Yesterday saw the publication of a new scientific research paper entitled: A review of the impacts of corvids on bird productivity and abundance. The paper is available for free download here.
If you want to skip over the technical details, the authors have helpfully issued a press release which provides a more general overview for the more casual reader. It reads as follows:
A MURDER OF CROWS?
They steal, raid nests, and keep the company of witches. But the unpopular crow may not be as big a menace as people think.
A new study has found that crows – along with their avian cousins the magpie and the raven – have surprisingly little impact on the abundance of other bird species.
Collectively known as corvids, these birds are in fact being menaced by mankind in the mistaken belief that removing them is good for conservation.
The new 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 percent), 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, the first of its kind, 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 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.
“These results have big implications for the likely benefits of corvid control,” Dr Amar said. “They suggest that killing corvids will be of most benefit to those interested in gamebird shooting rather than conservationists.” He added: “Bird hunters are usually most interested in increasing numbers of birds available to shoot immediately after the breeding season and this appears to be where corvids have most impact”. “Conservationists on the other hand, are usually interested in increasing a species population size and our results suggest that only in a very few cases did corvids have an influence on this aspect of their prey,” Dr Amar said.
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 review also found large differences between the impacts of crows, historically considered the most ‘cunning’ corvid, and magpies which are sometimes killed by home owners hoping to protect songbirds in their gardens. Crow species were six times more likely to have an impact on bird prey species than Magpies.
Mistaken assumptions about corvid predation were possibly explained by the birds’ diurnal nature and the fact that they are conspicuous nest predators: “Their importance in prey population regulation is often assumed prior to any assessment of the evidence,” the study warned.
Chrissie Madden, the lead author on the paper, 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.
The paper itself is an interesting bit of science, but of more interest (to us at least) is the potential application of the research results. Basically, this review paper has shown that in the vast majority of cases, corvids (including crows and ravens) have little effect on their prey populations, and thus this raises an important question about the validity (and legality) of Government-issued General Licences which allow the mass killing of corvids, supposedly for the purposes of ‘conserving wild birds’.
General Licences have long been an issue of concern to conservationists, and we have blogged about this a lot (e.g. see here & scroll down through the posts and links). General Licences are routinely used by gamekeepers and land managers for the largely unregulated practice of killing so-called ‘pest’ species, especially corvids, in Larsen traps, clam traps and crow cage traps, or by shooting them. However, General Licences are not permitted to be used to kill ‘pest’ species for the purposes of protecting surplus stocks of gamebirds, even though that is exactly what gamekeepers have been doing, although they don’t admit to that – they simply claim they are ‘controlling vermin’ to protect wild birds such as waders.
How will SNH deal with the results of this latest study, given the overwhelming evidence that corvid predation isn’t having a significant impact on wild bird species in the majority of cases?
Don’t expect a quick response from SNH. We are still waiting for them to deal with other concerns that have been raised about the use of General Licences, some of which were raised in a publication dating back 15 years:
Interestingly, SNH has recently announced that their suite of 2015 General Licences will shortly (this week) be published on their website, WITHOUT conducting a public consultation and WITHOUT any substantive changes to their 2014 licences. Robbie Kernahan (SNH licensing dept) said:
“From our previous consultations and discussions on the GL suite, I think we have a good understanding of the key issues and your outstanding concerns relating to General Licences“.
So that’s ok then. SNH understands the key issues and concerns but has decided not to address them. Brilliant.
Although, they are apparently addressing one aspect of trap use and have been conducting a questionnaire survey of trap-users (see here). As we blogged at the time, asking trap-users for a truthful account of their activities is, frankly, ridiculous. We all saw quite graphically last week, with the conviction of gamekeeper George Mutch, how some of these trap-users are operating.
We’ll be re-visiting this topic in the New Year.
Photo of trapped corvids by Walter Baxter