Estimate effect of introgressive hybridization on wild populations
All three datasets (mitochondrial, microsatellites and SNPs) show that captive stocks in both the UK and Greece have been predominantly derived from birds of western origin (Perdix perdix perdix ). However, there is also evidence of Eastern genotypes within both these captive populations, signalling that birds of the eastern lineage (Perdix perdix lucida ) have also been used for captive breeding. However, considering that in both the UK and Greece, individuals from captive stocks are released each year, very little evidence of hybrids within the wild individuals has been found; one hybrid individual was identified in the UK and five in Greece/North Macedonia based on microsatellites (Figure 4). Although they provide insight, the use of just eight microsatellites has limitations. As can be seen with the known hybrid individuals, only three out of the six can be identified as hybrids using this microsatellite data. However, even when considering the robust SNP data, there is no evidence of F1 hybrids in the wild (Figure 7), although the microsatellite data indicates that there may be one wild F1 hybrid in the UK dataset (Figure 4).
Obviously, Greece would be predicted to have a larger number of hybrids, considering the captive stocks being released are of Western origin, however, there is very little evidence to suggest that hybridization has occurred within the wild Greek birds. This contrasts with neighboring North Macedonia where all our genetic datasets (mitochondrial, microsatellite and SNPs) indicate that individuals of both eastern and western genealogies are present in the wild population. However, as already discussed, whether this is a natural transition zone betweenP. p. perdix and P. p. lucida or the result of captive releases requires further investigation. The differences between levels of hybridization in the captive stocks compared to the wild individuals in both the UK and Greece are intriguing. Several reasons could explain this: 1) Released birds do not survive and therefore do not contribute to the wild gene pool, 2) Released hybrid individuals do not survive and contribute to the wild gene pool, 3) Hybridization within the captive stocks is a relatively recent phenomenon and not many hybrids have been released yet, 4) The numbers of released birds have been very small compared to the wild population. We argue that the last point is unlikely, as the grey partridge has been the most important European game bird for centuries and there is historical evidence that stocking and translocations have been conducted throughout Europe as far back as the 1560s (Andersen and Kahlert, 2012). The other common name of the grey partridge (the Hungarian Partridge) is based on the widespread exporting of captive stock from Hungary across Europe and to North America during the 20th century (Browne, Buner and Aebischer, 2009). Estonian and French birds were more recently imported into Ireland for captive breeding between 2002-2005 (Buckley et al. , 2012). This widespread sharing of game bird stock across Europe is of routine and historical occurrence. However, the geographical origins of captive stock are rarely known. Regarding the third point, it does seem that captive populations are mainly of Western origin. This has been the finding of our study in both the UK and Greece but also of the two previous genetic studies in Finland and Denmark (Liukkonen-Anttilaet al. , 2002; Andersen and Kahlert, 2012) It is, therefore, feasible that hybridization is of limited success or has not occurred until recently. However, regarding the two first points, multiple studies have found that released birds have very low survival (Brittaset al. , 1992; Mallord et al. , 2024). This is the most likely explanation for Greece, where releases of captive birds have occurred since the 1980s, but without evidence of western genotypes in the wild populations yet. However, contrasting with Greece, in the UK we failed to find evidence of genetic structure between wild populations within the UK. These wild populations also share genetic similarities to the captive population based on mitochondrial (Figure 3) and microsatellite data (Figure S3_Appendix). Considering the low dispersal of Grey Partridge, this is indicative of widespread historic translocation of birds and/or the survival of captive releases.