A study by Graitson and Taymans (2022)1 investigating the relationship between gamebird releasing and reptile numbers in Belgium has recently been published in the Bulletin of the French Herpetological Society.
In summary the article concludes that in the area studied, reptiles were universally present at a large sample of sites with no or few released pheasants, and no reptiles were found in any of a small number of sites with released pheasants. The authors also point out that in one or two places, some reptiles such as common lizards and slow worms not observed when pheasants were released, were subsequently recorded after pheasant releasing ceased.
The study has been referred to as evidence that gamebird releasing is damaging to reptile populations and should be restricted. We thought it would be useful to look at this paper in a bit more detail and in the context of other evidence available on the matter.
What do we know about the relationship between pheasants and reptiles?
As a research-led organisation with expertise in gamebird management and biodiversity monitoring, the GWCT has previously reviewed the evidence relating to the relationship between pheasant releasing and reptile numbers alongside our wider work on the potential impacts of gamebird releasing on the environment. These include our What the Science Says fact check, peer-reviewed published research, Rapid Evidence Assessment for BASC and Natural England, and our 2021 Gamebird Releasing report reviewing best practice management and ecological impacts, as well as other relevant peer-reviewed scientific papers.
We know that there is overlap between some of the habitats occupied by reptiles and pheasants - such as woodland rides, edges, and clearings - and this may create potential for pheasants to have an impact on reptiles2. This is especially true during late summer when pheasants begin to disperse and reptiles are making the most of basking opportunities before beginning hibernation later in the year.
There is also potential for interactions to occur prior to and following hibernation, when reptiles are much more likely to be sluggish and need to bask to gain energy, but pheasants remain active2–4. As a result of gamebird monitoring however, we know that 90% of released pheasants stay within 1 km of their release site, meaning that potential impacts to reptiles or their habitats are likely to be localised 5.
Although it is recognised that pheasants have the potential to negatively impact all six native species of British reptile2,3,6, there is currently no robust scientific evidence published to confirm this7. There is only anecdotal information available, which suggests that pheasants can sometimes injure or kill reptiles such as adders. This suggestion is based on observations of pheasants seen pecking adders and reptiles being found with pecking injuries8–11.
Some observed declines in reptiles - for instance in the Wyre Forest, Worcestershire - have been attributed to the presence of pheasants9 although this is not the result of formal, published research but based on anecdotal observations. There is also the suggestion that disturbance from pheasants may interfere with important post-hibernation basking for reptiles, which could lead to potential health or breeding issues9.
The limited dietary evidence that is currently available has found little to no impact of released pheasants on reptiles, for example, the diets of >2,500 pheasant have been studied in the USA and UK and no reptile remains have been found5,12–14. A 2013 pilot study carried out by the University of Worcester studying 50 pheasant dropping samples also found no evidence of reptile DNA in the droppings8, however this study had too small a sample size to be considered conclusive.
A 2014 University of Exeter MSc project also found that only immature pheasants pecked at reptile-shaped objects, with this behaviour not displayed by adult pheasants15. As only adult pheasants are released for shoots and 0-10% remain by the following Spring2,5, this suggests this behaviour is not common or widespread.
The same study also found that fewer grass snakes and slow worms were found in woodlands containing pheasant release pens, but due to small sample sizes could not perform any data analysis to help confirm if the release pens were a significant factor affecting reptile presence.
As far as we know there is currently no robust evidence available to accept or reject the idea that released gamebirds have a negative impact on reptiles in the UK, and there is no documented mechanism whereby predation, killing, or disturbance from pheasants affects reptile populations, contributes to their declines, or causes local extinctions.
However, we recognise that a lack of evidence does not mean there is not a problem2. It is well understood that Britain’s native reptiles face many pressures, such as disturbance from recreational users, dogs, and livestock; fragmentation and loss of suitable habitat; inappropriate habitat management, particularly on delicate heathland habitats; and climate change7.
What does the latest study tell us?
The main part of the study by Graitson and Taymans (2022)1 reports an association between massive releasing of pheasants and reptile presence at 267 sites. In brief, based on long term reptile inventories at selected sites across a large part of Wallonia, a region of Belgium, 261 of 261 sites where nearby pheasant releases were not recorded post hoc, had between one and six reptile species present in one or more years, while six out of six sites with massive pheasant releasing had no reptile species present.
To interpret this result, it is important to have a look at the paper carefully to understand how the study selected sites and collected data. However, in some areas the methods used are not very clear. The paper (in French) is available online free to view and can be translated online if required. Here are a few key points that we think need to be considered.
Like any reported association between two variables, by definition no mechanism has been identified and anything that correlates with either of those two variables may cause or contribute to the association. For example in this study, the part of Wallonia where reptiles were universally reported by all inventories has low intensity agriculture, lots of forest and grassland habitats, and relatively low human population density.
The region where all the sites with no reptiles (and some with reptiles) were recorded is densely populated and dominated by intensive agriculture. The two regions are delineated by the Sambre Meuse valley that traverses Belgium. It is possible that this basic landscape factor is contributing to the association found.
More generally, the paper lacks a detailed methods description and because of this interpretation of the results is difficult. Firstly, we do not know the extent to which previous knowledge of reptile presence contributes to inventory site selection.
The Belgian guidelines16 on how to conduct inventories of reptiles (Graitson 2009) indicates that likely sites should be selected (e.g., quarries or wasteland) and that previous knowledge of reptile presence or absence should be sought when deciding and planning inventories. In relation to the study here, if previous knowledge of presence influenced site selection, this would have a significant effect on the quality or meaning of the reptile presence or absence variable used.
In addition, it is not clear how sites were categorised as having massive releases or not. The paper says that 6 sites were ‘involved with massive pheasant releases’, while 261 sites were ‘without pheasants or with anecdotal presence’. It would be useful to know what ‘massive pheasant releases’ and ‘anecdotal presence’ actually means in terms of on-site pheasant density.
able 1 in the paper describes the sites with released pheasants but adds confusion because it refers to only five sites, of which two contained rearing pens and three had pheasants released in their immediate proximity. In our experience, the likelihood of confirming the presence or absence of released pheasants on an area varies seasonally and absence is often difficult to ascertain.
The second part of the study considers a single site where reptile presence was assessed in 1999 and 2011. Massive pheasant releases took place until 2003, when they ceased after the site became a cycle route. No reptiles were detected in 1999, but they were recorded in 2011. In addition, at 2 of the 6 massive release sites, releasing ceased and reptiles were recorded on them some years later. These are interesting anecdotes that support the linkage between pheasant releasing and reptile absence. Being anecdotal, they do not provide evidence of an association but are observations that warrant further investigation.
In conclusion
The main value of these observations and the multi-site study described in this paper is to raise again the serious issue of released gamebirds potentially causing reductions or losses of reptiles at release sites. This possibility was not identified by Bicknell et al. (2010)17 - the review referenced by the latest study - but was reported in published documents by the reviews of Madden and Sage (2021)5 and Sage et al. (2021)18.
Partly as a consequence of these reviews, there is a new PhD study at DICE, University of Kent, which is going to look at the issue of gamebird releasing and reptiles over the next three years. In the meantime, despite the lack of robust evidence, we are clear that there are plausible mechanisms for an effect of pheasant release pens on local reptile populations.
We welcome more research being conducted and published on this important matter, and will continue to use our position as a research-led advisory body to identify, develop, and promote best-practice guidelines based on available data and information. These guidelines seek to mitigate any potential negative impacts of gamebird releasing, asking game managers to consider and avoid sensitive sites.
In relation to reptiles we now suggest to game managers that when they are siting pheasant release pens they avoid known reptile sites2 (Sage et al. 2021). We also suggest releases are done later in the season - this results in a reduced temporal overlap between the release in late summer, and when the reptiles hibernate in autumn. The Sustainable Gamebird Releasing Guidelines that include these points about reptiles can be found here.
References
1. Graitson, E. & Taymans, J. (2022). Impacts of massive releases of colchid pheasants (Phasianus colchicus L.) on squamates (Reptilia Squamata). Bulletin de la Société Herpétologique de France, 180. doi:10.48716/bullshf.180-2
2. Sage, R.B., Brewin, J., Stevens, D.C. & Draycott, R.A.H. (2021). Gamebird Releasing and Management in the UK. A review of ecological considerations, best practice management and delivering net biodiversity gain. Fordingbridge, UK.
3. Edgar, P., Foster, J. & Baker, J. (2010). Reptile Habitat Management Handbook. Amphibian and Reptile Conservation. Bournemouth.
4. What the Science Says. (2020). Are released pheasants driving adders to extinction? What the Science Says: Available at: https://www.whatthesciencesays.org/are-released-pheasants-driving-adders-to-extinction/. (Accessed: 25th August 2022)
5. 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. Peterborough, UK.
6. Foster, J. Game changer? Amphibian and Reptile Conservation: Available at: https://www.arc-trust.org/news/game-changer. (Accessed: 25th August 2022)
7. 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.
8. Dimond, R., Wheeler, M., Hand, N. & Westbury, D. (2013). An investigation into the relationship between pheasants (Phasianus colchicus) and reptiles as prey. Herpetological Journal, 24:3–6.
9. 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.
10. Rice, C.N. (2016). Abundance, impacts and resident perceptions of non-native common pheasants (Phasianus colchicus) in Jersey, UK Channel Islands. University of Kent. Canterbury.
11. Phelps, T. (2004). Population dynamics and spatial distribution of the adder Vipera berus in southern Dorset, England. Mertensiella, 15:241–258.
12. Stromborg, K.L. (1979). Pheasant Food Habits in Spring and Consumption of Seed Treatment Pesticides. The Journal of Wildlife Management, 43:214–219.
13. 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.
14. Fried, L.A. (1940). The Food Habits of the Ring-Necked Pheasant in Minnesota. The Journal of Wildlife Management, 4:27–36.
15. 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. Exeter.
16. Graitson, E. (2009). Guide de l’inventaire et dui suivi des reptiles en Wallonie.
17. Bicknell, J., Smart, J., Hoccom, D., Amar, A., Evans, A., Walton, P. & Knott, J. (2010). Impacts of non-native gamebird release in the UK: a review RSPB Research Report Number 40. The Lodge, Sandy, Bedfordshire.
18. 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.