Are released pheasants driving adders to extinction?

Claim

Game birds “could wipe out adders in most of Britain within 12 years”

What the science says

Unsupported.

Adder populations in the UK are declining. Most of Britain’s adder sites support small, isolated populations, many of which are at risk of extinction, but some larger populations are growing although still vulnerable. The main reasons for the decline are thought to be habitat loss and fragmentation, along with disturbance.

There is no robust evidence looking at whether pheasant predation affects adder populations, contributes to their decline, or is responsible for local extinctions, as suggested.

This is an important area of research for the future.

Comes from

European adder (*Vipera berus*) by Benny Trapp / CC

Pieces in both The Guardian on Thursday 1 October 2020¹ and The Telegraph on Friday 2 October 2020² report on adder declines and the possible link to pheasant releases. Both articles used quotes from Nigel Hand, a trustee of Amphibian and Reptile Groups of the UK (ARG UK):

The adder is on the brink of extinction in many sites across Britain … and it is the uncontrolled release of millions of pheasants by shooting estates which is pushing it over the brink. At this rate the adder will be extinct over much of its range by 2032, so it is essential we take action now to conserve it.

What the Science says – the fuller picture

Adder populations are in decline in the UK, and this is a serious concern. In 2007, the adder was added to the UK Biodiversity Action Plan list of priority species to help conserve its population. Adders prefer heathland areas, but they are also found in sunny areas of woodlands, grassy banks, sand dunes, and other habitats with warmth and well-drained soils³. Some of these habitats can overlap with the areas in which pheasants are found – making effects possible³.

Adder extinction

The claim that adders could be extinct in 12 years is based on the results of an 11-year adder count and survey published by Gardner in 2019, which analysed data from 129 sites across the UK⁴. Of these, 117 sites (90%) were classed as “small population” sites, with declining populations and the authors concluded that “the rate at which these small population sites are declining implies the average small adder population could be extinct by 2032”.

The remaining 12 sites in the adder survey were classed as “large population” sites with increasing numbers. However, these sites are also not secure as changes to these sites are thought to make the adder “increasingly vulnerable to extinction”.

So, the claim that adders may be extinct in 12 years specifically refers to some small, isolated adder sites rather than the whole population. Small populations do make up the majority of sites in the adder survey, and the future of the larger sites with more stable populations are also a cause for concern, but the 12-year time frame is not applied to these or Britain as a whole. This was suggested in the Guardian’s article which refers to “most” of Britain in its headline, but not in the Telegraph, which says adders may become “extinct in Britain”.

What is driving these declines?

This decline in adder numbers is concerning and there are thought to be several causes for it. However, there is only limited research examining the causes, the interaction between them, or the possible contribution of released pheasants.

The role of pheasants

As the habitat of adders and pheasants overlap, and there is a period in the late summer when released pheasants have begun to disperse from their pen but adders have not yet begun to hibernate, there is potential for pheasants to impact adders during this time. Furthermore, reptiles are known to be more sluggish on colder days, with slower defenses⁵, especially when entering or emerging from hibernation. Due to climate change, hibernation periods are getting shorter as winters become warmer and some adders might not fully hibernate⁶. A shorter hibernation period means they will only conserve smaller amounts of energy that will be used up quicker⁷. This sluggish period and lack of hibernation may leave reptiles more vulnerable to potential predation by pheasants when the weather begins to turn colder and they prepare to hibernate, but pheasants remain active^3,8.

Dispersal of released gamebirds has also been in the spotlight recently and is relevant to this discussion. After release, game managers try to control the movement of their birds because for a driven shoot to work effectively, pheasants need to be around the release points and nearby game crops. Evidence shows that on average 90% of released pheasants and red-legged partridges remain within 1km of their release points⁹. This means direct impacts of releases on the reptile populations are usually only possible close to release sites.

Although the potential is recognised for released pheasants to prey on, or affect the behaviour of reptiles, to date there is little published scientific evidence on the direct effect pheasants may have on adders (or other reptiles). A recent paper from Belgium studied reptile predation rates by birds and mammals using model snakes and found that mammals were responsible for more than twice as many predation events as birds. While they did not separate gamebirds from corvids or raptors in their analysis, predation by birds did not appear more common on sites with pheasants present¹⁸. More broadly, past papers have analysed pheasant droppings and stomach content to understand what may contribute to a pheasant’s diet in the UK and USA, but these also find no evidence of reptile remains^10-12. When discussing predators, Gardner concludes that the impact of pheasants is anecdotal at present and there is not enough research-based evidence⁴.

Extending the evidence base to include unpublished literature, three reports are available that study this subject – an MSc thesis from 2014¹³, a summary of a pilot study from Worcester University from 2013¹⁴, and an observational report published by Nigel Hands in 2020¹⁵ – these and many other sources of evidence were reviewed by Sage et al. in their 2020 paper on the effects of pheasant releasing¹⁶.

The MSc work found that young penned pheasants preferentially pecked at reptile-shaped plastic objects over non-reptile shapes, but adult pheasants did not do this, (released pheasants are young adults when they start to move from their release pen). This work also surveyed a small number of woodlands for reptiles and, although the sample was too small for proper statistical analysis, slightly fewer reptiles (grass snakes and slow worms) were found in a woodland that contained pheasant release pens¹³.

The Worcester University pilot study looked for adder DNA in 50 pheasant dropping samples. It found none, but we need more research to understand the meaning of this result. For example, it is unknown whether adder DNA can withstand digestion and be detected in pheasant droppings¹⁴.

Finally, in a report that was released in 2020, Nigel Hand reports his observations of adders near a well-known pheasant release site that has suffered a decline in adder numbers. The report does not record any pheasant-adder interactions, but individual adders were found with pecking injuries that are suggested to have come from pheasants. Anecdotal evidence of a pheasant seen pecking an adder and a photograph taken of another eating an adder are included. Hand makes multiple suggestions for why pheasants affect adders, for example, they are attracted to the wriggly movements of snakes. Furthermore, he suggests the presence of pheasants can reduce the time they have to bask after emerging from hibernation, which leads to potential health and breeding issues¹⁵. Overall, there is no robust evidence that pheasants impact adder populations, however, there is the potential for them to predate reptiles, possibly adding pressure to already vulnerable populations close to release sites, and this is an important area for further research. GWCT best practice guidelines ask game managers to consider and avoid especially sensitive sites and this should include not siting release pens on or next to known adder sites. The Sustainable Gamebird Releasing Guidelines can be found here.

Causes of adder declines

The results of the adder survey reported by Gardner et al. (2019)⁴ identified three key concerns from observations of adder sites that should be addressed to better protect their populations:

Public pressure was recorded on 48% of the sites examined. This includes dog walking, mountain biking, photographers, vehicles, and trampling vegetation while walking. The impact of public disturbance requires further investigation, especially as this was a new concern highlighted⁴.
Habitat fragmentation was reported on 17% of sites. When larger areas of more joined up habitat are broken into smaller patches, smaller populations become confined to specific locations and isolated from others. This creates smaller isolated populations that in the case of adders, are often fewer than 10 individuals, leading to inbreeding and eventually local extinctions¹⁷.
Habitat management had an impact on both large and small population sites. Each large population site had at least one conservation designation, with many being Sites of Special Scientific Interest and/or and National Nature Reserves. Gardner et al. suggest the positive habitat management of large sites could be why larger populations of adders are increasing, especially as habitat neglect was reported on 22% of the small sites⁴. Negative habitat management (for adders) includes overgrazing, the use of heavy machinery, and focusing on management for other species⁴.

Other pressures

Climate change may also impact adders by altering their hibernation habits. As the climate warms, reptiles may stay active for more of the year. Evidence for this is also limited, but an article in the Guardian from 2019 quotes Nigel Hand on the impact of climate change: “I’ve seen adders in every month of the year except December but have had reliable reports of them being active [then] in Wales”⁶. He went on to explain that the likely consequence of a shorter hibernation period is that it creates more opportunities to come in contact with predators including dogs and cats^3,6,15, but didn’t discuss pheasants.

Conclusion

Adder populations are under pressure, and there are likely to be many reasons for their decline. The release of pheasants may be one of these, but this is not currently supported by robust scientific evidence despite some smaller studies, anecdotal and observational reports. It is important to consider and address the possible effects of pheasants on vulnerable reptile populations, but the evidence does not support the statement that “it is the uncontrolled release of millions of pheasants by shooting estates which is pushing it over the brink”.

Habitat loss and fragmentation, disturbance, and climate change are suggested as key pressures for the adder, and other reptile populations. However, in the context of other modern land-uses, pheasants are a plausible contributing factor to local declines. Further research on the potential impacts of gamebird release on reptiles is required and, in the meantime, shoots should adhere to best practice guidelines to avoid known sensitive sites for release pens.

Addendum (10 March 2023)

In light of new research being published, the following information has been added:

A recent paper from Belgium studied reptile predation rates by birds and mammals using model snakes and found that mammals were responsible for more than twice as many predation events as birds. While they did not separate gamebirds from corvids or raptors in their analysis, predation by birds did not appear more common on sites with pheasants present¹⁸.

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References

Milton, N. (2020). Game birds ‘could wipe out adders in most of Britain within 12 years’. The Guardian.
Horton, H. (2020). Flood of pheasants could wipe out Britain’s adders in 12 years. Telegraph.
Edgar, P., Foster, J., Baker, J., Gleed-owen, C., Moulton, N., Powell, G., Preston, M., Webster, J. & Amphibian, B.W. (2010). Reptile Habitat Management Handbook. Amphibian and Reptile Conservation.
Gardner, E., Julian, A., Monk, C. & Baker, J. (2019). Make the Adder Count: population trends from a citizen Published by the British Herpetological Society science survey of UK adders. Herpetological Journal, 29:57–70.
Blanke, I. & Fearnley, H. (2015). The sand lizard: Between light and shadow. Laurenti Verlag Bielefeld.
Milton, N. (2019). Adders now active all year with warmer UK weather. The Guardian.
Henle, K., Dick, D., Harpke, A., Kühn, I., Schweiger, O. & Settele, J. (2008). Climate Change Impacts on European Amphibians and Reptiles. 1–51.
Beebee, T.J.C. & Griffiths, R.. (2000). Amphibians and reptiles: A natural history of the British Herpetofauna. Harper Collins. London.
Madden, J.R. & Sage, R.B. (2020). Ecological Consequences of Gamebird Releasing and Management on Lowland Shoots in England: A Review by Rapid Evidence Assessment for Natural England and the British Association of Shooting and Conservation. Natural England Evidence Review.
Fried, L.A. (1940). The Food Habits of the Ring-Necked Pheasant in Minnesota. The Journal of Wildlife Management, 4:27–36.
Wright, T. (1941). A Study of the Fall Food Supply of the Ring-Necked Pheasant and the Bob-White Quail in Washington County, Rhode Island. The Journal of Wildlife Management, 5:279–296.
Stromborg, K.L. (1979). Pheasant Food Habits in Spring and Consumption of Seed Treatment Pesticides. The Journal of Wildlife Management, doi:10.2307/3800656
Berthon, G. (2014). A study Investigating the possible impacts that pheasant shooting industry might be having on UK reptile species in South West England woodland ecosystem. University of Exeter.
Dimond, R., Warner, N., Wheeler, M.. & Westbury, D.. (2013). An investigation into the relationship between pheasants (Phasianus colchicus) and reptiles as prey.
Hand, N. (2020). The adder (Vipera berus) and the cultivation of the ring neck pheasant (Phasianus colchicus). Field observations from long term monitoring and adder radio telemetry projects across the Midlands and Southern England.
Sage, R.B., Hoodless, A.N., Woodburn, M.I.A., Draycott, R.A.H., Madden, J.R. & Sotherton, N.W. (2020). Summary review and synthesis: effects on habitats and wildlife of the release and management of pheasants and red-legged partridges on UK lowland shoots. Wildlife Biology, 2020:1–12.
Madsen, T., Stille, B. & Shine, R. (1996). Inbreeding depression in an isolated population of adders Vipera berus. Biological Conservation, 75:113–118.
Duchesne, T., Graitson, E., Lourdais, O., Ursenbacher, S. & Dufrene, M. (2022). Fine-scale vegetation complexity and habitat structure influence predation pressure on a declining snake. Journal of Zoology, 318:205–217.