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The article is about twice as long as what I'm quoting.

Bronx Zoo veterinary pathologists determined that in addition to the traumatic injuries, Flaco had two significant underlying conditions. He had a severe pigeon herpesvirus from eating feral pigeons that had become part of his diet, and exposure to four different anticoagulant rodenticides that are commonly used for rat control in New York City. These factors would have been debilitating and ultimately fatal, even without a traumatic injury, and may have predisposed him to flying into or falling from the building.

The identified herpesvirus can be carried by healthy pigeons but may cause fatal disease in birds of prey including owls infected by eating pigeons. This virus has been previously found in New York City pigeons and owls. In Flaco’s case, the viral infection caused severe tissue damage and inflammation in many organs, including the spleen, liver, gastrointestinal tract, bone marrow, and brain.

No other contributing factors were identified through the extensive testing that was performed.