I’m honestly not sure. West Nile virus produced a ton of papers about death from disease; but all rather unhelpful from the “so imagine I was an avian actuary trying to design a maximally useless health insurance plan for crows…” perspective; though, unshockingly, previously unexposed populations went down like wheat before the reaper when West Nile showed up. This one, at least, had the politeness to say what happened to the non-WN cases: “WNV-negative crows (n = 85) died from traumatic injuries (51.8%), predation (16.5%), avian pox (14.1%), pneumonia (11.8%), and poisoning (5.9%).” Non-predation traumatic injuries aren’t further specified; I assume windows are not helpful.
This one is pretty old; but has mortality rates by age and ranges for a number of species, which seemed somewhat relevant; though the ‘band and recover’ approach probably overestimates causes of death that leave a body, uneaten, to be discovered; looks like food stress is a thing for some species, disease/unspecified a major one, and getting shot a major one.
This one on crow inbreeding suggests that infectious disease must be a reasonably significant factor in the absence of getting shot or poisoned; but not clear how many die of disease vs. predators.
Also, if disease is the major risk, that would raise the question of whether association with a given area is helpful(yes if the local food items are coated with soil borne parasites or something; probably not if birds are dying of things with incubation periods that allow them to proceed without incident for some days or more and die pretty much anywhere within their normal range); and whether gathering around to observe the dead might actually be risky(probably not if close contact is required for transmission; possibly if it’s air or mosquito borne).
I will have to look further.
(edit: in the course of looking for crow mortality numbers I ran across this report of a cluster of poisonings in Portland early this year. Probably not terribly noteworthy in itself; but I looked up the ‘Avitrol’ agent believed responsible and its intended mechanism of action seemed to rely on a variant(though one that requires less cognitive sophistication) of this learned-fear behavior.
Our wiki overlords report that 4-Aminopyridine is fairly low toxicity(even has some human medical applications); but that it is a potent convulsant. The theory of using it for bird control is that seeing poisoned birds flailing and seizing and emitting distress calls will freak out the flock and inspire them to not return. From here:
Mode Of Action: Avitrol causes behaviors similar to an epileptic seizure. Birds eating the treated bait will emit distress signals used by their species when they are frightened or injured. This may include flying erratically, vocalizing, trembling, dilation of the pupils and other symptoms. This will frighten the flock and cause it to leave the site.