Should the Red Grouse be split from Willow Ptarmigan?

There’s some part of me that couldn’t help but get a bit patriotic when the IOC announced that the Red Grouse (Lagopus (lagopus) scotica) would be split from the Willow Ptarmigan (Lagopus lagopus). On face value, it seemed an obvious choice, with large moult and plumage differences between the two putative species.

The sources used by the IOC were Sangster and colleagues (2022), and Kozma and colleagues (2019). The paper by Sangster and colleagues provides a comprehensive history of research into the delimitation of the Red Grouse, concluding that it would best be treated as a separate species, which I would definitely recommend reading: it details the long, drawn-out, controversial road to species status of the iconic Red Grouse.

Here I will attempt to explain why I think that this is a premature decision. As much as I would like to see a British Isles endemic (I don’t know how many people still count the Scottish Crossbill), something tells me that this line of research is not quite over yet, given a lack of studies on vocalisation, as well as some clear problems with the genetic and morphological reasons given.

Red Grouse, Northumberland. CC BY-SA 3.0 by MPF: original here.

Molecular data

Kozma and colleagues (2019) wrote a fascinating paper which explored genes and loci under selection in Red Grouse. Nowhere did they propose a taxonomic reassessment. The paper looked at F_ST values; F_ST analysis is a way of measuring genetic differentiation between two or more populations; it essentially measures the reduction of heterozygotes in a population due to random genetic drift. A value of 0-0.05 generally indicates very little drift, and 0.05-0.15 indicates moderate levels of drift. For context, two hybridising North Pacific albatross species had a value of ≈0.63, while Collared and Pied Flycatchers had a mean F_ST of ≈ 0.30 (Burri et al., 2015; Huynh et al., 2023). Kozma and colleagues found an F_ST value of 0.08± 0.04… not a particularly astounding value. To be fair, the Willow Ptarmigan to Rock Ptarmigan F_ST value was 0.17 ± 0.07, which is surprisingly low as well. All of the Rock Ptarmigan samples came from Greenland – perhaps samples from Scandinavia would clear things up – perhaps they wouldn’t. To conclude that this is particularly decisive evidence for a Red Grouse split is a bit of a stretch.

Biogeography and the fossil record, what happened over the last 2 million years?

Yes, it shows that there has not been gene flow between Red Grouse and Willow Ptarmigan for at least 12,000 years, but that’s pretty obvious: of course, a highly sedentary grouse population doesn’t tend to cross the North Sea. I am not convinced that 12,000 – 125,000 years is enough time for speciation to have occurred, especially considering the biogeography of north-western Europe during the last glacial maximum. A broad belt of steppe-tundra habitat stretched from southern Ireland well into Siberia, until only ~16,000 years ago (see Ray and Adams, 2001; Fig. 1). Given the current habitat of Willow Ptarmigan in Siberia, and data from a different Kozma and colleagues paper (2016), which suggests the Scandinavian Willow Grouse did not go through a bottleneck during the last-glacial maximum, it is likely that Willow Grouse inhabited that broad belt and were then forced northward into Ireland, Britain and Scandinavia with subsequent warming. The fossil record backs this up, with lots of evidence suggesting the Willow Ptarmigan used to be a common bird of mammoth steppe, with fossils scattered all over Europe (Potapov, Potapova and Pavlova, 2003; Fig. 2).

Figure 1. GIS-based map of Europe from the last glacial maximum, ~16,000 years ago. From Ray and Adams, 2001.

Figure 2. Map of Europe with dark dots showing locations where fossil evidence of Willow Grouse, Lagopus lagopus has been found between 10,000 and 120,000 years ago. Compare with Figure 1, it matches up quite nicely. From Potapov, Potapova and Pavlova, 2003.

Meyer-Lucht and colleagues (2016) also ran an F_ST analysis between Scottish and Irish Red Grouse, finding F_ST values of ~0.09-0.11, comparable to that of Willow Ptarmigan to English Red grouse. Interestingly, the land-bridge between Ireland and Britain is believed to have been submerged at a relatively similar time to that of Doggerland, the land bridge between Britain and the rest of north-western Europe. Does this mean that perhaps the plumage similarities between Irish and British Red Grouse are convergent? Did they evolve separately or was it a shared characteristic of a briefly panmictic British and Irish population, which couldn’t have existed for very long (if at all) given biogeographic models explored above and the sedentary behaviour of grouse.

Morphology

It is generally believed that Red Grouse lost their all-white winter plumage due to milder winters in the British/Irish Isles. A white prey species in a sea of red moorland would likely stand out to a hungry fox! We are left with two ‘species’ which look very different on face value. However, strong selective pressures are well established to lead to rapid changes in colouration over relatively short periods of time, especially considering the low generation time in birds. Somewhat complicating this issue, maintaining autumn feathers in winter plumages is not a trait unique to Red Grouse – birds on Norwegian islands do the same. Interestingly, L. lagopus maior, a subspecies from northern Kazakhstan and south-central Russia, is also reported to maintain some autumn feathers in winter, although I couldn’t find any photos of this (Hannon, Eason and Martin, 2020). It isn’t even a trait unique Willow Ptarmigan, some Rock Ptarmigan populations also have reduced white in winter plumages (Potapov, Potapova and Pavlova, 2003).

Potapov and colleagues (2003), propose that colouration in grouse populations changed with glacial cycles during the last 2 million years, with peaks and troughs of warm and cold periods coinciding with redder and whiter winter plumages. Red plumage not being unique to Red Grouse doesn’t discredit their elevation to species status per se, but casts doubt when attempting to give its ‘unique’ plumage as evidence for a split. Furthermore, Potapov and colleagues (2003) further muddy the waters by claiming that all-white winter plumage British Red Grouse do occasionally occur, suggesting that this difference in winter plumage is not exactly the result of some large alterations to the genetic code, but a common mutation.

I would interpret the evidence that birds from Norwegian islands (ssp. variegatus) being very closely related to mainland Scandinavian birds further disputes the weight put on Red Grouse’s ‘unique’ plumage. I am not aware of a study on vocalisations, but that would greatly supplement our current understanding of the complex. An SFS or PSMC-based effective population size model performed on both Irish and British Red Grouse would also be very interesting to explore the biogeography of this species complex.

Conclusions

Maintaining autumn feathers in winter plumages is not a trait unique to Red Grouse – birds on Norwegian islands do the same, as do Kazakh Willow Ptarmigan and Aleutian Rock Ptarmigan. Looking at the biogeography of Europe during the last glacial maximum, it is hard to believe that what was very likely a panmictic population of Willow Grouse given historical effective population size data, the fossil record and biome GIS models as recently as 17,000 years ago could possibly have speciated by now. Potapov and colleagues (2003), believe that ‘Red Grouse’ phenotypes cyclically became more common during warm periods over the last 2 million years, it’s an interesting theory, and makes sense given the frequency with which the phenotype occurs in milder locations. Given that the ‘Red Grouse’ phenotype can evolve in populations with very little differentiation from ‘Willow Grouse’ phenotype populations, as seen in variegatus, there is nowhere near enough evidence to warrant a split (Kozma et al., 2019). Perhaps vocalisation studies would help; the primary colour of Red Grouse is another interesting feature, although, on balance, I can’t see how such a recently isolated island population could possibly be considered a separate species. British Rock Ptarmigan probably became isolated from mainland European Rock Ptarmigan around the same time, had a historically lower population size which allowed for more drift, and have comparable generation times but aren’t in the same conversation due to a lack of plumage divergence.

References

Burri, R. et al. (2015) ‘Linked selection and recombination rate variation drive the evolution of the genomic landscape of differentiation across the speciation continuum of Ficedula flycatchers’, Genome Research, 25(11), pp. 1656–1665. Available at: https://doi.org/10.1101/gr.196485.115.

Hoebe, P.W. et al. (2024) ‘Early Holocene inundation of Doggerland and its impact on hunter-gatherers: An inundation model and dates-as-data approach’, Quaternary International, 694, pp. 26–50. Available at: https://doi.org/10.1016/j.quaint.2024.05.006.

Huynh, S. et al. (2023) ‘Whole-genome Analyses Reveal Past Population Fluctuations and Low Genetic Diversities of the North Pacific Albatrosses’, Molecular Biology and Evolution, 40(7), p. msad155. Available at: https://doi.org/10.1093/molbev/msad155.

Kozma, R. et al. (2016) ‘Looking into the past – the reaction of three grouse species to climate change over the last million years using whole genome sequences’, Molecular Ecology, 25(2), pp. 570–580. Available at: https://doi.org/10.1111/mec.13496.

Kozma, R., Rödin-Mörch, P. and Höglund, J. (2019) ‘Genomic regions of speciation and adaptation among three species of grouse’, Scientific Reports, 9(1), p. 812. Available at: https://doi.org/10.1038/s41598-018-36880-5.

Laloë, D. and Gautier, M. (2012) On the genetic interpretation of Between-Group PCA on SNP data. Research Report HAL : hal-00661214, version 1. auto-saisine, p. 23 pages. Available at: https://hal.science/hal-01193689 (Accessed: 2 July 2024).

Lindgren, A. et al. (2016) ‘GIS-based Maps and Area Estimates of Northern Hemisphere Permafrost Extent during the Last Glacial Maximum’, Permafrost and Periglacial Processes, 27(1), pp. 6–16. Available at: https://doi.org/10.1002/ppp.1851.

Meyer-Lucht, Y. et al. (2016) ‘Adaptive and neutral genetic differentiation among Scottish and endangered Irish red grouse (Lagopus lagopus scotica)’, Conservation Genetics, 17(3), pp. 615–630. Available at: https://doi.org/10.1007/s10592-016-0810-0.

Potapov, R., Potapova, O. and Pavlova, E. (2003) ‘The genus Lagopus Brisson, 1760: taxonomy, paleontological dates, and evolution.’, Proceedings Zoological Institute, Russian Academy of Sciences, 299, pp. 101–120. Available here.

Ray, N. and Adams, J.M. (2001) ‘A GIS-based Vegetation Map of the World at the Last Glacial Maximum (25,000-15,000 BP)’, Internet Archaeology, 11. Available at: https://doi.org/10.11141/ia.11.2.

Sangster, G. et al. (2022) ‘The taxonomic status of Red Grouse’, British Birds, 115, pp. 28-⁠38. Available here.