Crows are a genus of highly-intelligent birds with populations spanning from Eurasia to the Americas.
As infected animals
After the Raccoon City Destruction Incident caused by James Marcus in the Arklay Mountains region, many crows became infected with the t-Virus after consuming smaller, infected animals. Unlike other animals infected by the t-Virus, crows showed no visible signs of infection aside from their increased size. If threatened, however, they would attack their prey in a flock. Otherwise, they were harmless. It was through the crows that S.T.A.R.S. Bravo team member Forest Speyer became infected with the t-Virus.
Infected crows would appear again after Raccoon City had become completely infected by the t-Virus. These crows were noticeably more aggressive than the ones found at the Arklay Laboratory, attacking anything they saw. A single crow managed to bring down Ed, a trained UBCS mercenary.
Cases of uninfected crows
Throughout his search for Ashley Graham in Europe, Leon encountered normal, uninfected crows on a regular basis. These crows posed no threat whatsoever. The crows had a tendency of collecting shiny objects, stealing pesetas; some even stole grenades.
Regular crows were also encountered by Parker Luciani and Jill Valentine while investigating on Globsters. The most notable location of these crows is in Resident Evil 4, whereas a large flock flies endlessly over the church in which Ashley is found. Uninfected crows can also be found in Resident Evil: Revelations 2, where in one of the episodes, killing them is required to earn one of the medals.
In Resident Evil: Extinction, a swarm of infected crows surrounds Claire Redfield's convoy with Claire realizing that they have become infected from feeding on Undead flesh. While the crows don't detect the convoy members at first, a dropped can draws their attention and they swarm the convoy, causing the school bus to crash. The crows kill a few of the convoy survivors including Otto and Betty before Alice telekinetically redirects a flamethrower blast to incinerate the crow swarm.
Upon arrival in Las Vegas, the convoy survivors are shocked to find it empty of Undead since the last they knew, it had been "crawling" with them. Alice realizes that the crows swept through Las Vegas and devoured all of the Undead.