Polecats Are On Their Way Back

Johnny Birks of The Vincent Wildlife Trust

The polecat, the mammalogists' spiritual equivalent of the quintessentially Welsh Red Kite, is recovering well in Britain. That is one conclusion from a major study recently published by The Vincent Wildlife Trust (VWT). The report, The Distribution and Status of the Polecat Mustela putorius in Britain in the 1990s, by Johnny Birks and Andrew Kitchener, includes a review of previous distribution surveys, the findings of a new survey, development of a method to monitor variations in abundance based on live-trapping by volunteers, a summary of the relationship between polecats and ferrets, and a description of a study of polecat ecology on lowland farmland in England (part of which was carried out in Worcestershire).

The VWT distribution survey produced over one thousand records from the 1990s, 68% of which were road casualties. Many of these were collected by naturalists (thanks to all who helped in Worcestershire) for us to confirm their identity as either true polecats or ferret hybrids. Records of true polecats came from 228 'new' 10-km squares (several of which were in Worcestershire), confirming continuation of the encouraging recovery after near-extinction at the beginning of this century. Worcestershire is now completely repopulated by true polecats (a process which probably began in the far west of the county in the 1960s), although feral ferrets and hybrids will always occur from time to time as a result of ferrety escapes or releases.

for the diagrams please buy a copy of the printed version of Worcs Record

Figure 1. The caption is on the figure. However the reproduction method may not show the populations derived from introductions clearly. They are clearly defined in Scotland, the Lake District, and the SE band running SW-NE across England away from the growing Welsh population.

The latest survey suggests a dramatic 93% increase since 1991 in the area occupied by polecats in Britain. However, only some 30% of this is thought to represent 'real' range expansion. The remainder amounted to 'catching up' following a period of conservative recording which had not reflected the true extent of the expanding range, nor the establishment of new populations derived from re-introduction's since 1970 (now occupying 87 10-km squares). The understandable confusion with feral ferrets and mink, and assumptions about the polecat being mainly confined to Wales probably explain past reticence to report polecats in the Midlands. Notably, the VWT survey indicates that the polecat occurs in more 10-km squares in England than in Wales. Reduced persecution and the post-myxomatosis recovery of rabbits, a favoured prey, are thought to be the main factors behind the polecat's continuing advance. Clearly the polecat's recovery has much in common with that of the buzzard.

for the diagrams please buy a copy of the printed version of Worcs Record

Figure 2. The caption is on the figure, which should be viewed in relation to figure 1

All possible polecat corpses collected were processed by the National Museums of Scotland where, among many valuable studies, work on the confusing 'ferret question' was carried out. Skins were scored and skulls measured from polecats and ferrets from many parts of Britain, and compared with those from 'pure' Welsh animals to determine where else true polecats still survived. Despite increasing evidence of hybridisation with ferrets as one moved East through the polecat's range, true Welsh-type animals were found throughout Wales, in the English Midlands, and in populations derived from re-introduction's. True polecats are now re-established in the Midlands by natural spread at least as far east as a line through Macclesfield, Northampton and Oxford, and through re-introduction's to Argyll, Cumbria, the Chilterns and parts of Hampshire and Wiltshire in central southern England.

Studies of mitochondrial DNA at the University of Leeds revealed two geographically distinct lineages in British polecats and feral ferrets that probably reflect the past history of hybridisation. However, this work also showed just how closely related the two forms are, adding weight to the view that the ferret is simply a domesticated variant of the wild polecat. Domestication in ferrets involved selection against many of the predatory and survival skills found in wild polecats. As a consequence the ferret is severely disadvantaged in the feral state, so there is likely to be selection for the polecat phenotype in all wild populations. This is expected to limit the damaging effects, in terms of polecat conservation, of hybridisation with ferrets.

To improve understanding of the polecat's recovery, a monitoring exercise based on live-trapping by volunteers was developed and tested. 136 1-km squares of the Ordnance Survey grid were each live-trapped for seven nights. Significant regional variations in trapping success were recorded, with squares near to the species' historical stronghold more likely to catch polecats. The trapping data revealed a positive association with rabbit abundance, and a negative association with areas of high road density. It seems likely that polecat mortality on Britain's roads may be high enough in some areas to cause population effects; this may explain why the species has still not re-established itself fully in the valleys of South Wales, nor around the fringes of the West Midlands conurbation. Estimates of population density suggest a modest abundance on farmland of between 0.7 and 1.0 polecats per square kilometer.

The apparent link between polecats and rabbits was backed up by diet and radio-tracking studies. Analysis of stomach contents of road casualty animals showed that rabbits comprised 85% of the bulk of prey remains. The radio-tracking study was carried out mainly in the Parish of Matron to the west of the Malvern Hills (in modern Herefordshire, but in vice-county 37), and around the Kemerton Estate on the edge of Bredon Hill on the Worcs/Glos border. Radio-tagged polecats spent half their time in rabbit warrens hunting the occupants and sleeping. Warrens comprised 80% of all daytime resting sites identified. In winter, farmyards were the most preferred habitat. Polecats visited them to prey upon rodents, leading to the risk of contamination with anticoagulant rodenticides. Analyses of liver tissues recovered from road casualty polecats shows that rodenticide contamination is widespread, and matches the pattern found in barn owls. The VWT is still studying the extent of this problem in collaboration with the Institute of Terrestrial Ecology.

The VWT report makes recommendations for research and action to promote polecat conservation. It recognises that the species is spreading from an upland, western refuge with relatively low human activity towards the intensively farmed lowlands where negative pressures are much greater. Areas of concern include the effects of road casualties, rodenticide poisoning, deliberate and accidental persecution, and habitat degradation. Maintaining rabbit populations is regarded as important, especially on intensive farmland where alternative prey are scarce.

For the time being we are no longer appealing for polecat records from Worcestershire, though a new survey may be organised early in the new millennium.

The VWT Polecat Report (152 pages) is available from The Vincent Wildlife Trust, 10 Lovat Lane, London EC3R 8DN, price 8.00 to include post and packaging.

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