Se puede hacer un análisis del cadáver para ver que daño hay y  según los sintomas ver agentes compatibles que provoquen esos daños. El veneno parathion, por ejemplo puede verse al hacer una autopsia del estómago y tracto digestivo. Creo que habria que hacerle un análisis toxicológico, preservando el cadáver en freeser. Si esta bien conservada se pueden detectar virus. 

I do not recommend to use columbiformes as a raptor diet, unless the columbiformes have been reared in captivity.  Feral pigeons can carry many diseases that can be dangerous for raptor birds such as candidiasis, trichomoniasis, Newcastle disease, among others.  If you need to use them anyway, it is better to cook the bodies before to give them to the birds of prey.

I agree with Claudia. Feral pigeons may serve as vectors for tracheal worms or other parasites that are potentially fatal to raptors (and other species). If you're interested in determining cause of mortality, I recommend looking up the National Wildlife Health Center (Madison, WI, USA) or an equivalent near you that may have protocols available or will perform the necropsy for you. The NWHC here in the states has a procedure in place for submitting animal carcasses for necropsy when cause of mortality seems suspicious or when extreme mortality events occurr (i.e. large localized die-offs of a particular species). I would imagine there may be something similar for European countries. I have recently necropsied several galliforme species here in the U.S. and conducting a thorough necropsy procedure is extremely time consuming. Good luck.

Hello Hanno,

 my falconry tutor is using feral pidgeon for feeding, he has a contract with one small city in the country and he is catching ´em alive by drop-trap, where is lure, and he is checking it 2-3 times per week.

 He didnt use them for feeding for some time, he was checking their condition, how they look etc,  and then he started to feed his falcons or goshawk after some months and few autopsies of first individuals and they were alright. Also the communication among other falconers from surroundings really help... 

 If you ll have them in your hand, you can check the quality of feathers or skin, ectoparasites, old/new wounds and infection on the surface...

 The autopsy... we dont feed by guts, so we are putting everything out. On GIT, livers you can check the colours, that should be there,  condition of throat, their diet or worms in the lungs, heart, intestines.

if it is good we are separeting on the pieces sometimes, and beware of sharp pices of broken bones.

Thanks to all for your kind responses. As it seems a fixed protocol hasn't been developed so far.