From 2000-2008, over 9500 corvids from Ontario, Canada, including 6897 American Crows (Corvus brachyrhynchos), were tested during surveillance for West Nile virus (WNv). A subset of 589 crows, 189 from 2000, prior to the establishment of WNv in 2001, and 400 testing negative for WNv over the period 2001-2008, were examined for gross and microscopic lesions, and tested retrospectively for agents that might explain mortality. Findings were evaluated in relation to age, sex, year, time of year and body condition, and statistical relationships between various pathologic syndromes, including severe lymphoid depletion, and viral, bacterial, mycotic, parasitic and toxic agents were assessed. Integrating history, gross and microscopic findings, and results of ancillary diagnostic tests, final diagnoses (mean = 4.4/bird) were established, and cause of death was inferred, where possible. Of the 125 final diagnostic terms, 106 were considered potentially to have a negative impact on health, or to be of epidemiological or other scientific relevance. A cause of death was identified in 72% of birds. Twenty causes of death other than euthanasia (9%) were recognized, in descending order of frequency: trauma (total 45%), caused by blunt force, electrocution, gunshot, predation; infectious (total 11%), involving unknown or identified bacterial, fungal, and viral agents, alone or in various combinations; toxicity (total 3%), involving Avitrol®, anticoagulants, cholinesterase inhibitors and botulism; starvation (3%); dehydration (<1%); and heart failure (<1%). Gastrointestinal parasitism (24%), microfilaremia (19%), and Leucocytozoon infection (12%) were common, but not associated with mortality. Compared with the overall prevalence of WNv (23.9% of 5728 crows tested 2002-2008), other infections were uncommon in the study group. Avian pox (2.7%), aspergillosis (2.4%) and sepsis/polyserositis associated with Gram positive and Gram negative bacteria and Mycoplasma (4.8%) were most prevalent. An avian reovirus-like agent was associated with fatal necrohemorrhagic enterits in <1% of crows.