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In most cases, it is important to identify individual animals in a study. Ideally, the method should ensure a permanent, indisputable identification of an individual and should not place a burden on the animal. Some methods of identification are listed below (however, this is not an exhaustive list):

• Collars with identification tags - for dogs and cats
• Dyes - for temporary or short-term marking of some birds and animals (especially used on white areas of fur or feathers, or on the tails of some rodents)
• Ear notching - for identifying small rodents and pigs, although tattooing is preferred (there is a universally recognized notching scheme for small rodents, but those using other animals have devised their own pattern of coding)
• Ear tags - commonly used on farm animals (e.g., sheep and cattle); for small animals (e.g., mice, rats) ear tags are not recommended as they are heavy and distort the ears
• Feather notching - an acceptable marking technique, provided such modification does not impair normal flight
• Fin clipping - for fish
• Implantable microchips - a unique and tamperproof means of permanently identifying animals
• Tattooing - may be done on any part of the body, but consideration should be given to a location where the tattoo may be read without excessive handling of the animal
• Wing and leg bands - commonly used to identify birds

Animal Welfare Issues

Several of the techniques described above are associated with pain to the animal (e.g., ear tagging, ear notching, wing banding, microchip insertion and tattooing). This can be alleviated with the use of an analgesic. Small animals (e.g., mice) may be anesthetized with a volatile anesthetic which allows for quick recovery from the procedure. In larger animals, a local anesthetic may be sufficient.

With birds, care must be taken when banding wings to ensure that no muscle tissue is pierced and that the band is not between the radius and ulna of the wing. The bands must be checked during growth of the bird, with replacement of poorly placed bands. Other banding systems, such as plastic leg bands or nylon bands placed in the wing web or the skin at the back of the neck, may provide adequate identification with fewer welfare concerns.

Some techniques for marking and tagging are no longer considered appropriate, as there are now less painful and/or less invasive alternative methods available. For example, toe clipping to identify newborn rodents in a litter or for some wild animals (e.g., salamanders), branding of cattle and horses, and the removal of combs of roosters.

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