Release of Casualty Mice and Voles (Wildlife Casualty Management)

Summary Information
Type of technique Health & Management / UK Wildlife Casualty Management / Techniques:
Synonyms and Keywords N.B.  This information should be read in association with Wildlife Casualty Release which contains background information together with links to the Electronic Library and Organisations (UK Contacts). The related Species pages contain similar linkages.
Description This page has been prepared for the "UK Wildlife: First Aid and Care" Wildpro module, and is designed for the needs of the following species: Apodemus flavicollis - Yellow-necked mouse, Apodemus sylvaticus - Wood mouse, Micromys minutus - Harvest mouse, Mus musculus - House mouse,  Arvicola terrestris - European water vole, Clethrionomys glareolus - Bank vole, Microtus agrestis - Field vole, Microtus arvalis - Common vole.

These species are within the family Muridae.


  • No pre-release preparation is required if these animals have been in care for a short period of time.

Release assessment/criteria:

  • Released mammals:

    • Must be able to recognise, catch, manipulate, consume and digest their natural diet.
    • Must be capable of normal locomotion (movement) and have sufficient fitness for sustained activity.
    • Must have adequate sensory ability (sight, smell, hearing, touch).
    • Must be of an appropriate weight for the age, sex, and time of year. 
    • Must have a satisfactory hair coat.
    • Must show appropriate wariness of humans and domestic animals.
    • (P19.1.w10, P24.233.w11, J35.147.w1, D27)
  • Appropriate health checks should be carried out prior to release. A careful assessment (risk analysis) must be made as to the risks of released animals introducing novel pathogens (disease agents) into the wild population/environment.
    • These pathogens may have been acquired from domestic animals, other wildlife casualties or humans whilst the animal was in captivity.
    • The health checks should be designed to minimise the risk that pathogens posing a threat to wild populations of this or other species will be introduced into the environment when the animal is released.
    • (V.w5, V.w6, P28.2000.w1, J15.20.w3)

Selecting a release site:

  • Release back into the site the animal came from if known, or the nearest safe location. (D27)
  • Release programmes for harvest mice use carefully selected and protected sites. (D40)

Timing of release:

  • Release predominantly diurnal species in daytime and nocturnal species at night; see individual species pages for information on activity patterns.
  • Release during a period of fine weather whenever possible.

Type of release:

  • Hard release is generally used when releasing wildlife casualties of small prey species.

  • However, it is important to recognise that these species rely on the presence of burrows/runs for protection from predators. Release in the absence of such burrows/runs may considerably increase the chance of the animal being caught by a predator in the short term. 

  • Soft release is used routinely when animals of these species are released during organised captive breeding and release programmes.

  • (P28.2000.w1)

Mice and voles:

  • May be released into hedgerows, cat-free gardens, graveyards, golf-courses etc. (B224)
  • Release by placing the animal's sleeping box under a hedge away from a road.(B199)
  • Release during a period of fine weather .(V.w26)

Micromys minutus - Harvest mouse:

  • Release near field edge with tall grasses for nest building sites.(B151)
  • Release by placing the animal's sleeping box under a hedge away from a road.(B199)
  • Release during a period of fine weather.(V.w26)
  • Information on keeping and release of harvest mice (Micromys minutus - Harvest mouse) may be obtained from Penny Rudd.(V.w18)

Arvicola terrestris - European water vole:

  • Contact The Mammal Society for details of any local water vole projects. (B151)
  • Mike Jordan (V.w30) may be contacted regarding the captive breeding and release programme for water voles. (V.w6)
  • Release in an aquatic environment.(B224)
  • Release near a stream.(B199)
  • Release in good water vole habitat, at side of small rivers or canals.(B151)
Appropriate Use (?)
  • Soft release is particularly important for hand reared animals, particularly of species which need to learn about their surroundings (e.g. food sources) and/or learn survival skills such as hunting.
  • Soft release is also suitable for animals which have been in care for prolonged periods.
  • Soft release may compensate for difficulties of newly released animals finding food and shelter, particularly in a new environment and/or at a time of reduced physical fitness/stamina.
  • Hard release is most appropriate for animals which have been held in captivity for only a short time, for adult animals and for animals being released back into their own territory.
  • The individual animal must, at the the time of release, be healthy, have a reasonable level of fitness and be able to fend for itself in the wild.
  • Diurnal species should be released in the morning, giving them a full day to explore and look for food and shelter before nightfall.
  • Nocturnal species should be released at night.
  • Release should preferably take place during a period of fine weather.
  • Releasing back into the site the animal came from may assist in ensuring that it is released into a suitable habitat. (B199)
  • Information on keeping and release of harvest mice (Micromys minutus - Harvest mouse)may be obtained from Penny Rudd (V.w18).
  • Mike Jordan (V.w30) may be contacted regarding the captive breeding and release programme for water voles. 
Complications/ Limitations / Risk
  • Mice and voles should not be released near busy roads or in the territory of a known hunting domestic cat.
  • Hard release is least appropriate for juveniles which have been hand reared, particularly species for which learning about their environment and/or social skills are important.
  • Hard release may also be inappropriate for adults which have been maintained in captivity for prolonged periods and/or are being released at a site distant from their original location.
  • Released animals may be at risk of contracting disease if there is an ongoing disease problem in the wild population at the time of release.
  • The wild population may be at risk from novel pathogens (disease agents) carried by a rehabilitated animal. These pathogens may pose the greatest threat to free-ranging populations if the animal is to be released at a site distant from its original location therefore increasing the likelihood of spread of disease. It is important to remember that the casualty wild animal may have acquired disease from domestic animals, other wildlife casualties or humans whilst in captivity. 
Equipment / Chemicals required and Suppliers
Expertise level / Ease of Use
  • Knowledge of the natural history of the species and its habitat requirements are important for release of mice and voles into an appropriate site at an appropriate time; this is particularly important for the release of rare species such as Micromys minutus - Harvest mouse and Arvicola terrestris - European water vole.
Cost/ Availability
  • Considerable time investment may be required to find suitable release sites for Micromys minutus - Harvest mouse and Arvicola terrestris - European water vole.
  • Costs of appropriate health screening.
Legal and Ethical Considerations
  • The potential risks to the individual being released and to the wild population into which it is being released (also to domestic animals) must be considered before release is undertaken.
  • The potential risk to humans and pets from habituated/tame individuals must be considered.
  • An offence may be committed under Section 1 of the Abandonment of Animals Act 1960 if a released rehabilitated animal does not have a reasonable chance of survival (i.e. a chance similar to its non-rehabilitated peers). 
    • This may include release at an unsuitable site, in the wrong territory, unfit, not having learned to hunt, at the wrong time of year etc. 

    (J35.147.w1, B156.21.w21, B223, W5.Jan01)

  • (J35.147.w1, B156.21.w21, B223, W5.Jan01)
Author Debra Bourne
Referee Becki Lawson and Suzanne Boardman

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