Most birds die naturally, of cold, hunger or predation, and we rarely see the intact carcass. When birds are found dead it is usually when they have had collisions with vehicles, overhead wires or have struck a window (something that can be minimised by the use of silhouettes or other means of making the glass more obvious).
What action to take if a dead bird is found depends very much on the circumstances at the scene. Where a cause is fairly evident, as mentioned above, there is no need to take any further action. If the circumstances in which a dead bird is found raise suspicion that a crime or disease may be involved then action is required accordingly as follows:
Wildlife Crime: If you find a dead bird that you think may have been shot illegally or poisoned, contact the Investigations Team of the RSPB (01767 680551) and/or contact West Mercia Police on 101 or get in contact with a Wildlife Crime Officer. A relatively new group called: Birders Against Wildlife Crime (BAWC): http://birdersagainst.org/ have produced a leaflet giving advice on the 3Rs: Recognise, Record and Report which gives the do’s and don’t’s about wildlife crime. If the circumstances indicate that a significantly severe wildlife crime is still in progress, this warrants a 999 call to the Police and it is very important to follow RSPB or BAWC cautionary guidelines to safeguard yourself. Raptor persecution is in particular a topical and prevalent issue at present and the RSPB have a Raptor Crime Hotline: 0300-999-0101.
Disease: reporting disease in wild birds is important in relation to Avian Flu, which is a notifiable animal disease. These links to Government, Defra and APHA pages provide guidance on the disease. Poultry keepers in particular need to be aware of the signs of Bird Flu, but wild birds are rarely found dead from the disease. If wild waterfowl (swans, geese or ducks) or other dead wild birds, such as gulls or birds of prey, are found dead, especially, if in the cases of flocking species, multiple carcasses are involved they should be reported to the Defra helpline (03459 33 55 77). Defra will collect carcasses and test them to help understand how the disease is distributed geographically and in different types of bird. Reports are published and regularly updated on findings of highly pathogenic avian influenza (bird flu) in wild birds in Great Britain.
The BTO and RSPB have a more general interest in receiving reports of other diseases of birds which are not important to report to other agencies, The BTO Garden Birdwatch provides a means of reporting disease in garden birds. Avian pox, Trichomonosis and Salmonellosis are the more prevalent and are the subject of scientific publications https://www.bto.org/volunteer-surveys/gbw/publications/papers/disease.
The BTO and RSPB give advice on prevention of these diseases especially in relation to hygiene in the use of garden feeders.