We had two dead collared doves in the garden within a week of each other about a year ago (well, actually the 2nd one we had to dispatch, as it wasn't quite dead, but obviously very unwell). Internet research revealed that there was a disease affecting certain types of bird (doves, pigeons etc) where their throats swell and they can't swallow any food & starve (nasty!) then the food they can't swallow is contaminated and if eaten by another bird, they catch the disease. It recommended not feeding the birds for a few days if you find a dead or obviously unwell bird to stop the spread.