A woman in my bible class recently visited Europe and noticed that the protestant churches have a rooster on them. My first impression was that it is based on Peter's denial and more in general our denial of Jesus too often. In a library I found a German lexicon with a comprehensive description. Church fathers are very interested in the cock. Set on top of the cross on a church, it a rooster means the symbol of the victory of Christ.
Dutch books have other interesting information.
First the cock was a symbol of light, later it became a symbol of the light of mercy. The power of the night demons stops when the cock crows. For this reason Early Church fathers used the cock as symbol of resurrection. A rooster on top of a church reminds to the denial of Peter, to preach the gospel and to be alert. I think the people of the reformation used the rooster to distinguish from the catholics by using Greek and Roman symbolism.