, and we need to seriously consider any action that we take in our own lives that may impact any further.

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  14. Slightly of topic ~ but this is one of the best videos I have seen on interconnectedness for a long time:

    http://www.theguardian.com/environment/georgemonbiot/2014/dec/12/how-whale-poo-is-connected-to-climate-and-our-lives

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  15. It is beyond me how anyone could damage a corvid. They are intelegent and may manifest as the Buddhist protector Mahākāla, or the Hindu deity Shiva.

    http://en.wikipedia.org/wiki/Mahākāla

    http://www.khandro.net/animal_bird_raven_crow.htm

    Why harm something that could save your life?

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  16. Norfolk newspaper the Eastern Daily Press has published my letter on this subject. Let's see how "Crow Country" responds.

    Many's the time I've read on these pages of people's concern that some of our most beloved garden and farmland birds are being decimated by corvids such as crows and magpies. Anyone who has spent more than two minutes in the Norfolk countryside will be well aware that crows are pretty much top bird round here - seemingly able to thrive in any habitat and eat anything, more than capable of mobbing the buzzards and red kites that might compete for their food and nest sites.
    Perhaps understandably, given their success while so many other species have declined, corvids have felt the harsh glare of suspicion. They could only look more guilty by wearing a napkin, licking their lips and clutching a knife and fork. And yet we tend to feel at least a grudging respect for this group of birds. They are clearly among the most intelligent of not just birds but all living things, with the capacity to use tools and solve complex problems.
    So it's a cheering early Christmas present for corvids that arrives in the form of a landmark study showing that their impact on numbers of other species is far smaller than generally assumed. Indeed, it may be almost negligible in most cases. That's not to say we shouldn't be concerned about house sparrows, song thrushes and other birds whose remembered abundance blinds us to their shocking declines. However, we can at least remove the noose from the crow's neck and celebrate its qualities without guilt.
    You can read the full study here: http://ow.ly/FT3Qg

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