How will you know which animals have been neutered after you have put them back?
And how will the local authorities know that they are part of a control programme?

Identification of dogs
For control of a dog population to be effective, it must be possible to identify each dog after neutering.
There are various methods available which vary in how reliable, permanent, visible and easy to apply they are.

Microchipping – This is the best method to use. A microchip can be implanted easily and with minimal discomfort. The unique number is logged on to a database that also records the details of the animal and its owner. However, this method only works if there is a reliable and accessible computerised database available and if enough people have a suitable scanner which is needed to read the microchip number. Unfortunately, there is no external/visible indicator that the animal is chipped. There is also a small risk of the microchip migrating subcutaneously or failing.

A collar with a tag is a simple way of identifying a dog that is used to being handled. However, they can break or be deliberately or accidentally removed.

Ear tags are also a useful identification method, but it is a painful procedure and should be performed under anaesthetic. Handling of the dog is required to read the tag and tags can be deliberately or accidentally removed.

Tattoos in the earflap are also used. This requires specialist equipment and must be done under anaesthesia. The disadvantage of tattoos is that they will fade with time, do not show up well on pigmented skin and are only visible from a short distance.

Earflap-notching can also be used to identify dogs which have been neutered. This must be performed under anaesthesia. Ear notches are not easily visible in long haired dogs and the wounds can be difficult to distinguish from bite wounds and other injuries.

Identification of cats

Ear-tipping is a very clear and effective sign to show that a stray/feral cat has been neutered when viewed from a distance. It is also good for public relations as it is very visible and shows that the cat is part of a managed colony.
Ear-tipping is done by removing 1 cm from the tip of the left ear while the cat is anaesthetised, using forceps to stop bleeding after the cut has been made. Cats show no sign of pain or discomfort after ear-tipping. In some European Union countries there are laws against causing mutilations to animals, but the procedure of ear-tipping has in most cases been exempted, because it is of practical use and prevents the worse situation of a cat being subjected to trapping, anaesthesia and surgery for a second time.

Microchipping – see above for further information. Ear-tipping is not unique to the individual, so it is best to combine it with microchipping to allow each individual cat to be identified. The microchip can be read while the animal is conscious (eg a cat in a trap) but not from a distance which is why it is useful to also ear-tip the cat.