The quicker you can bring the dead bird directly to the vet's office, preferably within six hours, the better the diagnosis. You can help to preserve the bird in good condition by wetting it thoroughly as soon after death as possible. First, wash the bird's body in warm, soapy water. Mild dish detergent is fine. This will break down the insulating ability of the feathers.
Feathers keep body heat in, allowing the body to decompose faster.
If there is too much tissue decomposition, a necropsy will not give the answers that you are looking for. This can help preserve the bird for up to a week, although as time passes the tissues will deteriorate. Make sure that you do not put the bird in the freezer, since freezing allows ice crystals to form. These crystals can rupture cells, making it harder to find lesions that may give clues as to the cause of death. In addition, if the body is frozen, bacterial cultures can not be taken as many pathogenic bacteria may have been killed.