One of the Popes decreed that cockerals or roosters like this one should adorn the spire of each place of worship to remind the flock that Jesus said that Peter would deny Jesus thrice before the cock crowed. Since then the rooster weather vane has found it's way into more general use on farm and home.
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http://en.wikipedia.org/wiki/Wikipedia:Reference_desk/Archives/Humanities/2007_September_3#Weathercock_on_churchtowers "Please tell me the origin of the weathercock on churchtowers..." и ответы.