Murderous – but not so bad after all? By Dr. Mark Avery.
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. 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 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. 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.